Algae as nutritional and functional food sources: revisiting our understanding

Abstract

Global demand for macroalgal and microalgal foods is growing, and algae are increasingly being consumed for functional benefits beyond the traditional considerations of nutrition and health. There is substantial evidence for the health benefits of algal-derived food products, but there remain considerable challenges in quantifying these benefits, as well as possible adverse effects. First, there is a limited understanding of nutritional composition across algal species, geographical regions, and seasons, all of which can substantially affect their dietary value. The second issue is quantifying which fractions of algal foods are bioavailable to humans, and which factors influence how food constituents are released, ranging from food preparation through genetic differentiation in the gut microbiome. Third is understanding how algal nutritional and functional constituents interact in human metabolism. Superimposed considerations are the effects of harvesting, storage, and food processing techniques that can dramatically influence the potential nutritive value of algal-derived foods. We highlight this rapidly advancing area of algal science with a particular focus on the key research required to assess better the health benefits of an alga or algal product. There are rich opportunities for phycologists in this emerging field, requiring exciting new experimental and collaborative approaches.

Full text

Algae as nutritional and functional food sources: revisiting
our understanding
Mark L. Wells
1
&Philippe Potin
2
&James S. Craigie
3
&John A. Raven
4,5
&
Sabeeha S. Merchant
6
&Katherine E. Helliwell
7,8
&Alison G. Smith
7
&
Mary Ellen Camire
9
&Susan H. Brawley
1
Received: 6 June 2016 /Revised and accepted: 25 September 2016
#The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Global demand for macroalgal and microalgal
foods is growing, and algae are increasingly being consumed
for functional benefits beyond the traditional considerations of
nutrition and health. There is substantial evidence for the
health benefits of algal-derived food products, but there re-
main considerable challenges in quantifying these benefits,
as well as possible adverse effects. First, there is a limited
understanding of nutritional composition across algal species,
geographical regions, and seasons, all of which can substan-
tially affect their dietary value. The second issue is quantifying
which fractions of algal foods are bioavailable to humans, and
which factors influence how food constituents are released,
ranging from food preparation through genetic differentiation
in the gut microbiome. Third is understanding how algal nu-
tritional and functional constituents interact in human metab-
olism. Superimposed considerations are the effects of harvest-
ing, storage, and food processing techniques that can dramat-
ically influence the potential nutritive value of algal-derived
foods. We highlight this rapidly advancing area of algal sci-
ence with a particular focus on the key research required to
assess better the health benefits of an alga or algal product.
There are rich opportunities for phycologists in this emerging
field, requiring exciting new experimental and collaborative
approaches.
Keywords Algal foods .Antioxidants .Arsenosugars .
Experimental design .Microalgal supplements .Nutritional
minerals .Omega-3-fatty acids .Polysaccharides .Sea
vegetables .Vitamins
Introduction
Algae have been part of the human diet for thousands of years,
based on archaeological evidence from 14,000 yBP in Chile
(Dillehay et al. 2008)andearlywrittenaccounts(e.g.,in
China, 300 A.D.; in Ireland, 600 A.D.; Newton 1951;
Tse ng 1981; Aaronson 1986;Turner2003;Gantarand
Electronic supplementary material The online version of this article
(doi:10.1007/s10811-016-0974-5) contains supplementary material,
which is available to authorized users.
*Susan H. Brawley
brawley@maine.edu
1
School of Marine Sciences, University of Maine, Orono, ME 04469,
USA
2
Integrative Biology of Marine Models, Station Biologique Roscoff,
CNRS-Université Pierre et Marie Curie, Place Georges Teissier,
29680 Roscoff, France
3
National Research Council of Canada, 1411 Oxford Street,
Halifax, NS B3H 3Z1, Canada
4
Division of Plant Sciences, University of Dundee (James Hutton
Inst), Invergowrie, Dundee DD2 5DA, Scotland, UK
5
Plant Functional Biology and Climate Change Cluster, University of
Technology Sydney, Ultimo, NSW 2007, Australia
6
Department of Chemistry & Biochemistry, University of
California-Los Angeles, 607 Charles E. Young Dr., East, Los
Angeles, CA 90095-1569, USA
7
Department of Plant Sciences, University of Cambridge, Downing
St., Cambridge CB2 3EA, UK
8
Present address: Marine Biological Association of the UK, Citadel
Hill, Plymouth PL1 2PB, UK
9
School of Food and Agriculture, University of Maine,
Orono, ME 04469, USA
J Appl Phycol
DOI 10.1007/s10811-016-0974-5

Svircev 2008;Craigie2010). In North America, the Tsimshian
First Nations’people named the month of May for the time of
year when they harvested the important food crop of Pyropia
(Fig. 1). More contemporaneously, the global harvest of sea-
weeds in 2013 was estimated at US $6.7 billion, and over 95 %
was produced in mariculture, with China and Indonesia being
the top producers (FAO 2015). In addition to macroalgae,
some microalgae are cultivated for foods and food additives
(Switzer 1980; Jassby 1988;Fournieretal.2005;Gantarand
Svircev 2008; Chacón-Lee and González-Mariño 2010;FAO
2016). The FAO (2014)estimatedthat38%ofthe23.8million
t of seaweeds in the 2012 global harvest was eaten by humans
in forms recognizable to them as seaweeds (e.g., kelps,
nori/laver), not counting additional consumption of hydrocol-
loids (e.g., agars, alginates, carrageenans) used as thickening
agents in foods and beverages. Human consumption of algal
foods varies by nation, with Japanese diets representing a re-
cent (2010–2014) annual per capita consumption ranging from
9.6 (2014) to 11.0 (2010) g macroalgae day
−1
(MHLW 2014).
Overall, the trend towards increasing nutritional demand for
algal products on a global basis stems from a greater focus on
health and wider use of food additives.
In addition to their nutritional value, algae increasingly are
being marketed as Bfunctional foods^or Bnutraceuticals^;
these terms have no legal status in many nations but describe
foods that contain bioactive compounds, or phytochemicals,
that may benefit health beyond the role of basic nutrition (e.g.,
anti-inflammatories, disease prevention; Bagchi 2006;
Hafting et al. 2012). The path from algal research to the
launching of new food products or dietary supplements is
strongly affected by industrial, regulatory, and nutritional con-
siderations (e.g., see Borowitzka 2013a; Finley et al. 2014).
The widespread interest in algal foods and/or their functional
food potential is evident in numerous recent reviews (Warrand
2006; MacArtain et al. 2007; Kulshreshtha et al. 2008;
Bocanegra et al. 2009;Mendesetal.2009;Cottinetal.
2011;HarnedyandFitzGerald2011;HoldtandKraan2011;
Lordan et al. 2011; Pangestuti and Kim 2011; Stengel et al.
2011; Cornish et al. 2015; Hafting et al. 2015) and books
(Rhatigan 2009;Mouritsen2013;TiwariandTroy2015;
Fleurence and Levine 2016). Many studies report the potential
nutritional or bioactive content of different algae but many
fewer studies quantify the bioavailability of nutrients and phy-
tochemicals from algal foods. Our purpose is to review and
assess what is known about different food components (i.e.,
proteins, polysaccharides, lipids, vitamins, minerals, and anti-
oxidants, potential toxicants) in the context of improving
knowledge about the efficacy of algal foods. There are rich
opportunities for phycologists to collaborate with other scien-
tists and clinicians in this emerging field from algal
Bprospecting^to defining nutritional value, bioaccessibility,
and subsequent bioactivity, to the design and construction of
mid-large cultivation systems for production of commercial-
scale product.
Digestion and bioavailability
In this article we use the term bioavailability, as defined by
Carbonell-Capella et al. (2014)Bas a combination of bioactiv-
ity and bioaccessibility,^where bioaccessibility refers to the
release from the food matrix, transformations during diges-
tion, and transport across the digestive epithelium, while bio-
activity encompasses uptake into tissues, metabolism, and
physiological effects. Because of the difficulties, both practi-
cal and ethical in terms of measuring bioactivity, the fraction
of a given compound or its metabolite that reaches the system-
ic circulation (Holst and Williamson 2008) can be considered
bioaccessible, but not necessarily bioactive. Most published
evaluations of bioactivity of algal foods are based on short-
term in vitro tests using algal extracts that frequently are of ill-
defined composition and purity, so a clear understanding of
Fig 1 a Pyropia spp. being dried in squares in the intertidal zone by First
Nations’people at Pearce Island, British Columbia (2009). Harvesters
would traditionally lay the seaweed out to dry on warm rocks while
waiting for those fishing to return with the canoes (photo credit, Amy
Deveau). bChecking the seaweed squares after transfer to cedar racks for
final drying (photo credit, Victoria Wyllie-Echeverria)
J Appl Phycol

their food value is highly constrained. Particularly lacking is
information on the behavior of algal food components in the
gut. For example, can the purported active metabolites identi-
fied in in vitro studies be transferred from the gut lumen into
the body? Likewise, are observed in vivo biological effects the
consequence of biological uptake or instead indirect outcomes
stemming from improved functionality or composition of the
intestinal microbiome? It is important then to consider the
process of digestion and transformation in the human system.
Digestion refers to the physical and biochemical degrada-
tion of foods and the nutrients therein in preparation for ab-
sorption into the body. Digestion begins in the mouth with
chewing, which reduces particle size and mixes food with
saliva (Lovegrove et al. 2015). The predominant salivary en-
zyme is alpha (α)-amylase, which is specific for α(1→4) glu-
cose linkages, and human salivary amylase is more active than
that from other primates (Boehlke et al. 2015). Hardy et al.
(2015) hypothesized that cooking to increase digestibility and
sensory quality of starch-rich foods helped drive human evo-
lution by providing more glucose to growing brains. Studies
of the effect of human saliva on algae and specifically algal
starch are lacking, however. The relative importance of sali-
vary versus pancreatic amylase in starch digestion also is not
clear (Lovegrove et al. 2015). Pepsin and the pepsinogens
begin protein digestion in the stomach, aided by hydrochloric
acid that denatures proteins and releases nutrients from the
food matrix. Lipases produced in the mouth and stomach be-
gin the process of digesting triacylglycerols. The stomach also
releases intrinsic factor that is essential for vitamin B
12
absorp-
tion in the small intestine. Gastric peristalsis further reduces
food particle size, preparing macronutrients for additional
chemical breakdown and absorption in the small intestine.
The pancreas discharges a mixture of trypsin, chymotrypsin,
carboxypeptidases, α-amylase, lipase, and other enzymes that
respectively digest proteins and peptides, starches, triacylglyc-
erols, and other compounds in the small intestine (Gropper
and Smith 2013). The mixture of proteases, amylase, and li-
pase are collectively known as pancreatin; porcine pancreatin
is often used to model human digestion in in vitro systems.
The small intestine itself releases a variety of enzymes acting
on peptides, amino acids, monoacyglycerols, disaccharides,
and α(1→4) and α(1→6) linkages in oligosaccharides, dex-
trins, and polysaccharides such as starch. Micronutrients such
as vitamins and minerals also are absorbed in the small intes-
tine once they are solubilized from the food matrix.
Fucoxanthin, a key algal carotenoid, may be better absorbed
if other lipids are present (Peng et al. 2011).
Humans lack the ability to digest β(1→4) linkages in glu-
can polysaccharides, as in cellulose and hemicelluloses such
as xyloglucan, and this indigestible material is referred to as
dietary fiber. The undigested materials continue on to the large
intestine (colon) where microbial co-metabolism ferments
substrates such as non-starch polysaccharides, resistant starch,
and oligosaccharides to short-chain fatty acids, and proteins
into a wider variety of compounds. These bacterial-dependent
enzymatic processes are not considered Bdigestion,^although
the fermentation products can provide nutritional or functional
benefits either by being absorbed and transported via the
bloodstream or by shaping more healthful gut microbiomes
and chemical conditions in the colon (MacFarlane and
MacFarlane 2012). Indigestible, fermentable carbohydrates
and sugar alcohols are referred to as FODMAP (fermentable,
oligo-, di- mono-saccharides and polyols) (Gropper and Smith
2013). Algal proteins and carbohydrates that escape complete
digestion in the small intestine may benefit humans by stimu-
lating immune response indirectly via promotion of microbial
responses (Cian et al. 2015). Dietary modulation of the colon-
ic flora and the impact of bacterial fermentation products on
human health are rapidly evolving areas of research (Duffy
et al. 2015) and are likely to be especially important consid-
erations in assessing the health benefits of algal-derived foods.
Not all human gut microbiomes have equal competencies,
as algal polysaccharide fermentation differs among humans
from different regions. The arsenal of polysaccharide-
degrading enzymes exhibited in the common gut bacterium
(Bacteroides plebeius) of Japanese people, but not Americans,
appears to result from horizontal gene transfer (HGT) from
Zobellia galactanivorans (Bacteroidetes), a marine bacterium
inhabiting the surfaces of algae such as nori (Hehemann et al.
2010). HGTalso may explain the presence of a gene cluster in
Japanese gut Bacteroides that enables fermentation of algi-
nates in brown algal cell walls (Thomas et al. 2012).
Similarly, a small cohort of Spaniards possesses gut
microbiomes with apparently HGT-provided porphyranases
and agarases (Hehemann et al. 2012). Such striking differ-
ences emphasize the complex interactions among food cus-
toms, dietary history, and gut microbiomes that complicate
study of the nutritional and functional benefits of algal foods
(Paulsen and Barsett 2005; Costello et al. 2012;Gordon2012;
Nicholson et al. 2012).
The importance of assessing the biological availability of
nutritional and functional food components cannot be
underestimated. Bioavailability has critical relevance to both
the proportional digestion and uptake of nutrients and func-
tional food components, but also the degree of fermentation
and nature of the host-microbial co-metabolism in the colon.
While there exists a vast literature on the food content of
microalgal and macroalgal foods and supplements, extrapolat-
ing these findings to assess their quantitative contribution to
human health is more tenuous. The analytically determined
concentration of constituents in food can differ, sometimes
substantially, from that actually crossing from the digestive
tract into the blood (i.e., the bioaccessible fraction).
Moreover, current analytical approaches give even less insight
to the complexity of interacting effects that regulate the bac-
terial flora of the colon, and hence the nature of fermentation
J Appl Phycol

products. Confounding issues stem from the food itself (e.g.,
the presence and nature of intact cell walls, soluble fiber char-
acteristics, and the presence of other substances that may inhibit
or facilitate the uptake of metabolites), the harvest season (e.g.,
altered metabolite and biomass composition, environmental var-
iability of essential precursors, and anthropogenic factors), and
the food preparation methods (Sensoy 2014). Analytical
methods such as simulated gastrointestinal digestion (Moreda-
Pineiro et al. 2011;Maehreetal.2014), xenobiotic animal
models, and molecular biological and genetic techniques can
provide a sound basis for improvedassessmentofbioavailabil-
ity; however, their use is not yet widespread in the studyof foods
of algal origin. As a consequence, and despite highly accurate
and precise analytical determinations of food content, current
knowledge of the nutritional or functional food value of algal
products remains largely qualitative. The development of appro-
priate model systems and use of rigorous experimental design
thus is essential in order to verify the bioavailability of nutrition-
al and functional components of algae used in all foods.
Proteins
Protein content differs widely across groups of algae (Online
Resource 1). The filamentous cyanobacterium Arthrospira
platensis (Bspirulina^) and various commercial species of the
unicellular green alga Chlorella (Fig. 2) contain up to 70 %
dry wt protein; these microalgae also have an amino acid
profile that compares well with egg, notably containing all
of the essential amino acids (EAA) that humans cannot syn-
thesize and must obtain from foods (Online Resource 2).
Historically, Bspirulina^was wild-harvested as a protein-rich
whole food in many cultures outside Europe and North
America (Gantar and Svircev 2008). Today, domesticated
Bspirulina^and Chlorella from several large producers have
BGRAS^designations [Generally Recognized As Safe (FDA
2016)]. Large-scale production of both Bspirulina^and
Chlorella occurs throughout the world, and these well-
domesticated crops are added to many foods to increase their
protein and other nutritional contents (e.g., salad dressings,
beverages, baked goods), and/or sold as protein supplements
(e.g., Lubitz 1963; Ciferri 1983; Jassby 1988;Belay1997;
Gantar and Svircev 2008; Szabo et al. 2013;Safietal.2014).
Among the marine macroalgae, red and green algae [e.g.,
Porphyra spp. (Blaver^), Pyropia spp. (Bnori^), Palmaria
palmata (Bdulse^), Ulva spp. (Bsea lettuce^)] often contain
high levels of protein (as % dry wt) in contrast to lower levels
in most brown algae (Online Resource 1;Dawczynskietal.
2007; Holdt and Kraan 2011; Pereira 2011; Taboada et al.
2013;A
ngell et al. 2016). During periods of nutrient limitation
such as during the summer stratification of coastal waters,
however, macroalgal protein content decreases, and the rela-
tive proportions of amino acids change (Online Resource 2;
Galland-Irmouli et al. 1999; Johnson et al. 2014; Schiener
et al. 2015). Historic harvesting times and current harvesting
regimes usually occur at times when protein contents are fa-
vorable (e.g., Butler 1936; Black 1950; Turner 2003), but
there is remarkably poor documentation of seasonal changes
in protein content and amino acid profiles. Strong conclusions
about nutritional content also depend upon good biological
sampling (=simultaneously collected replicates) combined
with appropriate laboratory analyses (=analytical replicates
of each biological sample). Galland-Irmouli et al. (1999)ana-
lyzed one dulse blade/month (except August) from the
Brittany coast with three technical replicates and found
∼15 % of dry mass as protein in a June blade while a
November blade contained ∼23 % protein. A seasonal study
(October 2010–October 2011, 3–8monthssampled/species)
of protein content of four kelps (Laminaria digitata,
Laminaria hyperborea,Saccharina latissima,Alaria
esculenta) based on three technical replicates/species recently
demonstrated an inverse relationship between protein con-
tent—higher in winter—and polysaccharide content—higher
in summer, as well as clearly showing the higher protein con-
tent of Alaria esculenta (Online Resource 1)comparedtothe
other kelps (Schiener et al. 2015). There is a pressing need for
better replication of protein and amino acid analyses, as for all
nutritional components in macroalgal studies, as well as better
Fig 2 a Haematococcus pluvialis cells showing droplets of red astaxanthin within the cells; bChlorella vulgaris;cArthrospira maxima SAG 21-99
(also known as spirulina). Scale bar =15μm. (photo credits, Maria Zori)
J Appl Phycol

definition of the natural intertidal or commercial sites from
which analyzed samples were obtained (N.B. Shuuluka et al.
2013 as example). Some new sea vegetable products will ben-
efit from complementary food constituents (Woolf et al. 2011;
see vProtein software).
Protein concentration in algae is often estimated using a
total nitrogen-to-protein (NTP) conversion factor (6.25) based
on the assumption that most N in the sample occurs as protein.
This conversion factor, however, often over-estimates the pro-
tein content because of the presence of variable amounts of
non-protein-N in the sample (Lourenço et al. 2002;Safietal.
2013;Angelletal.2016). For example, the conversion factor
calculated for crude biomass for Chlorella vulgaris (walled)
was 6.35, whereas it was 5.96 based on direct protein extracts
(Safi et al. 2013). Similar studies of 19 tropical marine algae
yielded even lower average factors of 4.59 (red algae), 5.13
(green algae), and 5.38 (brown algae) (Lourenço et al. 2002),
perhaps related to seasonally lower N inputs to tropical surface
waters. Zhou et al. (2012 [see her Tables 3–5]) reported similar
findings. These conversion factors certainly will vary with
season based upon varying amino acid composition, empha-
sizing the need for protein and amino acid studies to determine
the seasonal optima for harvest among algal foods. Angell
et al. (2016) argued for a new universal conversion factor, after
finding a median nitrogen-to-protein value of 5 in a literature-
based meta-analysis of 103 macroalgae; however, the range of
values in their analysis was high (see their Fig. 4). The algae
have polyphyletic origins and this, too, is reflected in the ab-
sence of a universal N to protein conversion factor.
In most analyses of amino acid composition in marine al-
gae, glutamic acid, and aspartic acid represent the highest
proportions of amino acids (e.g., Fleurence 1999b; Lourenço
et al. 2002;OnlineResources1,2;HoldtandKraan2011).
These amino acids occur as protein constituents and as free
amino acids or their salts. For humans, glutamate is the major
component of the savory, the fifth basic taste called umami
from its characterization in kelp (Ninomiya 2002;Mouritsen
2013). Glutamic acid content may decrease after several suc-
cessive harvests of Pyropia yezoensis (nori; Niwa et al. 2008).
Other amino acids (alanine and glycine) also contribute to
distinctive flavors of some marine algae (e.g., see Holdt and
Kraan 2011).
The non-proteinaceous amino acid taurine is especially
abundant in marine red algae (e.g., ∼1–1.3 g taurine per
100 g DW of nori, Niwa et al. 2008). Although taurine is not
an EAA for adults, it is a component of bile acids that complex
and lower cholesterol in the bloodstream (Medeiros and
Wildman 2015).
In general, protein in most algae is digested less completely
than reference proteins such as casein (a milk protein) in
in vitro model systems containing digestive enzymes such as
pepsin, pronase, and pancreatin, with evidence that this is due
especially to inhibitory soluble fibers (e.g., Fujiwara-Arasaki
et al. 1984;Fleurence1999a;UrbanoandGoni2002;Marrion
et al. 2003,2005;Won g a n d C heung 2003;DeMarcoetal.
2014). Inclusion of pre-analytical steps such as freezing, mill-
ing, digestion of crude sample with polysaccharide-digesting
enzymes, and/or osmotic rupture ofcellstofreeintracellular
compounds is an active area of research (e.g., Harnedy and
FitzGerald 2013;Safietal.2014;Ursuetal.2014;and
references therein). Importantly, a recent study (Maehre et al.
2016) with excellent biological and technical replication
shows the beneficial effect of cooking on amino acid avail-
ability from dried dulse (Online Resource 3); however,
cooking did not significantly increase the total amino acids
measured from Alaria (Online Resource 3). Furthermore,
Maehre et al. (2016) demonstrated that the apparent amino
acid bioaccessibility from both raw and 30 min-boiled dulse
was higher than from an equivalent dry weight of wheat, rice,
or corn flour in a simulated in vitro gastrointestinal digestion
model with analysis at each sequential digestive step (amy-
lase/saliva buffer; pepsin/gastric buffer; pancreatin/duodenal
buffer) (Fig. 3). Future research on microalgal and macroalgal
protein bioavailability might incorporate measures such as the
protein digestibility-corrected amino acid score (PDCAAS),
which involves urinary and fecal determinations of N absorp-
tion in rats, as well as the FAO recommended replacement of
PDCAAS by the digestible indispensable amino acid score
(DIAAS) (Medeiros and Wildman 2015; Rutherfurd et al.
2015).
Lipids
Lipids are essential for all living organisms as components of
membranes, energy storage compounds, and as cell signaling
molecules (Eyster 2007). Although humans and other mam-
mals synthesize lipids, some essential lipids must be obtained
from dietary oils or fats. Phospho- and glycolipids, important
for membrane function, contain a polar head group with two
fatty acid chains, while the triacylglyceroles (TAGs), impor-
tant energy stores in the cell, are non-polar (neutral) lipids
containing three fatty acid chains (Fig. 4). Lipid membranes
contain sterols such as fucosterol and β-sitosterol (Fahy et al.
2005)thatalsohavereportedhealthbenefits(Aruletal.2012).
Embedded in algal lipid fractions are the nutritionally valuable
carotenoid pigments that will be discussed in the
Bphytochemicals^section (below). TAGs have attracted great
attention in recent years as a source for biodiesel, with some
microalgae accumulating up to 40–60 % of their dry weight
as TAGs (Georgianna and Mayfield 2012). However, marine
macrophytes typically do not exceed 2–4.5 % dry wt as lipids,
mainly as phospholipids and glycolipids (Holdt and Kraan
2011). Of these, the long-chain polyunsaturated fatty acids
(PUFAs) and carotenoids are most noteworthy as functional
foods (Holdt and Kraan 2011).
J Appl Phycol

Long-chain PUFAs
There are two general families of PUFAs: the linoleic acids (n-
6 or omega 6 fatty acids) and the α-linolenic acids (n-3 or
omega 3 fatty acids). Long-chain PUFAs comprise a substan-
tial portion of marine algal lipids, with planktonic algae being
the source of most omega fatty acids in fish. The most impor-
tant of these PUFAs are the essential fatty acids (EFAs)
eicosapentaenoic acid (EPA; 20:5 n-3) and docosahexaenoic
acid (DHA; 22:6 n-3) along with their precursors α-linolenic
acid (ALA; 18:3 n-3) and docosapentaenoic acid (22:5 n-3)
(Cottin et al. 2011). The first product of ALA in the synthesis
pathway to C20–22 PUFAs is stearidonic acid (SA, 18:4n-3),
and this fatty acid can represent a significant portion of PUFAs
in some edible macroalgae (sea vegetables) (Guil-Guerrero
2007). EPA is the predominant PUFA in many sea vegetables
(Fig. 4), along with arachidonic acid (20:4 n-6), particularly in
red algae (Norziah and Ching 2000; Wen et al. 2000;Ortiz
Fig. 4 Structures of some key
vitamins and lipids mentioned in
review
Fig. 3 A comparison of essential fatty acids liberated from 1 g dry weight
of Palmaria palmata (raw and boiled for 30 min) wheat, rice, and corn
flours in simulated gastrointestinal digestion. The mean values ± 1 SD
(n= 5) are shown in mg g
−1
. Significant differences between species
(p> 0.05) are indicated by different letters. (Used with permission from
Maehre et al. (2016))
J Appl Phycol

et al. 2009) where EPA comprises up to 50 % of the total fatty
acid content (e.g., Palmaria palmata, van Ginneken et al.
2011). Humans and other animals cannot convert ALA to
EPA and DHA at required levels, so dietary sources of these
EFAs are critically important for the health of humans (Cottin
et al. 2011) and many animals (Li et al. 2009).
Numerous epidemiological and controlled interventional
trials (N.B., the excellent reviews of Conquer and Holub
1996;Holub2009; Cottin et al. 2011)supportthehealthben-
efits to humans of DHA and EPA long-chain omega-3 fatty
acids from fish oils and algal sources (mainly extracts). In
contrast to most other algal food constituents, the bioaccessi-
bility of DHA and EPA in algal-derived oils and extracts is
well quantified for humans, ranging from ∼50 to 100 % de-
pending on the matrix (Haug et al. 2011; Schuchardt et al.
2011). While clinical research to date strongly supports a nu-
tritional need for oils that are enriched in DHA and EPA, there
is more understanding about the bioactivity of DHA than of
EPA (Conquer and Holub 1996; Holub 2009; Cottin et al.
2011). There is a considerable literature (Cottin et al. 2011)
on the cardioprotective effects of DHA-containing TAG from
Crypthecodinium cohnii (a dinoflagellate, Mendes et al. 2009)
and Schizochytrium sp. (a thraustochytrid stramenopile, Li
et al. 2009; Barclay et al. 2010), and as a consequence, infant
formula, infant foods, and certain other food categories (dairy,
bakery, eggs, and non-alcoholic beverages) and marketed nu-
tritional products now are supplemented with algal-derived
DHA. There is evidence that enhanced DHA intake may im-
prove infant cognitive performance and enhance visual acuity
(Jensen et al. 2005,2010; Imhoff-Kunsch et al. 2011), al-
though more recent data raises question about this linkage
(Delgado-Noguera et al. 2015). There also is some under-
standing about the bioaccessibility of DHA in different algal
products. Algal oil capsules based on a patented commercial
source (Martek) and cooked salmon are reported to represent
nutritionally equivalent sources of DHA (see in Cottin et al.
2011). A similar human trial showed that DHA from two
different strains of Schizochytrium sp. (DHASCO-T and
DHASCO-S) supplied in capsules generated equivalent
dose-dependent DHA levels in plasma phospholipids and
erythrocytes (Arterburn et al. 2008). Fortified snack bars also
delivered equivalent amounts of DHA on a DHA dose basis
(Arterburn et al. 2007). A systematic review of plant omega-3
fatty sources by Lane et al. (2014) concluded that further re-
search on algal sources was warranted based on promising
preliminary work.
Nonetheless, the relative health benefits of commercial al-
gal supplements that tend to be DHA-rich versus natural fish
oils that contain both DHA and EPA are uncertain. Cottin et al.
(2011) found that BRecent evidence from randomized con-
trolled trials has produced equivocal results. Heterogeneity
of the studies in terms of dosage, duration, population target,
sample size, as well as the relative amounts of EPA and DHA
used in supplements could account for the variability of the
results.^Even so, important trends stand out. While both EPA
and DHA reduce TAG levels in humans (Wang et al. 2006;
Bernstein et al. 2012), DHA appears responsible for the blood
pressure and heart rate-lowering effect of fish oils (Valera et al.
2014). DHA also seems to be beneficial for endothelial and
platelet function, although a direct role for EPA in regulating
TAGs has not beenruled out. Algal DHA extracts can produce
other cardiovascular protective effects in humans by altering
plasma lipoproteins at reasonably small doses (2 g algal
DHA day
−1
over 4.5 months: Neff et al. 2011). The health
benefits of algal DHA supplements for subgroups such as
vegetarians, who otherwise may have low essential fatty acid
intakes, remains a high research priority (Geppert et al. 2005;
Cottin et al. 2011).
Fish oils also have demonstrated anti-inflammatory and
insulin-sensitizing properties in vitro and in animal studies
(Nauroth et al. 2010; Cottin et al. 2011); however, human
trials often yield conflicting findings. Neither EPA nor DHA
alone showed any effects on inflammation in double-blind
trials with cystic fibrosis patients (Van Blervliet et al. 2008)
or insulin sensitivity inhuman subjects, despite indications for
potency in vitro (critically reviewed in Cottin et al. 2011).
Without better quantification of the biological uptake of EPA
or DHA, the reason for this discrepancy remains unknown.
Microalgae are the primary sources of DHA and EPA for
zooplankton, fish, and other multicellular organisms, and
these essential fatty acids (EFAs) become increasingly con-
centrated up the food web (e.g., Legezynska et al. 2014).
Therefore, fish oils are rich in both DHA and EPA because
they represent the trophic integration of DHA-rich flagellates
and EPA-rich diatoms in the foodweb (Viso and Marty 1993).
There is emerging evidence that ocean acidification, the result
of changing coastal processes and increased atmospheric CO
2
,
can negatively change the supply of these essential fatty acids
to higher trophic levels (Rossoll et al. 2012). This and other
factors affecting EFA production in algal assemblages will
be an important area of future research (Chrismadha and
Borowitzka 1994; Pasquet et al. 2014).
Concern over the sustainable supply of fish oils and the
commercial dominance of algal-based DHA-only supple-
ments has led to a large industry effort towards developing
alternatives to fish oil-derived EPA (Zeller 2005). One exam-
ple is Lovaza
TM
, a prescription pharmaceutical containing
purified DHA and EPA synthesized from fish oils that report-
edly have anti-hyperlipidemic properties (Weintraub 2014),
although there are some negative indicators for this product
(Spindler et al. 2014). A new promising biotechnological
source of EPA has been proposed by Řezanka et al. (2010)
from the Eustigmatophyceae Trachydiscus minutus;however,
its commercial production is not developed yet. Other biotech-
nological production of EPA is provided by the diatoms
Phaeodactylum tricornutum grown in tubular
J Appl Phycol

photobioreactors (Chrismadha and Borowitzka 1994)or
Odontella aurita co-cultivated in raceway ponds with the
red macroalga Chondrus crispus in France by the Innovalg
company (Braud 2006). Commercial production of DHA and
EPA is one of the main targets of producers and has benefited
from the development of microalgal cultivation via fermenta-
tion technology (Branger et al. 2003; Barclay et al. 2013).
Several recent studies analyzed the constituent fatty acids
of large numbers of red, brown, and green macroalgae from
polar (Graeve et al. 2002, 20 species), temperate (Schmid et al.
2014, 16 species; McCauley et al. 2015, 10 species), and
tropical (Kumari et al. 2010, 27 species; Kumar et al. 2011,
22 species) habitats, and, despite some species variability, red
(Rhodophyta) and brown (Phaeophyceae) macroalgae had a
high proportion of total FAs in EPA and arachidonic acid
across latitudes, whereas the green (Chlorophyta) algae had
low EPA (as % of total FA) but some DHA, and, were
enriched in C18 LC PUFA. Phytoplankton contain more
PUFA, as expected, when grown at low temperature (e.g.,
DHA in Crypthecodinium, Jiang and Chen 2000), and higher
temperatures good for maximal biomass production can be
lowered for as little as 12 h to induce maximal EPA content
in the diatom Phaeodactylum (Jiang and Gao 2004).
Whether omega-3 FA content can be manipulated by the
timing of wild harvest or grow-out of sea vegetable crops in
winter to increase EFA of whole foods needs much more
work. Marine macrophytes generally contain low total lipid
contents, so their comparative value as a food energy source
likely is small (Holdt and Kraan 2011;Maehreetal.2014),
and at realistic daily consumption levels (e.g., 8 g dry wt.,
Blouin et al. 2006), even red algae such as Porphyra
umbilicalis (laver) and Palmaria palmata (dulse) (Fig. 5)that
have a high proportion of their total fatty acids as EPA (Mishra
et al. 1993;Graeveetal.2002;Blouinetal.2006; Schmid
et al. 2014) will not meet dietary recommendations for daily
EFA alone (Blouin et al. 2006), although higher levels of
macroalgae can support EFA needs for animal aquaculture
feeds (Mulvaney et al. 2015; Wilke et al. 2015). Thus, relevant
growth conditions should be manipulated for promising
macroalgae in the laboratory to see if further increases are
possible; unialgal cultures will be important because recent
PUFA studies of green algae in a Bgreen tide^in the Gulf of
Finland showed that the high EPA content was largely due to
epiphytic diatoms (Gubelit et al. 2015).
The bulk of research on omega-3 long chain PUFAs in
microalgae and sea vegetables has been empirical, testing dif-
ferences among species under different growth conditions. It
will be important to understand their biosynthetic pathways
and metabolic controls, and the increasing availability of
microalgal genomes should provide excellent opportunities
in this goal. One recent example is five genes functionally
characterized in the haptophyte Emiliania huxleyi that are pre-
dicted to underpin omega-3 LC-PUFAs synthesis (Sayanova
et al. 2011). Additional putative genes for functionally redun-
dant pathways for the synthesis of omega-3 EPA and DHA
were also annotated in the E. huxleyi pan genome sequence
(Read et al. 2013).
Further investigation to produce biomass or extracts of sea
vegetables containing EPA at a range of doses compatible
with functional foods would enable research to examine the
protective effects of consuming this source of long chain n-3
PUFA. Such trials could provide clear evidence for the
clinical therapeutic potential of consuming EPA rich
macroalgae in combination with supplementation of
microalgal DHA.
Sterols
Algae vary in their total sterol content and in the variety of
sterols present (Holdt and Kraan 2011). Older analytical tech-
niques may have misidentified algal sterols as cholesterol
since their structures are similar (Pereira et al. 2016).
Fucosterol occurs in many algae, especially red and brown
macroalgae (Pereira et al. 2016), and this compound may have
value in treating complications of diabetes and hypertension,
as well as other major health concerns (Abdul et al. 2016).
However, like other studies of algal foods, the linkages are
implied but little is known about the actual in vivo effects of
fucosterol when algae are consumed by humans. Nonetheless,
as for long-chain PUFAs, understanding the seasonal, environ-
mental, and species-specific factors that alter the abundance
and composition of algal sterols, such as in the recent studies
in Antarctic seaweeds (Pereira et al. 2016), will be fundamen-
tal to understanding their potential effects in human diets.
Polysaccharides
Polysaccharides are used for energy storage and as structural
elements in marine algae and terrestrial plants. Humans pos-
sess enzymes that degrade algal starches to mono-and di-sac-
charides for transport across the gut lumen, but generally can-
not digest the more complex polysaccharides, as was first
recognized more than a century ago (Saiki 1906). These resis-
tant polysaccharides, known as dietary fiber, may be
fermented in the large intestine to varying degrees depending
on the enzymatic competence of the microbiome (Terada et al.
1995; Michel and MacFarlane 1996;Hehemannetal.2010;
Cian et al. 2015). Algal cell walls differ from those of terres-
trial plants as they contain uncommon polyuronides and poly-
saccharides that may be methylated, acetylated, pyruvylated,
or sulfated (Paulsen and Barsett 2005; Pal et al. 2014; Rioux
and Turgeon 2015; Stiger-Pouvreau et al. 2016). It is fair to
say that algal polysaccharides are the most widely, and often
unknowingly, consumed food of algal origin. Small amounts
are incorporated into beverages, meat and dairy products, and
J Appl Phycol

fillers (Cofrades et al. 2008; Gupta and Abu-Ghannam 2011a,
b;Griffin2015) at levels generally deemed to be beneficial
and safe by regulatory agencies (extensively reviewed in
Mabeau and Fleurence 1993; MacArtain et al. 2007; Watson
2008;HoldtandKraan2011; Barlow et al. 2015;Fleurence
and Levine 2016).
Edible macroalgae contain unusually high amounts of die-
tary fiber, ranging from 23.5 % (Codium reediae)to64.0%of
dry weight in Gracilaria spp., values that frequently exceed
those for wheat bran (Ruperez and Saura-Calixto 2001;
McDermid et al. 2005;BenjamaandMasniyom2012). The
nomenclature of food-derived fiber is confusing because there
is no consensus on its definition among scientists and
regulatory agencies. Dietary fiber, considered a nutrient in
the USA under the Nutrition and Education Act of 1990
(Thomas.loc.gov/ H.R. 3562.ENR), comprises Bnondigestible
carbohydrates and lignin that are intrinsic in intact plants.^
Some fraction of this human-inert matter is considered by
some as Functional fiber; that fraction of isolated, non-
digested carbohydrates having apparent beneficial physiolog-
ical effects beyond nutrition in humans (Institute-of-Medicine
2005;MedeirosandWildman2015). In this case, Tot al f i b e r is
the sum of dietary and functional fiber (Institute-of-Medicine
2005;MedeirosandWildman2015). In contrast, the
European Food Safety Authority, following the CODEX
Alimentarius Commission definition of dietary fiber (Jones
2014), acknowledges that benefits beyond nutrition can occur
but does not formally distinguish functional from dietary fiber
because no analytical methods exist for this differentiation
(EFSA 2010). Regardless of these semantics, non- or partially
fermented fiber that generates physiological benefits, through
either physical or chemical pathways, meets the definition of
dietary fiber (Jones 2014).
BSoluble fiber^comprises 52–56 % of total fiber in com-
monly used green and red macroalgae and 67–85 % in brown
macroalgae (Lahaye 1991). Much of it can be fermented to
short-chain fatty acids (SCFAs) such as acetate, propionate,
and butyrate (see Table 1 in Michel and MacFarlane 1996;
Cantarel et al. 2012) which both nourish the epithelia of the
large intestine and offer other benefits to the host (Terada et al.
1995; Michel and MacFarlane 1996). For example, acetate
and propionate are transported in the blood to many organs
where they are oxidized for energy or utilized in signaling to
help regulate aspects of energy homeostasis and immune
function (reviewed by Nicholson et al. 2012). The fermenta-
tion process and SCFA products also nourish and modify the
microbial consortia in the large intestine, thereby exerting pre-
biotic effects and influencing digestive outcomes (e.g.,
Fernando et al. 2008;O’Sullivan et al. 2010;Cianetal.
2015). Investigating the coupling of algal (and other) polysac-
charides to the health of intestinal microbiomes and their an-
imal and human hosts is an active and needed area of research
(Bäckhed et al. 2005; Hehemann et al. 2010;Cantareletal.
2012). These beneficialresponses may include reduced risk of
Fig. 5 Sea vegetables used in
European cuisine include
marinated kelp (Alaria esculenta)
in a cannelloni bean salad (a),
laver/nori (Porphyra umbilicalis/
Pyropia yezoensis) in chocolate
molasses meringues (b), the
traditional Welsh laver-bread
cakes, with dulse (Palmaria
palmata) crisps (c), and
dulse-cheese scones (d). These
additions add texture, protein,
vitamins and minerals, and flavor.
(Used with permission of Prannie
Rhatigan from The Irish Seaweed
Kitchen)
J Appl Phycol

diabetes, hypertension, and cardiac heart disease (Institute-of-
Medicine 2005). However, the complexity of interactions
among functional and dietary fiber and the intestinal
microbiome challenges efforts to demonstrate the functional
food and biomedical benefits of algal polysaccharides (de
Jesus Raposo et al. 2015; Dhargalkar 2015).
The evidence for bioactivity of algal polysaccharides de-
rives largely from in vitro experiments using isolated oligo-
mers/polymers, with fewer data on testing any whole alga in
animal or human trials. Compositional analysis of Chlorella
and similar microalgae began more than 60 years ago, and an
impressive number of biological processes are now reported
to be influenced by ingestion of whole algae or polysaccharide
extracts as food or supplements (Pulz and Gross 2004; Plaza
et al. 2009; Chacón-Lee and González-Mariño 2010;Lordan
et al. 2011;Voetal.2011). Microalgal genera (Fig. 2) com-
monly considered as beneficial dietary supplements include
Chlorella,Arthrospira (spirulina), Dunaliella,
Haematococcus,Scenedesmus,Aphanizomenon,Odontella,
and Porphyridium,withspeciesofChlorella being recognized
as particularly rich in polysaccharides (Chacón-Lee and
González-Mariño 2010). This putative bioactivity includes
anticancer properties, cytokine modulation, anti-
inflammatory effects, macrophage activation, and inhibition
of protein tyrosine phosphatase (Hasegawa et al. 1997;
Cheng et al. 2004; Kralovec et al. 2005; Sheng et al. 2007;
Hsu et al. 2010). Algal polysaccharide extracts can possess
strong immunomodulating effects both in vitro and in vivo
(Watanabe and Seto 1989;PascoandPugh2010;Suárez
et al. 2010). Kwak et al. (2012) observed an
immunostimulatory effect in 30 Korean volunteers fed
5 g day
−1
Chlorella vs. placebo in a double-blinded 8-week
trial. Acidic polysaccharide extracts from Chlorella
pyrenoidosa have been patented (Chlon A and Respondin
TM
)
as potentially useful anti-tumor and immunostimulating sup-
plements (Umezawa and Komiyama 1985; Komiyama et al.
1986; Kralovec 2005; Kralovec et al. 2005). Even so, the
molecular structures responsible for such observed physio-
logical functions are poorly understood because of fragmen-
tary and sometimes conflicting information on the chemistry
of these large, highly complex cell wall polymers (Řezanaka
and Sigler 2007). Research also has focused on strikingly
few algal species, leaving a broad window of opportunity
for more expansive assessment of potential sources of bio-
active compounds (Admassu et al. 2015).
The study of extracted polymer sub-fractions of structural
polysaccharides provides a useful exploratory tactic for
assessing the potential functional benefits of consuming
macroalgal foods, and it establishes a quantitative means to
determine the seasonal or environmental effects on food qual-
ity (Stengel et al. 2011; Mak et al. 2013). The predominant
algal polysaccharides are the alginates in brown macroalgae,
and the sulfate-esterified polysaccharides of macro- and
microalgae that are widespread in red, brown, and green sea-
weeds (Aquino et al. 2005; Popper et al. 2011). The cellular
quantities and compositions of these polysaccharides vary
among species and with seasonal and environmental changes
(Bourgougnon and Stiger-Pouvreau 2011; Mak et al. 2013).
Alginate
Alginate is the major polysaccharide of brown algae, compris-
ing 14–40 % of its dry mass (cf. Ramberg et al. 2010), and was
first isolated in 1881 as algin from kelp (Laminaria sp.) by E.
C. C. Stanford. The direct consumption of brown algae as
human food is of long standing (Tseng 1981;Druehl1988;
Dharmananda 2002; McHugh 2003). The purported benefi-
cial effects of alginate include its ability to absorb toxins,
decrease cholesterol uptake, alter the colonic bacterial pro-
files, and generate SCFAs (Brownlee et al. 2005). The metal
chelating abilities of alginates makes them valuable scaven-
gers of toxic elements in the human gut, but this scavenging
also may lead to nutritional deficiencies of essential di- or
polyvalent trace metals (Hollriegl et al. 2004;Brownlee
et al. 2005). Most studies have investigated the effects of
polysaccharide extracts rather than consumption of intact sea-
weeds. Although the extent of alginate dissociation from algal
cell walls after ingestion is not well studied, there is little or no
digestion of sodium alginate from Ascophyllum nodosum in
humans (Percival and McDowell 1967; Painter 1983;Aarstad
et al. 2012). Dietary alginates also provide a sense of satiety
and so have been explored as a weight control measure, al-
though there remains uncertainty about its efficacy in this role
(Yavorska 2012).
Sulfated heteroglycans—ulvans
The abundant, heavily sulfated ulvans are extracted from
members of the Ulvales. They are the best studied of the green
seaweed polysaccharides, in part because the high production
of Ulva spp. in eutrophic coastal waters has sparked research
for new uses of these algae (Alves et al. 2013). Ulvans owe
their bioactive properties to their unusual hydrophilic
polyanionic features and structural analogies with animal gly-
cosaminoglycan regulators (dermatan sulfate, heparin/heparin
sulfates) and L-rhamnose specific lectins in humans. The re-
ported bioactivities of ulvan extracts in vitro include antibac-
terial, anticoagulant, antioxidant, antiviral, anti-tumor, anti-
hyperlipidemic, and immunoregulatory effects (Kaeffer et al.
1999;Yuetal.2003; Mao et al. 2006,2008; Leiro et al. 2007;
Zhang et al. 2008,2010; Lee et al. 2010;HoldtandKraan
2011; Matloub et al. 2013).
Although the ingestion of green macroalgae by humans is
rather widespread, the potential health benefits of food sup-
plements of native ulvans or their chemically modified deriv-
atives, let alone the direct consumption of the whole algae, are
J Appl Phycol

not well understood (Taboada et al. 2010; Wijesekara et al.
2011). Fermentation of Ulva and ulvan by human colonic
bacteria was slight (16.6 and 8.9 % of organic matter, respec-
tively) (Durand et al. 1997), indicating that they would be
poor sources of SCFA production in the colon (Bobin-
Dubigeon et al. 1997). However, these results cannot be gen-
eralized because only two individuals provided the bacterial
inocula, and their prior dietary history relating to algal foods
was unknown. A cautionary note here though is that Ulva can
be rich in free sulfate which is readily converted to sulfide
during fermentation, so consumption of more than 20 g day-
1of the dry, unprocessed seaweed may have adverse (and
odiferous) health effects (Durand et al. 1997).
Sulfated galactans—carrageenans
Red algal polysaccharides include the nutritionally important
floridean starch, and their sulfated galactans are known to
initiate or modulate a large number of biological activities of
significance to human health (Prajapati et al. 2014). The most
studied are the sulfated agarocolloids and the carrageenans
derived from macroalgae in the orders Gelidiales,
Gigartinales, and Gracilariales. Anti-viral activities include
those against herpes simplex, herpes zoster, dengue-2, vaccin-
ia, rabies, and vesicular stomatitis virus with patents and some
commercial projects resulting (Richards et al. 1978; Baba et
al. 1988; Vedros 1993;Bourgougnon2003; Eccles et al. 2010;
Talarico et al. 2011; Levendosky et al. 2015; Luo et al. 2015).
Whether consumption of the relevant red algae or their ex-
tracts in foods is protective against viruses does not appear
to be known and deserves study. Carrageenan extracts that are
depolymerized to various degrees have potential as tumor in-
hibitors and as immunostimulators in cancer therapy.
Oligomers from acid hydrolyzed κ-carrageenan injected into
mice increased macrophage phagocytosis and stimulated sev-
eral immune-related markers while significantly inhibiting the
growth of sarcoma S180 cells (Yuan et al. 2006).
Phosphorylation or further sulfation of these oligomers in-
creased the activity of natural killer cells, the cytotoxic lym-
phocytes critical to immune system function (Yuan et al.
2011). Similarly, transplanted human sarcoma S180 tumors
were inhibited significantly in mice fed fractionated λ-
carrageenan extracts of Chondrus ocellatus (200 mg kg
−1
dai-
ly) (Zhou et al. 2004). Although seaweeds containing carra-
geenans act as prebiotics when supplied as supplements in
both poultry and rat diets (Kulshreshtha et al. 2014;Liu
et al. 2015), the potential for sulfated galactans from algae to
benefit human health remains to be established.
Carrageenans have the potential to be harmful (Tobacman
2001). Carrageenan extracts generate proinflammatory agents
in mice (Hansra et al. 2000), and the resulting public health
concerns have led to several actions regarding carrageenans in
food products (Watson 2008). Carrageenan is prohibited in the
EU for use in infant formulas, and, although it is permitted in
the USA, it must be of high molecular mass (i.e., >100 kDa
with <5 % of 50 kDa fragments). High doses of low molecular
mass carrageenan cause ulceration in the guinea pig colon
(Watson 2008) and lead to marked increases in the chemokine
interleukin-8 and B-cell CLL/lymphoma 10 activities in the
normal human colonic mucosal epithelial NCM460 cell line
(Bhattacharyya et al. 2010). Oral introduction of undegraded
λ-κcarrageenan in drinking water of 12-week-old mice also
caused significantly higher peak glucose levels in the blood,
leading to concern that carrageenan-induced insulin resistance
might contribute to human diabetes (Bhattacharyya et al.
2012). However, a comprehensive examination of in vivo di-
etary κ-carrageenan effects in rats revealed no effects on blood
glucose (Weiner et al. 2007). More recent appraisals of carra-
geenans as food additives could find no hazards to human
health as they are currently used (McKim 2014;Weiner2014;
Barlow et al. 2015;Weineretal.2015). The potential benefits
and negative effects of including algae or their refined products
in the diet require continuing research on a case-by-case basis.
Beta-(1-3)-glucans—laminarans
The main polysaccharides after the alginates in brown algae
include β-glucans (laminarans), cellulose, and heteroglycans,
the first being an energy reserve while the others are structural
components of the cell wall, fitting the definitions of dietary
fiber (Jones 2014). The concentrations and composition of the
β-glucans vary substantially with season and growth rates
(Rioux et al. 2009). The most studied β-glucans are those
from cereals and fungi, but these differ significantly in struc-
ture from those of algal origin (Rioux et al. 2010). The biolog-
ical responses elicited by algal β-glucans depend strongly on
details of their primary structures (Bohn and BeMiller 1995;
Mueller et al. 2000;Williamsetal.2005;HoferandPospíšil
2011). For example, brown algal M-series laminaran showed
significant hepatoprotective effects when ingested orally by rats
(Neyrinck et al. 2007). The protection was organ specific and
appeared to act via the Kupffer cells in the liver, thereby estab-
lishing an immunoregulatory function of this orally ingested
functional fiber. These and other biological effects of β-
glucans have been reviewed (Novak and Vetvica 2008;
Ramberg et al. 2010;LehtovaaraandGu2011;Kadametal.
2015), and certain cautions have been expressed about the func-
tional effects of soluble and particulate forms of these com-
pounds (Young and Castranova 2005;HoferandPospíšil 2011).
Sulfated fucose-containing polysaccharides—fucoidans
The fucoidans are a subset of marine fucose-containing poly-
saccharides (FCPs) found in brown algae (Painter 1983)that
are now attracting widespread interest (Shanmugam and
Mody 2000; Berteau and Mulloy 2003;Kusaykinetal.
J Appl Phycol

2008;Lietal.2008; Pomin and Mourão 2008;Courtois2009;
Pomin 2009,2012; Fitton 2011;Jiaoetal.2011; Kim and Li
2011;KimandWijesekara2011; Wijesinghe et al. 2011;
Wijesinghe and Jeon 2012). Double-blind clinical trials with
fucoidan extracts show anti-aging effects on skin and other
benefits in cosmetic applications (Fitton et al. 2015). A com-
mon source of FCPs used in experimental studies is Fucus
vesiculosus, but fucoidans also are found in edible species
such as Cladosiphon okamuranus,Saccharina japonica (as
Laminaria japonica), and Undaria pinnatifida (Fitton 2011).
The highly sulfated nature and molecular weights of FCPs
appear to be responsible for many demonstrated biological
activities in vitro (Croci et al. 2011; Ustyuzhanina et al.
2014). The FCP structures are species-dependent and can be
modified by environmental variables and the developmental
status of the seaweed fronds, all of which can affect their
bioactivities (Honya et al. 1999; Zvyagintseva et al. 2003;
Rioux et al. 2009;PieleszandBiniaś2010; Skriptsova et al.
2010; Stengel et al. 2011;Anastyuketal.2012; Mak et al.
2013). More recently, in vitro studies have provided insight
into some structure-function relationships of FCPs (Cumashi
et al. 2007;Ushakovaetal.2009; Ustyuzhanina et al. 2013,
2014).
It can be concluded that knowledge of the beneficial effects
of algae and their extracts as food additives for humans lags
far behind that on which diets have been formulated for com-
mercially important species in aquaculture and agriculture.
The number of species exhibiting benefits is wide ranging
from invertebrates (nematodes, shrimp, abalone) and finfish
(sea bream to salmon) to farm animals including poultry and
mammals (both ruminants and monogastric species) (reviews:
Craigie 2010;O’Sullivan et al. 2010;Rajauria2015;Heuzé
et al. 2016;Makkaretal.2016). Algal-based products
Tas co™from Ascophyllum nodosum and Ocean Feed™(a
blend of brown, green and red macroalgae) are commercially
marketed as feed additives to improve performance, stimulate
immune reactions, mitigate sea lice damage in salmonids, and
other benefits. Notable is the Alternative Feeds Initiative to
develop alternative dietary ingredients (NOAA 2011). In ad-
dition to conventional methods of measuring animal perfor-
mance, molecular techniques have been applied to buttress
claims of efficacy (cf. Kulshreshtha et al. 2014; Liu et al.
2015). Bearing in mind ethical considerations, similar ap-
proaches may be adapted to facilitate the assessment of the
benefits of macroalgal ingestion by humans.
Vitamins
Vitamins are essential organic micronutrients, which an organ-
ism cannot synthesize directly in sufficient quantities and so
instead must obtain from the diet. Well-known human
vitamin-deficiency diseases include beriberi (deficiency in
thiamine, vitamin B
1
), pellagra (niacin), pernicious anemia
(cobalamin, vitamin B
12
), and scurvy (ascorbic acid, vitamin
C) (Stabler and Allen 2004;Martinetal.2011). These com-
pounds serve as precursors for essential enzyme cofactors and
are needed for essential metabolic functions (Fig. 4). Since
animals have lost the capacity to synthesize these cofactors,
they must obtain them from external sources. Algal foods are
rich in vitamins. Several sea vegetables—laver (Porphyra
umbilicalis), sea spaghetti (Himanthalia elongata), and
Gracilaria changii—contain levels of vitamin C comparable
to common vegetables such as tomatoes and lettuce (Norziah
and Ching 2000; Ferraces-Casais et al. 2012), while the vita-
min C content described for the brown seaweed Eisenia
arborea (34.4 mg (100 g)
−1
dry wt) approaches those reported
for mandarin oranges (Hernandez-Carmona et al. 2009). The
vitamin content of individual algal species discussed in this
section, including details of sample origin and handling, is
compiled in Online Resource 4.
Sea vegetables also are a good source of B-group vitamins
(particularly B
1
,B
12
), as well as the lipophilic vitamin A
(derived from the carotenoid β-carotene) and vitamin E (to-
copherol). Kelp (Macrocystis pyrifera) can contain levels of
α-tocopherol (the most biologically active form of vitamin E)
at par with plant oils rich in this vitamin, such as palm, sun-
flower seed, and soybean oils (Ortiz et al. 2009;Skrovankova
2011). Moreover, values of β-carotene (pro-vitamin A) found
in the seaweeds Codium fragile and Gracilaria chilensis can
exceed those measured in carrots (Ortiz et al. 2009). The vi-
tamin composition of microalgae can be equally remarkable.
Fabregas and Herrero (1990)showedthatTetraselmis s ue c ic a ,
Isochrysis galbana,Dunaliella tertiolecta,andChlorella
stigmatophora were particularly rich in lipid-soluble (A and
E) and B-group vitamins [including vitamins B
1
,B
2
(ribofla-
vin), B
6
(pyridoxal), and B
12
]. The concentrations of several
vitamins, including E, B
1
,andβ-carotene, exceeded those in
conventional foods considered to be rich sources of these
compounds (Fabregas and Herrero 1990). It is clear then that
algal foods can be an excellent source for a wide range of these
essential micronutrients.
Even so, variability between samples can make direct com-
parisons among studies difficult (e.g., Chan et al. 1997;
McDermid and Stuercke 2003; Hernandez-Carmona et al.
2009). Part of the variability may lie in the sample processing
methods (Skrovankova 2011) as observed for other phyto-
chemicals (Ling et al. 2015); for example, analysis of freeze-
dried and oven dried samples of Sargassum hemiphyllum
yielded substantially different vitamin C contents (Chan
et al. 1997). But differences also can be due to environmental
and seasonal factors. For instance, there are notable variations
in the levels of β-carotene and vitamin C between samples of
Ulva fasciata collected from different sites (McDermid and
Stuercke 2003) (Online Resource 4). Monthly quantitation of
vitamins C, B
2
,B
1
,andAconcentrationsinEisenia arborea
J Appl Phycol

over a 12-month period revealed levels were lowest in the
summer months (June, July, August) and reached the highest
concentrations in April/September (for vitamins C, B
2
,B
1
)
and January (for provitamin A) (Hernandez-Carmona et al.
2009). The proximate cause for these patterns is unknown,
as is the effect of growth conditions on the content and com-
position of vitamins in algal foods, so this is an important topic
for future research.
Algal foods offer one of the few vegetarian alternatives for
cobalamin (vitamin B
12
) in the diet. Cobalamin is not required
or synthesized by higher plants (Croft et al. 2005)sofruitsand
vegetables are poor sources of vitamin B
12
, which explains
why vitamin B
12
-deficiency is common among people follow-
ing strict vegetarian or vegan diets (Haddad et al. 1999;
Waldmann et al. 2004;Allen2008). Over half of microalgal
species surveyed have a metabolic requirement for B
12
,and
contain large amounts (Online Resource 4), but they cannot
synthesize it (Croft et al. 2005; Helliwell et al. 2011).
Cobalamin is synthesized only by prokaryotes (Warren et al.
2002), and it has been shown that B
12
-synthesizing bacteria
are closely associated with or reside on eukaryotic algal sur-
faces (Croft et al. 2005; Wagner-Döbler et al. 2010). Pyropia
yezoensis (nori) contains up to ∼0.06 mg vitamin B
12
(100 g)
−1
algal dry wt, comparable to that found in beef liver
(Watanabe et al. 1999b; Takenaka et al. 2001). Lower levels
are found in other sea vegetables such as kelps (including
wakame) and hijiki, although reported concentrations vary
among studies, possibly reflecting differences among strains,
growing conditions, or harvesting periods (Watanabe et al.
1999a;Miyamotoetal.2009). Given that the ultimate source
of vitamin B
12
is bacteria, changes in the character and mag-
nitude of the epiphytic prokaryotic communities related to
region or algal physiological state may contribute to variation
in vitamin content; these factors currently are poorly
quantified.
There is uncertainty about whether the magnitude of vita-
min concentration in algal foods reflects their nutritional val-
ue. Dagnelie et al. (1991) investigated how sea vegetables
affected the hematological status of B
12
-deficient children
and concluded that the algal-derived vitamin B
12
was not bio-
accessible to humans. However, their very small treatment
group (n= 5) may have been insufficient to draw firm conclu-
sions. Takenaka et al. (2001) showed that feeding nori to vi-
tamin B
12
-deficient rats yielded a 1.9-fold increase in hepatic
levels of total B
12
compared to those without nori supplemen-
tation. Similarly, increased consumption of Chlorella or nori
by vegan participants prevented B
12
deficiency (Rauma et al.
1995). However, there are few data on which to base quanti-
tative estimates of the portion of algal vitamins that are
absorbed during digestion.
One approach to assessing the availability of vitamins is to
distinguish among their different chemical forms. The uptake
of cobalamin-based compounds, referred to more broadly as
corrinoids, is largely governed by the gastrointestinal
absorption system rather than their chemical liberation via
digestive chemical processes (Russell-Jones et al. 1999).
Pseudovitamin B
12
(or pseudocobalamin) differs from cobal-
amin in its lower axial ligand (Stupperich and Krautler 1988),
and this affects affinity of the mammalian B
12
-binding protein
intrinsic factor (IF) for the compound, thereby limiting its
absorption in the intestine (Stupperich and Nexo 1991). This
difference has human health implications because commer-
cially produced vitamin B
12
supplements derived from the
cyanobacterium Arthrospira sp. (spirulina) consist largely of
pseudovitamin B
12
(Watanabe et al. 1999b; Watanabe 2007a),
reducing their nutritional value. In contrast, Bgreen^(Ulva
[formerly Enteromorpha sp.]) and Bpurple^(Pyropia [former-
ly Porphyra] sp.) laver contain substantial amounts of biolog-
ically available B
12
(Watanabe et al. 1999b), and indeed, rats
fed purple laver improved their B
12
status (Watanabe et al.
1999b). A recent study has established that the vast majority
of cyanobacteria synthesize pseudocobalamin, whereas eu-
karyotic algae that are dependent on B
12
for growth are like
animals in that they require cobalamin (Helliwell et al. 2016).
Thus, sea vegetables are likely to be a more reliable source of
the appropriate form of this vitamin, although again this will
be determined by the prokaryotic community associated with
the algae.
These findings highlight the need for rigorous care in the
analytical determinations of the vitamin content of algal foods.
Bioassays using B
12
-dependent bacteria such as Lactobacillus
delbruekii ssp. lactis (ATCC7830) are inadequate because,
unlike humans, these bacteria do not discriminate between
vitamin B
12
and pseudovitamin B
12
. An alternative radioiso-
tope dilution assay (RIDA) is likely to better reflect the func-
tional B
12
content (Watanabe 2007a). Distinguishing among
bioavailable and non-bioavailable vitamin forms will be cru-
cial (Watanabe 2007b). Complicating these analyses further is
evidence that commercial processing methods can alter the
vitamin chemistry sufficiently to affect uptake. For example,
Yamada et al. (1999) showed that air-drying Pyropia tenera
(asakusa-nori) produced B
12
analogs that are biologically in-
active. Drying by lyophilization might have better nutritional
outcomes (Takenaka et al. 2001), although this has yet to be
rigorously demonstrated. Other factors of particular impor-
tance to preserving vitamin content include washing methods,
storage temperature, light, and moisture content (Online
Resource 1,Brown1995;Jimenez-Escrigetal.2001; Lage-
Yus t y et a l. 2014). There is a strong need for more detailed
investigations into how the nutritional quality of sea vegeta-
bles is affected by processing methods suited for commercial-
scale production.
The bioavailability of other algal-derived vitamins is also
underexplored. Vitamin E encompasses eight different forms
(tocopherols and tocotrienols) that differ in their biological
activity (α-andγ-tocopherols are the most active).
J Appl Phycol

Although much less is known about their relative bioavailabil-
ity compared to the vitamin B
12
analogs, it is clear that their
relative contributions affect the nutritional quality of foodstuffs
(Ortiz et al. 2009). An additional concern with fat-soluble vi-
tamins is that they must be consumed with lipid-rich foodstuffs
to ensure efficient intestinal absorption (Skrovankova 2011).
Although this co-dependence is understood, there currently are
few data on this dependence for edible-algal species.
Most studies on algae and vitamins often focus either on
analysis of vitamin concentrations in algae (e.g., Ortiz et al.
2006,2009; Hernandez-Carmona et al. 2009;Matanjunetal.
2009; Ferraces-Casais et al. 2012)ortestingthevalueofan
algal product as a functional food (e.g., Dagnelie et al. 1991;
Rauma et al. 1995; Takenaka et al. 2001), but not both. Ideally,
studies combining these two approaches should be adopted to
gain meaningful insights on the true quality of algal foods as
vitamin sources (Takenaka et al. 2001).
Finally, there are the ecological challenges to gaining a
broad picture of algal foods as a nutritional source of vitamins.
Vitamin production and metabolism can vary considerably
across diverse algal lineages (Croft et al. 2006;Helliwell
et al. 2011,2013). One approach that may help reveal this
complexity would be a high-throughput screening of promis-
ing algal food candidates with next-generation sequencing
techniques coupled with bioinformatics to search for
vitamin-biosynthesis pathways. Nevertheless, there will be
continued the need for careful analytical characterizations
and bioavailability testing because the up- or down-
regulation of gene expression almost certainly will be envi-
ronmentally regulated.
Antioxidants
It is not surprising that there is a very broad literature on
marine algae as sources of antioxidant compounds for human
diets. Photosynthetic energy acquisition and transformations
necessarily involve continuing redox disequilibria, with the
production of reactive species that can decrease lifespan and
evolutionary fitness. Microalgae and macroalgae, like other
life forms, contain antioxidant organic compounds and en-
zymes that limit this oxidative damage, which results primarily
from reduced states of oxygen—the Breactive oxygen
species^—including the superoxide radical anion (O
2
−·
;
O
2
+1e
−
), hydrogen peroxide (H
2
O
2
;O
2
+2e
−
), the hydroxyl
free radical (HO
·
;O
2
+3e
−
), and singlet oxygen (
1
O
2
)
(Halliwell and Gutteridge 2007). Whereas the antioxidant ben-
efits of several terrestrial plant foods are established, much less
is known about whether algal foods provide similar benefits.
The reactive oxygen metabolism in marine algae is diverse
and complex, given the wide range of antioxidant compounds
(Cornish and Garbary 2010), but an extension to any benefi-
cial response from human consumption of these substrates is
far less certain. Antioxidant activity can have two forms: the
activity of antioxidant enzymes or the production of
molecules that serve as sacrificial scavengers of reactive
oxygen species. There also are two broad categories of anti-
oxidant activity: limiting reactive oxygen species within the
digestive tract, thereby decreasing oxidative stress on the gut
microbiome and epithelial cells, or transport into the blood for
distribution throughout the body. Evidence for direct transport
is very limited, as there seems to have been no systematic
study of digestive uptake of these compounds. In one study,
Okada et al. (2009) examined the bioaccessibility of
astaxanthin extracted from the green alga Haematococcus
(Fig. 2) as judged from the concentration in blood serum, as
a function of the timing of the ingestion of astaxanthin relative
to a meal, and whether the subjects were smokers or non-
smokers. Astaxanthin increased more in serum when the dose
was taken 10 min after a meal rather than 2 h before, evidence
of complex factors affecting its bioaccessibility. The ingestion
(and topical application) of polyphenols of brown algae
inhibited UVB radiation-induced skin carcinogenesis in mice
(Hwang et al. 2006), and while this bioactivity remains to be
determined for humans, it provides evidence that algal foods
have significant functional food potential.
The foremost enzymes that restrict oxidative damage in
algae and terrestrial foods include the superoxide dismutases
that remove superoxide radical anions, and catalases and per-
oxidases, that convert hydrogen peroxide towater. Superoxide
dismutases in cyanobacteria have Ni, or mixtures of Fe, Mn,
and Ni, as the metal, whereas eukaryotic algae have Mn or Fe,
or some combination of Fe, Mn, and Cu + Zn (Wolfe-Simon
et al. 2005). Catalase has an Fe-containing heme cofactor
while peroxidases use a reductant to convert hydrogen perox-
ide to water. Of these enzyme cofactors, Cu and Zn, and par-
ticularly Fe are used in numerous human metabolic pathways.
Since the ingested antioxidant enzymes are digested in the
intestine, the only effect the enzymes can have in the animal
is through uptake of the metal cofactors across the intestinal
epithelium. The possible effects on the intestinal microbiome
of any undigested enzyme, or of the released metal cofactors,
have not been investigated.
There is a stronger linkage between selenium in food and
antioxidant capacity in metazoans such as mammals. Selenium
is an essential metal in metazoans and some algae for the pro-
duction of Se-requiring glutathione peroxidase, used to metab-
olize hydrogen peroxide and lipid hydroperoxides (Halliwell
and Gutteridge 2007;Perezetal.2007;Gobleretal.2011).
Analyses of the elemental contents of microalgae (Quigg et al.
2011) and macroalgae (Tuzen et al. 2009;Pereira2011)rarely
include Se, even though it is present in both (Fournier et al.
2005). Se readily bioaccumulates in algae (Cases et al. 2001;
Fournier et al. 2005), and Se-deficiency in rats can be alleviat-
ed by oral supplementation with Se-rich Arthrospira (spiru-
lina), as indicated by increased activity of (Se-containing)
J Appl Phycol

glutathione peroxidase in the kidneys and liver (Cases et al.
2001). However, increases in this enzyme activity were greater
in rats supplied selenite or selenomethionine (more reactive
species) than with the same dosage of Se-rich cyanobacterium,
likely due to lower bioavailability of the cyanobacterial Se.
The factors regulating Se content of algal foods and its avail-
ability are prime research topics for the future.
Under normal metabolic conditions, the production of hy-
droxyl radicals and singlet oxygen cause almost immediate
damage, essentially reacting with the first oxidizable molecule
that they encounter. In these cases, Bsacrificial^scavengers (of
HO
·
)andquenchers(of
1
O
2
)oftenaretheonlyrecoursefor
limiting damage once the free radicals are produced (Smirnoff
and Cumber 1989;Telferetal.1994a,b;Sundaetal.2002;
Ledford and Niyogi 2005; Halliwell and Gutteridge 2007;
Ledford et al. 2007). Algae contain a wide range of molecules
capable of free radical scavenging activity in vitro and in vivo.
These include the water-soluble ascorbate (vitamin C) and cer-
tain compatible solutes (osmoprotectants), and the lipid-soluble
α-tocopherol (vitamin E) and carotenoids such as astaxanthin
(Halliwell and Gutteridge 2007). Mycosporine-like amino
acids, mainly considered as UV screening compounds, are also
antioxidants (Oren and Gunde-Cimerman 2007)asarearange
of other solutes that act as scavengers and quenchers of reactive
oxygen species in algae (Cornish and Garbary 2010). HO
·
scavengers include glycerol (Smirnoff and Cumber 1989),
mannitol (Smirnoff and Cumber 1989;Shenetal.1997;
Larson et al. 2002), L-proline (Smirnoff and Cumber 1989),
dimethylsulfoniopropionate (Sunda et al. 2002), and
floridoside and isofloridoside (Li et al. 2010), although glycine
betaine (or betaine: trimethylglycine) does not have this prop-
erty (Smirnoff and Cumber 1989;Shenetal.1997). Given that
algal osmoprotectants are necessarily present in high concen-
trations (≥0.1 mol L
−1
)inmetabolicallydiversecompartments
(cytosol, plastid stroma, and mitochondrial matrix), there is
potential for them to have functional food roles. However,
preliminary experiments showed that none of these compounds
interact with O
2
−·
(Smirnoff and Cumber 1989), unlike β-
carotene and other carotenoids such as fucoxanthin that quench
1
O
2
as well as scavenging HO
·
and O
2
−
·
(Halliwell and
Gutteridge 2007;Sachindraetal.2007). Other algal compo-
nents that scavenge free radicals are phenolic compounds
(Ragan and Globitza 1986) including halophenols (Li et al.
2011) and phlorotannins (Shibata et al. 2007) and, as noted
above, alginate (Zhao et al. 2012;Zhouetal.2012)andsulfat-
ed polysaccharides (Barahona et al. 2012).
Most studies of the bioavailability of algal antioxidant
products remain at the entry level with respect to human ef-
fects: in vitro testing of extract bioactivity on cell lines. Nwosa
et al. (2011)confirmedandextendedpreviousworkshowing
the antioxidant activities of polyphenolic extracts from four
species of edible marine algae in inhibiting Caco-2 colon can-
cer cell proliferation and α-glucosidase activity (see below):
the green alga, Ulva lactuca, the brown algae Alaria esculenta
and Ascophyllum nodosum,andtheredalgaPalmaria
palmata. Ulva lactuca had a low yield of polyphenols relative
to the other algae, but the brown and red algal polyphenolic
extracts performed as well as antioxidants. However, Nwosa
et al. (2011) illustrated that the method of preparing the ex-
tracts from marine algae can significantly alter their antioxi-
dant efficacy (see also Ling et al. 2015), highlighting the need
for caution in comparisons of antioxidant performance among
studies. With this possible caveat, most work on antioxidant
activity of algal phenols has involved red algae; some
bromophenols from the marine red alga Rhodomela
confervoides have greater in vitro antioxidant activity than
ascorbate (Li et al. 2011). Olsen et al. (2013) showed that
bromophenols extracted from the red alga Vertebrata lanosa
significantly inhibited oxidant effects and lipid peroxidation in
cultures of human fetal lung (MTC-5) and human hepatocel-
lular liver carcinoma (HepG2). In this case, it was shown that
bromophenol can enter cells, and thus potentially can move
from the gut lumen into the blood stream. Overall, there is a
strong need for more work on the in vivo effects of the anti-
oxidant properties of phenols and other algal food constituents
in mammals, and humans in particular.
Instead of serving to facilitate the control of reactive oxygen
species, some algal componentscaninhibittheirproduction,
but most studies do not adequately distinguish between the
decreased production and increased removal of oxidants. For
example, dietary ingestion of phycocyanin, taken up from the
gut as the chromophore component phycocyanobilin, and relat-
ed bile pigment metabolites inhibits the generation by NADPH
oxidase of O
2
−
, which has a key role in numerous disease
syndromes, e.g., antigen expression, angioplasty, cancers, gly-
cemia and lipidemia, hypertension, immunostimulation, and
age-related maculopathy (reviewed by McCarty 2007). This
industry-sponsored but balanced and authoritative review
shows that Spirulina spp. (most now transferred to
Arthrospira)areaprominentcyanobacterialsourceofphyco-
cyanobilin, a dietary supplement worthy of in-depth study.
A class of compounds attracting increasing attention are the
phlorotannins found inbrown algae, which have extraordinary
though inconsistent antioxidant properties (see in Wang et al.
2014), in part due to the methods of extraction (Nwosa et al.
2011). The vast bulk of this work was done in vitro, much of it
studying the effects of phlorotannin on carbohydrate-
hydrolyzing enzymes. Nwosa et al. (2011) found that extracts
of Ascophyllum and Alaria inhibited Caco-2 colon cancer cell
proliferation, α-amylase activity and, to a lesser extent, α-
glucosidase activity, with mass spectrometric evidence indi-
cating that the active principal(s) were phlorotannins.
Kawamura-Konishi et al. (2012) also found that phlorotannin
extracts of four species of Sargassum significantly inhibited
the salivary enzyme α-amylase in vitro, and that a novel
phlorotannin from Sargassum patens inhibited rat pancreatic
J Appl Phycol

α-glucosidase action on amylopectin. Iwai (2008) showed
that oral administration of extracts inhibited lipid peroxidation
in the plasma, red blood cells, liver, and kidney of KK-A
y
mice, indicating that the antioxidant activity of phlorotannins
had beneficial properties for reducing diabetic oxidative
stress. Important recent work (Corona et al. 2016)investigated
the effect of food grade phlorotannins from Ascophyllum
nodosum in trials on human subjects. The work showed the
in vitro gastrointestinal modification of phlorotannins, the oc-
currence in plasma and urine of metabolites of phlorotannins,
and a significant increase in cytokine IL-8. To conclude con-
sideration of phlorotannins, while in vitro studies on
phlorotannins are valuable, more work along the lines of that
of Iwai (2008) and Corona et al. (2016)isneededtounder-
stand the uptake and systemic properties of phlorotannins, and
to determine whether the in vitro effects occur in vivo and
relate to their antioxidant properties (Bohn et al. 2015).
There remain substantial knowledge gaps about the effica-
cy of antioxidant properties of macroalgal and microalgal
foods at all levels, from characterization among species
through effects on gut microbiota and transport across the
gut lumen to their impacts on human physiology. This will
be a valuable area of emerging research over the next decade.
Inorganic elements
Processed seaweeds are widely used as mineral and metal
nutritional supplements (e.g., Kay 1991), but the efficacy of
these supplements is poorly quantified. Most studies suffer
from serious experimental limitations, including short dura-
tion of the study, small sample size, and inadequate documen-
tation of active ingredients. There is a comparatively small
literature describing mineral contents of macroalgal and
microalgal foods (Cabrita et al. 2016), and very little informa-
tion about seasonal variations for naturally harvested sea
vegetables.
The best evidence of the human nutritional benefits of sea
vegetable inorganic elements is for iodine and iron, which can
be highly enriched in marine macroalgae. Nutritional general-
ization about algal mineral contents is difficult because of
sometimes large seasonal, geographic, and taxonomic varia-
tions in mineral contents of marine algae (e.g., Jensen 1993).
For example, Indonesian green, brown, and red algae contain
high levels of potassium, calcium, and sodium, but significant-
ly lower levels of iron and zinc than reported for Japanese
Pyropia (as Porphyra)yezoensis,Ulva (Enteromorpha)
intestinalis,andSargassum (Hijikia)fusiformis (Takeshi
et al. 2005). These findings may indicate that macroalgal har-
vests from warm equatorial areas have lower mineral nutrition-
al value than higher latitude regions (e.g., Cabrita et al. 2016);
however, there are remarkably few data on which to assess the
validity of such generalizations.
Iodine
There is a long history linking seaweed consumption by
humans and the reduced incidence of goiter and other thyroid
disorders. Iodine deficiency causes hypothyroidism while ex-
cess iodine uptake can induce either hyper- or hypothyroidism
(Miyai et al. 2008). Seaweeds are a good nutritional source for
iodine, particularly in regions where other foods are deficient,
but the iodine content of commercially available sea vegeta-
bles varies dramatically among species, the methods of prep-
aration (many iodine compounds are water soluble), and the
duration of storage (iodine may vaporize under humid condi-
tions) (Teas et al. 2004a). Many macroalgae are washed and
dried for storage. These processing steps did not significantly
reduce iodine content in three common species (Alaria
esculenta,Palmaria palmata,andUlva intestinalis), but rehy-
dration followed by boiling in water lowered iodine content
by 14–75 % (Nitschke and Stengel 2016).
Some kelps (Laminaria spp., Saccharina spp.) have high
levels of iodine, and salts that include kelp powder are avail-
able commercially as a source of this vital nutrient. Not all
brown algae accumulate high levels of iodine; for example,
the kelps Undaria (wakame) and Alaria (BAtlantic wakame^)
have lower iodine levels that are comparable to Palmaria
palmata (dulse, a red sea vegetable) (MacArtain et al. 2007;
Rhatigan 2009; Holdt and Kraan 2011;Schieneretal.2015)
(Fig. 6). In contrast, high levels of iodine in other brown
macroalgae (e.g., Laminaria;Saccharina;Teasetal.2004b;
Miyai et al. 2008) have led to a strong concern that overcon-
sumption of these particular sea vegetables can be unhealthy.
Michikawa et al. (2012) reported that consumption of unde-
fined seaweed more than 2 days per week appeared to corre-
late with increased risk of thyroid cancer in Japanese postmen-
opausal women, although Wang et al. (2016)didnotfindthis
relationship to be significant. Nevertheless, there is clear evi-
dence that algal food consumption leads to elevated iodine
levels in humans. Miyai et al. (2008) measured serum levels
of thyroid hormones in conjunction with well-defined inges-
tion rates of kombu (Saccharina japonica) over short (7–
10 days) and longer term (∼90 days) exposure. Urinary excre-
tion of iodine increased significantly over the short term with
increasing intake (15–30 g day
−1
of kombu—anormalcon-
sumption for some Japanese) and suppressed thyroid function
for at least 3 months. Thyroid hormones returned to normal
levels when seaweed intake ceased. In this case, the absorbed
iodine (20–50 mg) exceeded the recommended upper daily
intake of iodine (0.2 mg day
−1
by more than an order of mag-
nitude; WHO 1989; Dawczynski et al. 2007; Miyai et al.
2008). Food preparation (e.g., cooking, pickling) can reduce
the iodine intake, but, even then, high-iodine sea vegetables,
which account for only a portion of those consumed, should
be restricted in the diet (Teas et al. 2004a). Research is needed
on how food preparation alters the bioavailability of iodine in
J Appl Phycol

different sea vegetables. Indeed, Lightowler and Davies
(2002) found that established food tables (UK Department
of Health, 1991) gave poor estimates of dietary iodine intake
and recommended developing more reliable data on iodine in
foods, including the variation within food groups.
Iron
Macroalgae are a potentially rich source of iron for human
diets. Garcia-Casal et al. (2007,2009)measuredseasonaldif-
ferences in the iron content of four seaweed species common
to Venezuelan waters (Ulva spp., Sargassum spp., Porphyra
spp., and Gracilariopsis spp.). The Gracilariopsis spp. and
Sargassum spp. had substantially higher iron content with
Porphyra spp. having the lowest, and there was a distinct
seasonal cycle whereby iron content was highest in spring
and summer and lowest in fall and winter. Sargassum,Ulva,
and Porphyra spp. have high iron content, and 15 g day
−1
of
Sargassum provided substantially higher amounts than daily
recommended intakes (≤18 mg Fe day
−1
, Institute-of-
Medicine 2001). Cabrita et al. (2016) found that Fe (and other
metal) contents varied, in some cases substantially, among
macroalgal species collected at the same sites and time, pre-
sumably linked to differences in metabolic requirements.
Recognizing that iron consumption in macroalgae likely does
not equate with uptake, the researchers quantified iron incor-
poration into the blood of 93 volunteers fed diets of radioac-
tive (gamma-emitting)
59
Fe-labeled Ulva spp., Sargassum
spp., and Porphyra spp. (Garcia-Casal et al. 2007,2009).
Uptake of
59
Fe was dose-dependent on sea vegetable added
to maize or wheat-based meals (Garcia-Casal et al. 2007), but
generally was greater than the total iron content of the differ-
ent macroalgal species. Thus, in addition to enhancing total
dietary iron content, the seaweeds appeared to act synergisti-
cally to facilitate iron uptake from wheat and maize, possibly
related to high levels of ascorbic acid (vitamin C) that converts
iron to the more readily absorbed ferrous form.
The Fe content of wild algae varies seasonally and geo-
graphically depending on the metal content of coastal waters,
in addition to being species-specific (Garcia-Casal et al. 2007;
Fig. 6 Examples of sea vegetable use in Japanese cuisine: aBOnigiri^is
aJapanesericeballusuallywrappedbynori(Pyropia yezoensis)with
several other ingredients: from right to left, rice wrapped in nori, with
Japanese apricot (umeboshi) and preserved kombu (tsukudani,
Saccharina japonica and other species of the genus Saccharina),
wrapped with shredded kombu (torero-kombu), and wrapped in nori; b
Tempura soba with wakame (Undaria pinnatifida). Wakame is used in
Japan, noodles, soups, salads, pickles, and more. cTraditional Japanese
dishes with sea vegetables: preserved kombu (tsukudani) on rice, miso
soup with wakame, sliced kombu with vegetables, and sliced raw fishes
(sashimi) with wakame and cucumber. dBKobu-maki^is simmered food,
often salmon or herring, wrapped in kombu, which is usually prepared for
the New Year’s holidays. Kombu is used in several dishes and soup stock.
(Courtesy of Kazuko Sato and Yoichi Sato)
J Appl Phycol

Cabrita et al. 2016). Wild harvesting must then be optimized
for each locale known to produce algae for optimal concen-
trations of inorganic nutrients. Similar constraints may apply
to aquaculture crops unless they are fertilized, but there are
very few data currently available on sea vegetable nutritional
metal content relative to species, region, or season.
Phytochemicals
The main non-acyl glyceride compounds in lipid membranes
attracting nutritional and commercial interest are the caroten-
oids, especially because of their dietary importance for vision
(Ben-Amotz and Levy 1996). Carotenoids, among the most
widespread pigments in prokaryotic and eukaryotic photosyn-
thetic organisms (Britton et al. 2004), function as light energy
harvesters, as photoprotectants, and as antioxidants (Halliwell
and Gutteridge 2007,andseebelow).Onespecificketo-carot-
enoid, siphonaxanthin, but not fucoxanthin, induced apoptosis
in human leukemia cell lines, suggesting its potential as a
chemopreventative or chemotherapeutic agent (Ganesan
et al. 2011), although its efficacy remains to be determined
in vivo. A subset of the carotenoids, the β-carotenes, are pre-
cursors for vitamin A (Minguez-Mosquera et al. 2008), and
natural β-carotene may act as a lipophilic antioxidant in vivo,
providing some protection in children exposed to radiation
from the Chernobyl accident (Ben-Amotz et al. 1998).
Various strains of the green microalga Dunaliella salina
can accumulate about 8 % of their dry weight as β-carotene,
which has been marketed as a functional food (Ben-Amotz
and Levy 1996; Borowitzka 2013b). The US FDA recently
(2011) had no questions about a filing for the powdered form
of the microalga Dunaliella (bardawil)salina as GRAS for
use as an ingredient in food products (Agency Response
Letter GRAS Notice No. GRN 000351, Borowitzka 2013b).
Another main source of carotenoids is the green alga
Haematococcus pluvialis,forwhichcultureshavedeveloped
industrially in several countries (Cysewski and Lorenz 2004).
Carotenoid supplements, including β-carotene, are effective
in improving the carotenoid supply in breast milk at early
lactation (Nagayama et al. 2014). However, the optimal levels
of β-carotene remain controversial. Undefined β-carotene
supplements increased lung cancer risk in smokers or people
exposed to asbestos (Druesne-Pecollo et al. 2010), while ear-
lier work showed no reduction in the incidence of lung cancer
among male smokers after 5 to 8 years of dietary supplemen-
tation with alpha-tocopherol or β-carotene (Albanes et al.
1995).
Phenolic compounds such as flavonoids and phenolic acids
serve as antioxidants. Brown macroalgae (Alaria esculenta,
Ascophyllum nodosum,F. vesiculosus,Saccharina latissima)
had 2–15 times the total phenolics (as mg gallic acid equiva-
lents g
−1
dry matter) as red species (Chondrus crispus,
Meristotheca papulosa,Palmaria palmata,Sarcodiotheca
gaudichaudii) (Tibbetts et al. 2016). Total phenolic content
was inversely related (r=−0.81) to in vitro protein digestibil-
ity. Phenolic compounds are challenging to extract and char-
acterize, but improvements in analytical methodology are ex-
pected to facilitate more detailed investigation of these anti-
oxidants in algae. Drying Kappaphycus alvarezii in sunlight
significantly reduces total phenolic, flavonoid, anthocyanin,
and carotenoid content compared with samples dried in ovens
or in the shade (Ling et al. 2015).
Safety
Consumption of any food is not without risk, so the promotion
of algal consumption must also consider potential harm to
consumers.Possible risks associated with algae include excess
intake of toxic metals, allergenicity, cyanotoxins, and certainly
secondary metabolites (e.g., prostaglandins, kainoids) as well
as contamination with pathogens, radioisotopes, and toxic
synthetic compounds.
Metal toxicity
Under normal, pristine conditions, metal uptake improves the
nutritional quality of algal foods, but excessive uptake can
lead to toxicity. Moreover, a substantial amount of metal as-
sociated with macroalgae can exist as colloidal-sized particles
sorbed to algal surfaces (Gadd 2009; Turner et al. 2009), so
surface chemistry and algal physical structures can affect met-
al content in addition to metabolic processes. The efficacy of
macroalgae as metal collectors is why they are effective sen-
tinel organisms for detecting anthropogenic signatures in
coastal waters (Melhuus et al. 1978;Lealetal.1997; Brown
et al. 1998; Mardsen and De Wreede 2000); a feature in con-
flict with their use as a food source.
Despite these caveats, some general patterns appear in the
literature. Metal content tends to differ among phylogenetic
groups, with brown algae typically possessing higher levels of
most metals in comparison with red or green algal species
(Foster 1976;MundaandHudnik1991;StengelandDring
2000;Al-Masrietal.2003; Benkdad et al. 2011). But there
are exceptions. Phaneuf et al. (1999) observed that the green
alga Ulva lactuca and Ulva (as Enteromorpha)spp.intheSt.
Lawrence River estuary had greater concentrations of Co, Cr,
and Cu, while the brown algae F. v e s i c u l o s u s ,Laminaria
longicruris and Fucus distichus had higher As and Cd con-
centrations (though all below regulatory levels). These
species-specific differences among algae from the same envi-
ronment may reflect differing metabolic affinities for these
metals. Algal uptake of Cd, Cu, Co, Zn as well as Pb occurs
via transporter-mediated processes, as indicated by their up-
take kinetics (e.g., Garnham et al. 1992;Knaueretal.1997;
J Appl Phycol

Mehta et al. 2002; Francois et al. 2007; Costa et al. 2011). As
noted, however, colloidal sorption to algal surfaces can be a
contributing factor, and there are few data on colloidal metal
concentrations in coastal waters (Wells 2002). Regardless,
macroalgae clearly can be a vector for toxic metal transfer to
humans, especially if harvested from contaminated habitats.
Perez et al. (2007) studied the concentrations of many ele-
ments in Pyropia (as Porphyra)columbina and Ulva sp. in
two Argentinian regions having different exposure to human
activities. They found a wide range of metal content between
these species with Pyropia columbina being a stronger accu-
mulator of As, Cd, Mo, and Se, while Ulva spp. tended to
accumulate more Cr, Pb, and Ni. There also were significant
seasonal variations in metal loading with Pyropia columbina
having maximum Cd concentrations during winter while Ulva
sp. showed highest Pb concentrations during summer. This
seasonality may result not only from metabolic controls but
also oceanographic influences. For example, Riosmena-
Rodríguez et al. (2010) found elevated concentrations of Cd
in several algal species during April that appeared to be related
to local upwelling events. It is unclear to what extent the
presence of these metals could affect human health, as their
bioavailability was not assessed.
Metal accumulation will be influenced by both the geo-
chemical conditions and also by metabolic control as a func-
tion of ecological growth strategies or seasonally determined
productivity (Stengel et al. 2004). Schiener et al. (2015)found
the concentrations of major metal salts of Ca and K and the
trace element Fe to more than double during spring and sum-
mer months in the brown algae Laminaria digitata,Laminaria
hyperborea,andSaccharina latissima, but concentrations of
other trace metals did not vary in a seasonal pattern. Although
concentrations of Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, or Zn
differed greatly among F. vesiculosus,Ascophyllum nodosum,
Laminaria longicruris,Palmaria palmata,Ulva lactuca,and
F. distichus collected from the St. Lawrence River estuary,
Canada, all were at very low levels and there were no metal-
related health risks (Phaneuf et al. 1999). Similarly, a study of
the metal content of seaweeds washed up on the Brazilian
coast also showed that some toxic metals were at levels con-
sidered harmful, although with remediation of nearby indus-
trial contamination sources these macroalgae represent a po-
tential food alternative for humans (de Oliveira et al. 2009). In
one of the more comprehensive recent analyses, Dawczynski
et al. (2007) measured concentrations of six trace elements
(Fe, Mn, Zn, Cu, Se, and I) and four ultra-trace elements
(As, Pb, Cd, and Hg) in 34 commercially available red and
brown macroalgal products originating from China, Japan,
and Korea. They found that for normal consumption, daily
intakes of Fe, Mn, Cu, and Se are well below daily intake
recommendations of the German Society of Nutrition (DGE)
and the provisional tolerable weekly intake (PTWI) values of
the World Health Organization (WHO), and the ultra-trace
elements were present at low, harmless concentrations.
Turner et al. (2008) measured the uptake of the trace metals,
Pd, Cd, Hg, and Pb, by Ulva lactuca along the salinity gradi-
ent (S = 15–35) under well-controlled laboratory conditions.
Only Cd displayed salinity-dependent uptake rates, with rates
decreasing at higher salinity. Their findings also showed that
the presence of environmentally relevant concentrations of
dissolved humic substances (3 mg L
−1
) suppressed slightly
the sorption of Pd and Hg, while moderately enhancing Pb
sorption via adsorption to the algal surfaces; the uptake
(internalization) of Pb was inhibited by humic substances. It
is clear that processing methods that help to eliminate metals
bound to algal cell walls/surfaces would be advantageous, but
there is little information available on this topic.
There is thus potential for some metals to reach harmful
concentrations in edible seaweeds, but there is no information
on how bioaccessible or bioactive most algal metals are in
human digestion. Worse, there is no consensus on a uniform
or even optimal approach to quantify the bioavailability of
metals. Recent advances using bio-digestive reactor ap-
proaches (e.g., Moreda-Pineiro et al. 2012) likely provide
the way forward, but this remains to be established.
Arsenic
Chronic exposure to inorganic arsenic (iAs: arsenite, As(III),
arsenate, As(V)) leads to a higher incidence of several cancers
including skin, lung, and urinary tract cancers. The
International Agency for Research on Cancer (IARC) clas-
sifies iAs as human carcinogens (Group I), while the biolog-
ical metabolites dimethylarsinic acid (DMA) and
monomethylarsonic acid (MMA) are classified as possibly
carcinogenic (Group 2B) in humans (IARC 2012). Toxic
effects of As include disruption of oxidative phosphorylation,
generation of reactive oxygen stress (ROS), enzyme inhibi-
tion, and epigenetic changes (IARC 2012;NRC2013).
Exposure to iAs occurs as humans drink water enriched in
As by natural geochemical processes or poor agricultural/
manufacturing/mining practices and through the diet includ-
ing from foods (e.g., rice) grown in contaminated soils (NRC
2013; Zhu et al. 2014;Lietal.2016). All seafoods contain
arsenic, which enters cells through phosphate transporters and
aquaglyceroporins (Bhattacharjee et al. 2008; Cooney et al.
1978;Zhaoetal.2010). The World Health Organization
(WHO)’s provisional maximum level of iAs in drinking water
is 10 µg L
−1
(WHO 2016).
There are more than 50 arsenic species in seafood, but the
absolute arsenic content of a seafood does not predict health
risk because marine organisms have evolved detoxification
strategies in which iAs is converted to methylated (organic)
forms. Fish and crustaceans convert most iAs into
arsenobetaine; humans excrete arsenobetaine, and it is not
considered to be toxic (Francesconi 2010, Molin et al. 2015
J Appl Phycol

[see their Table 1 for structures]). Algae, and mollusks that eat
algae (e.g., oysters, clams), convert most iAs to arsenosugars,
and there is some evidence that when phosphate levels are
low, iAs may be converted to As-phospholipids that have a
role in algal membrane function (Cooney et al. 1978; Garcia-
Salgado et al. 2012). It is reassuring that fractionation studies
show most macroalgae contain very little inorganic As in
comparison to arsenosugars, although measurement of iAs
in algae may be less reliable than in plants (de la Calle et al.
2012; Diaz et al. 2012;Hansenetal.2003).
One of the major, excreted metabolites from algal
arsenosugars in humans is dimethylarsinic acid (DMA),
which the IARC (2012) considers to be possibly carcinogenic
to humans (Group 2B) (IARC 2012). In vitro trials with hu-
man HepG2 cells showed that DMA toxicity occurred only at
levels that were 400×the maximum DMA found in the urine
of a human volunteer during clearance of arsenosugar (Raml
et al. 2005). However, the rate of clearance of DMA and other
metabolites of an arsenosugar that was experimentally
ingested appears to vary in different individuals; for example,
although 4-day urinary excretion removed ≥85 % of the As
represented in an ingested arsenosugar in four individuals, two
other individuals excreted only 4–15 %. This difference might
reflect either retention of As, or perhaps that they absorbed
less arsenosugar from the digestive tract (Raml et al. 2009).
Although most algae naturally synthesize arsenosugars
from the iAs they take up from seawater, a few brown
macroalgae contain a significant fraction of total As as iAs
(Laminaria digitata, Laminaria hyperborea: Hansen et al.
2003, Taylor and Jackson 2016;Sargassum spp.: Nakamura
et al. 2008, Magura et al. 2016). The sea vegetable hijiki
(Sargassum fusiforme) contains unusually high levels of iAs
(e.g., 60 µg g
−1
dry wt hijiki, 0.4–2.8 µg g
−1
cooked hijiki) in
comparison to its arsenosugar content (Francesconi 2010;
Nakamura et al. 2008;Roseetal.2007). Nevertheless, there
are at least three issues that bear upon As toxicity to humans:
the chemical speciation of As in the food item, the bioacces-
sibility after cooking (Devesa et al. 2008; Ichikawa et al.
2006), and the metabolism of As in the individual (Choi
et al. 2010;Ramletal.2009). Ichikawa et al. (2006)reported
that 88.7–91.5% (w/w) of As in hijiki was removed by
cooking. Nakamura et al. (2008) determined inorganic As
(III+V) extracted during a simulated gastric digestion (pepsin)
from cooked hijiki donated by 14 families who also provided
information on their monthly consumption of hijiki.
Nakamura et al. (2008) estimated that this would result in an
iAs intake of 1.1 µg kg
−1
human body weight per week and
could cause a non-negligible increase in skin cancer cases by
their model. Currently, the WHO does not have a Provisional
Tolerable Weekly Intake level of iAs, after withdrawing an
earlier PTWI of 15 µg kg
−1
human body weight per week
(WHO 2010). More research is needed on health risks from
lower dose iAs intake following consumption of the few
brown algae that store iAs. The National Research Council
(USA) recommends data-driven statistical approaches vs. lin-
ear extrapolation to estimate low-dose As effects from studies
where higher doses are used (NRC 2013). As Nakamura et al.
(2008) pointed out, calculations of excess cancer risk assume
that the mechanism of carcinogenesis has no threshold dose;
i.e., the incidence of cancer is linearly related to As intake in a
low dose range,and this may not be the case. Inorganic As and
arsenosugar content of hijiki varies geographically and with
manufacturing method (Shimoda et al. 2010). Canada and the
UK advise consumers to avoid eating hijiki (CFIA 2012;UK
FSA 2016).
By estimating As bioaccessibility through the human GI
tract using modifications of methods developed by nutrition-
ists for estimating Fe uptake, Garcia-Sartal et al. (2012)con-
cluded that only a fraction (7–15%) of the inorganic As and
arsenosugars in cooked algae such as kombu, wakame, nori,
and sea lettuce is bioaccessible. More research is needed to
understand how different cooking processes, the particular
algal food matrix, and the gut microbiome modify arsenosugar
bioaccessibility and then better definition of the interaction of
other As species produced in the body with cellular metabo-
lites and macromolecules before their excretion (NRC 2013;
Molin et al 2015; Carlin et al 2016). Experiments at all levels
are important but it is particularly important to move beyond
tests of toxicity using in vitro cell cultures in order to under-
stand whether there is any risk of consuming any sea vegeta-
ble. The quantities of bioaccessible arsenic as iAs and/or
arsenosugars in most sea vegetables appear too low to pose
risk to individuals unless there is co-exposure to much higher
levels of arsenic through high iAs-drinking water (≥10 µg L
−1
)
or foods or environments contaminated by As pollutants.
Bromine
Less well-recognized toxic effects can arise from excess in-
take of sea salt minerals, such as bromine, which can cause
nerve, DNA, and organ damage in mammals (e.g., Boyer et al.
2002). Bromine concentrations in the urine of human female
subjects in China, Japan, and Korea have been shown to cor-
relate with their seafood intake, with macroalgae apparently
being a major source (Kawai et al. 2002). However, this cor-
relation was skewed by the consumption of terrestrial crops
treated with methyl bromide, which can significantly increase
total bromine intake. It is an important reminder for the need
to quantify total exposure of an individual to metal intake to
evaluate the effects of algal consumption.
Allergenicity and macroalgal toxins
Relatively little has been published on the subject of possible
allergenicity of algae and their products. Ayoung man devel-
oped anaphylaxis after the first-time consumption of a
J Appl Phycol

spirulina tablet (Le et al. 2014), but the source of the spirulina
in this tablet, or its purity, was not given. Well-known epi-
sodes of human poisoning events have occurred after con-
sumption of wild-harvested spirulina that contained
Microcystis and other freshwater cyanobacteria that produce
neurotoxins and hepatotoxins. This emphasizes the impor-
tance of developing large-scale, controlled cultivation and dai-
ly testing of future supplement crops (e.g., Gellenbeck 2012),
similar to those used now by companies that supply large
quantities of GRAS-spirulina (e.g., Cyanotech, Earthrise
Farms) to the food and supplement markets (Belay 2008). A
dried, milled protein preparation, Whole Algalin Protein, from
Chlorella protothecoides produced by Solazyme, Inc. was
demonstrated to be unlikely to cause food allergies in nutri-
tional studies with rats (Szabo et al. 2013).
The amino acid kainic acid, which is found in dulse and
some other red algae (e.g., Digenia simplex), is structurally
similar to glutamate, a neurotransmitter in the brain. At high
doses, kainic acid is neurotoxic and used experimentally to
produce disease models in mice and other animals.
Concentrations of kainic acid that damage neurons are much
higher than those consumed by eating dulse, but Mouritsen
et al. (2013) report that no human safety standard has been
established. This is both a dose and a bioavailability issue, and
it needs more study because a few dwarf individuals of
Palmaria palmata had high levels of kainic acid (>10 mg g
-
1
dry wt, Ramsey et al. 1994), even though it was undetectable
or at very low level in most Atlantic dulse (Ramsey et al.
1994;HigaandKuniyoshi2000;Mouritsenetal.2013).
Another amino acid in the kainoid family is domoic acid
(DA), which is also found at low levels in some red algae
(e.g., Chondria armata, in the same Rhodomelaceae as
Digenia simplex, the first discovered kainic acid-producing
red alga). DA is a strong health risk during blooms of a few
diatoms (e.g., Pseudo-nitzschia) that are bioaccumulated by
filter-feeding mollusks and become a human health risk, but
DA poisoning from red algae is unknown (Higa and
Kuniyoshi 2000). Traditionally in both Europe and Asia,
worms were eliminated from humans and animals with red
algae that contain KA or DA (Mouritsen 2013).
Many Gracilaria (Rhodophyta) and Caulerpa (Chlorophyta)
species are eaten as sea vegetables, especially in the western
Pacific (de Gaillande et al. 2016), but illness and some deaths
have occurred when a few toxic species of these genera were
sold or collected by mistake (Higa and Kuniyoshi 2000;
Cheney 2016). Toxic prostaglandins (PGE
2
)arefoundin
the Asian Gracilaria vermiculophylla, which invaded
North American and European shores in recent decades
(Hammann et al. 2016). Wounding of G. vermiculophylla
increases synthesis of PGE
2
from arachidonic acid within
minutes (Noguchi et al. 1994;Nylundetal.2011).
Hammann et al. (2016) showed that this effect was enhanced
in invasive populations, finding that this appears to be a
defense against naïve herbivores in the non-native habitat.
Their study characterized many additional metabolites
present in G. vermiculophylla. Fatalities have resulted from
consumption of fresh Gracilaria edulis (Polycavernosa
tsudai in Navarro et al. 2015), due to toxic polycavernosides
(Yotsu-Yamashita et al. 2007). Navarro et al. (2015)’s recent
analysis suggests, however, that the polycavernoside is pro-
duced by filamentous cyanobacteria (Okeania sp.) that are
sometimes associated with the Gracilaria.
Summary
The considerable breadth and depth of literature on algae as
nutritional and functional foodsfrustratesattemptsforafully
comprehensive assessment of the field. It is clear that there is
substantial evidence for algae as nutritional and functional foods,
yet there remain considerable challenges in quantifying these
benefits, and in assessing potential adverse effects. The limits
to our understanding fall broadly into three areas. First is the
variation of nutritional and functional composition of algae
across species, seasons, and different coastal environments. The
scant evidence to date suggests this variability can be substantial
but it is only possible to speculate about the scope of this incon-
sistency. That is true also for quantification of toxic, or potentially
harmful constituents present within, or adsorbed to algae.
Assessments should also consider the effect of processing
methods, which can increase or decrease the nutritional quality.
Of the challenges ahead, these issues are the most tractable to
address, given that the analytical methods are well developed.
The second, and perhaps most pressing limitation, is quan-
tifying the bioavailability, or fraction of nutritional or function-
al components that actually have effect in relation to their
residence time in the digestive system. These effects can man-
ifest via translocation across the small intestinal epithelial cells
into the blood, by direct interactions with the digestive epithe-
lia, by altering uptake of other substances, by regulating the
microbial consortium, or by direct contact with colonic epithe-
lial cells in the large intestine (e.g., colon cancer). There is an
increasing literature on digestive Breactor^analytical methods,
and much more effort in this direction would be beneficial, but
it must be recognized that bioaccessibility will be a complex
function of the chemistry of the substance, the processing
methods used to prepare the alga as food, the specific algal
matrix containing it, the consortium of bacteria and their en-
zymatic competency, and the presence of other foods that may
interfere or enhance uptake (or within-gut effects). Also nec-
essary is more rigor in semantics; bioavailable, bioactive, ac-
tivity, digestible, gastrointestinal absorption, and utilization are
not equivalent terms but are used interchangeably in the liter-
ature. Advances in understanding bioavailability of foods in
general will continue piecemeal until analytical methods and
studies encompassing all of these factors become routine.
J Appl Phycol

The third limitation lies in understanding how algal nutri-
tional, and particularly functional, constituents interact in hu-
man metabolism and intermediary metabolic processes. Most
investigators studying these questions report in vitro experi-
ments or in vivo experiments employing direct introduction of
purified algal constituents. These methods are useful probes to
identify and mechanistically understand potential effects of
the consumption of algal-based supplements, but they are in-
adequate to assess nutritional and functional foods.Their
shortcomings most often include unrealistic or uncertain doses
relative to normal algal consumption, a focus on a single or
narrow range of metabolic responses, and inability to assess
concurrent, or synergistic effects arising from co-occurring
constituents during digestion. Coupling in vivo studies with
more holistic digestive research will provide for a better as-
sessment of algae as nutritional and functional food sources.
We envisage a rare opportunity to develop a rich and
rewarding collaboration among phycological, nutritional,
medical, analytical, and industrial groups investigating algae
as nutritional and functional foods. Part of the challenge ahead
for algal scientists is understanding the complexity of merging
of basic research through clinical trials and regulatory
requirements to create marine food products. The recent
comprehensive reviews by Finley et al. (2014)and
Borowitzka (2013b) help to clarify this pathway. But the most
dramatic advances will require a rethinking of experimental
and collaborative approaches, and the impetus for this re-
search will only increase as human pressures on the climate
system lead us to turn more to the oceans for food that we can
harvest and grow sustainably.
Acknowledgments We thank Eva Bradford for clerical assistance in
the initial stages of our literature review, and to Dr. Yoichi Sato (Riken
Food Co., Ltd.) and Dr. Keiko Yoshinaga (Riken V