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Diatoms are unicellular, mainly photosynthetic, eukaryotes living within elaborate silicified cell walls and believed to be responsible for around 40% of global primary productivity in the oceans. Their abundance in aquatic ecosystems is such that they have on different occasions been described as the insects, the weeds, or the cancer cells of the ocean. In contrast to higher plants and green algae which derive from a primary endosymbiosis, diatoms are now believed to originate from a serial secondary endosymbiosis involving both green and red algae and a heterotrophic exosymbiont host. As a consequence of their dynamic evolutionary history, they appear to have red algal-derived chloroplasts empowered largely by green algal proteins, working alongside mitochondria derived from the non-photosynthetic exosymbiont. This review will discuss the evidence for such an unusual assemblage of organelles in diatoms, and will present the evidence implying that it has enabled them with unorthodox metabolisms that may have contributed to their profound ecological success.
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Journal of Experimental Botany, Page 1 of 15
Chloroplast-mitochondria cross-talk in diatoms
Judit Prihoda
, Atsuko Tanaka
, Wilson B. M. de Paula
, John F. Allen
, Leı¨la Tirichine
and Chris Bowler
Environmental and Evolutionary Genomics Section, Institut de Biologie de l’Ecole Normale Supe
rieure (IBENS), CNRS UMR 8197
INSERM U1024, Ecole Normale Supe
rieure, Paris, 46 rue d’Ulm, 75230 Paris Cedex 05, France
School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
* To whom correspondence should be addressed. E-mail:
Received 5 October 2011; Revised 2 December 2011; Accepted 9 December 2011
Diatoms are unicellular, mainly photosynthetic, eukaryotes living within elaborate silicified cell walls and believed to
be responsible for around 40% of global primary productivity in the oceans. Their abundance in aquatic ecosystems
is such that they have on different occasions been described as the insects, the weeds, or the cancer cells of the
ocean. In contrast to higher plants and green algae which derive from a primary endosymbiosis, diatoms are now
believed to originate from a serial secondary endosymbiosis involving both green and red algae and a heterotrophic
exosymbiont host. As a consequence of their dynamic evolutionary history, they appear to have red algal-derived
chloroplasts empowered largely by green algal proteins, working alongside mitochondria derived from the non-
photosynthetic exosymbiont. This review will discuss the evidence for such an unusual assemblage of organelles in
diatoms, and will present the evidence implying that it has enabled them with unorthodox metabolisms that may
have contributed to their profound ecological success.
Key words: Alternative oxidase, chloroplast, diatom, mitochondria, secondary endosymbiosis, urea cycle.
Diatoms are eukaryotic unicellular organisms inhabiting
aquatic and other humid ecosystems. Most diatom species
are photosynthetic, and be cause they represent a signi ficant
component of both planktonic and benthic ecosystems in
the contemporary ocean they are believed to be responsible
for about 20% of primary productivity on Earth (Falkowski
et al., 1998; Field et al., 1998) and to produce around 40%
of marine organic carbon (Nelson et al., 1995). Another
well-known characteristic of diatom s is their ornate cell wall
constructed of amorphous silica, which makes the cells
heavy and susceptible to sinking when they die or when they
are grazed. By generating oxygen through photosynthesis
and sequestering atmospheric carbon into the ocean in-
terior, they are therefore likely to have influenced signif-
icantly the Earth’s atmosphere and climate (Brzezinski
et al., 2002).
The reason s underlying the evolutionary and ecological
success of diatoms are largely unknown, but the availability
of whole genome sequences from two divergent diatom
species (Armbrust et al., 2004; Bowler et al., 2008) has
provided a genetic context for exploring their evolutionary
trajectory and for predicting their biochemical potential.
Studies that have exploited these genome sequences and the
enabling resources that allow their dissection suggest un-
orthodox interactions between diatom plastids and mito-
chondria. The results have implications for understanding
the basics of diatom cell physiology and their response to
environmental change, as well as for exploiting these
organisms in applied applications such as nanotechnology
and for biofuels (Dismukes et al., 2008; Kro
ger and
Poulsen, 2008). These recent results have been reviewed
here and their potential evolutionary and metabolic
significance is discussed.
Evolutionary aspects
Based on evidence from the fossil record and from
molecular clocks, diatoms are believed to have appeared
after the Permian–Triassic mass extinctions (250 million
years ago), an d the first reliable fossils date to around
ª The Author [2012]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
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at Queen Mary, University of London on March 25, 2012 from
180 million years ago (Sims et al., 2006; Kooistra et al.,
2007; Armbrust, 2009). The two major diatom divisions
that are now recognized based on their symmetry, the
centrics (radially symmetrical) and pennates (bilaterally
symmetrical), diverged during the Cretaceous around 90
million years ago and their populations expanded and
diversified enormously around 30 million years ago at the
Eocene–Oligocene boundary, from where we can trace back
the appearance of many modern diatoms (Bowler et al.,
2010). It is now believed that there may be as many as
100 000 extant species, comprising two groups each within
the centric (radial and bi/multipolar) and pennate (raphid
and araphid) lineages. But from where did diatoms arise
and what do we know about their origins?
The secondary endosymbiont hypothesis of diatom
Plants and algae are believed to have originated through
a process where a non-photosynthetic eukaryote engulfed (or
was invaded by) a cyanobacterium, thereby acquiring
a photosynthetic apparatus that became housed within an
organelle surrounded by two membranes (Fig. 1). This event,
known as a primary endosymbiosis, is believed to have
occurred around 1.8 billion years ago, and it conveniently
explains the monophyletic origins of all plastids within
eukaryotic cells (Gould et al.,2008; Keeling, 2010). The host
cell can be termed the exosymbiont (Hamm and Smetacek,
2007), whereas the cyanobacterium in known as an endo-
symbiont. This initial endosymbiotic event gave rise to the
green and red algal lineages, as well as to the glaucophytes
(Fig. 1). Land plants arose following the evolution of
multicellularity within the green algal lineage.
The endosymbiotic process also involved the transfer of
thousands of genes from the cyanobacteria l genome to the
host eukaryotic nucleus (Martin et al.,2002), whereas the
genome within the chloroplast (a photosynthetic plastid)
became reduced to only a few hundred genes. In contrast to
the small genome of the chloroplast, this organelle contains
a large number of plastid proteins, probably between 2000 and
5000 (vanWijkandBaginsky,2011), the majority of which
are nuclear encoded, synthesizedoncytosolicribosomes,and
targeted to the plastid using amino-terminal targeting peptides
(Soll and Schleiff, 2004). Interestingly, dual protein targeting
to mitochondria and plastids is also quite a common phenom-
enon and, in some cases, these plastid proteins do not even
have targeting sequences (Jarv is, 2004; Jarvis and Robinson,
2004; Millar et al.,2006; Radhamony and Theg, 2006)
By contrast with the evolution of land plants and green
algae, the evolutionary history of diatoms is believed to
have followed a rather different path (Fig. 1). Specifically,
diatoms and their relatives are thought to have originated
when a second non-photosynthetic eukaryote engulfed
a photosynthetic eukaryote about 1.4 billion years ago
(Yoon et al., 2002). This is known as a secondary endosym-
biosis, and a single event is currently believed to be at the
origin of the whole Chromalveolata supergroup, which
comprises heterokonts (also known as stramenopiles, and
to which the diatoms belong), alveolates, haptophytes,
cryptophytes, and perhaps also rhizaria (Bhattacharya
Fig. 1. Schematic representation of the secondary endosymbiont hypothesis of diatom evolution. The upper panel shows the
conventional primary endosymbiosis at the origin of green and red algae and Glaucophytes. By contrast, diatoms are now believed to be
derived from a serial secondary endosymbiosis (lower panel) in which a heterotrophic host cell (exosymbiont) combined first with a green
alga and subsequently with a red alga (Moustafa et al., 2009). EGT, endosymbiotic gene transfer.
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et al., 2007; Cavalier-Smith, 1999; Keeling, 2010). Impor-
tant evidence for the secondary endosymbiosis is that the
plastids in many of these organisms are surrounded by four
rather than two membranes, corresponding to (from outside
to inside) the exosymbiont endomembrane, the plasma
membrane of the engulfed alga, and the two membranes of
the primary plastid. These nested cellular compartments
provide clues about the evolutionary history of these
organisms. For example, a vestige of the nuclear genome of
the endosymbiont (known as a nucleomorph) can some-
times be found between the inner two and the outer two
membranes surrounding the plastid (Ben Ali et al., 2001),
albeit not in diatoms. The nucleomorphs of cryptophytes
still encode 18S rRNA, and it has been shown that the
endosymbiont was most probably related to red algae (Van
de Peer et al., 1996). By extension, it would therefore appear
that the chromalveolate secondary en dosymbiosis involved
a red alga (Yoon et al., 2002). This further supports earlier
molecular phylogenetic studies, which place the plastids of
heterokonts as close relatives to those of red algae and
cryptophytes (as reviewed in Keeling, 2010; Green, 2011).
Diatom chloroplasts are typical secondary plastids sur-
rounded by four membranes (Fig. 2). The outer envelope is
termed the chloroplast endoplasmic reticulum (CER) and is
continuous with the nuclear envelope. Diatom plastids are also
characterized by a ‘girdle lamella’ that runs in parallel with the
four membranes that surround them (Fig. 2D). The girdle
lamella and the thylakoids are always found in bundles of
three but are never partitioned between stacked and unstacked
grana, as in some green algae and higher plants. A further
contrast with green algae is that all the algae within the
chromalveolate groupin g have chlorophyll a and c,whereas
green algae contain chlorophyll a and b (Delwiche, 1999;
Green, 2011). Furthermore, diatoms do not use state 1/state 2
transitions to balance absorbed excitation energy distribution
between Photosystem I (PSI) and Photosystem II (PSII)
(Owens, 1986), most likely because the photosystems are not
distributed between stacked and unstacked grana (Pfannsch-
midt et al., 2009). The ultrastructure of red algal thylakoids is
the simplest among eukaryotes (
48292/Algae.html’’>Algae-Prokaryotic Algae), with single thy-
lakoids that are not stacked. They use chlorophyll a and
phycobilins, and the phycobilisomes that house them are
attached to the stromal surface of the thylakoids (http:// lgae.html’’>Algae-Prokaryotic
Algae). Phycobiliproteins have been lost in diatoms but they
are still present in cryptophytes (Delwiche et al., 1995; Gibbs,
1981). Evidence for the derivation of diatoms and other
chlorophyll c-containing algae from the red lineage is therefore
somewhat anecdotal, but molecular phylogenetic analyses are
consistent with this hypothesis (Van de Peer et al., 1996;
Delwiche et al., 1995).
The light-harvesting peripheral antennae of the diatom
photosynthetic machinery are composed of fucoxanthin
chlorophyll a/c binding proteins (FCPs) structured into
oligomeric complexes (Bu
chel, 2003; Beer et al.,2006). These
complexes possess a large number of carotenoids only found
within photosynthetic heterokonts and haptophytes, such as
fucoxanthin (Fx), as well as diadinoxanthin (Ddx) and
diatoxanthin (Dtx) pigments that are involved in their unique
xanthophyll cycle (Lepetit et al.,2011). Besides the FCP
proteins, three other antenna protein families have been
found: Lhcf (the classical light-harvesting proteins), Lhcr (red
algal-related proteins), and the less abundant (Westermann
and Rhiel, 2005) Lhcx proteins (previously known as
LHCSR and Li818) (Eppard et al.,2000; Green, 2007;
Koziol et al.,2007; Lepetit et al.,2011). Diatoms also have
a PSI-specific antenna (Veith and Bu
chel, 2007; Veith et al.,
2009; Lepetit et al.,2011) that has a higher amount of Ddx
and Fx than the main FCP proteins (Lepetit et al.,2011).
Diatom thylakoid membranes contain the same classes of
lipids as higher plants and green algae (Goss and Wilhelm,
2009; Lepetit et al., 2011), although in contrast to plants
and green algae which have mostly neutral galactolipids,
diatom thylakoid galactolipids are mainly negatively
Fig. 2. Chloroplasts and mitochondria are closely juxtaposed
inside diatom cells. The images show the spatial relationship
between mitochondria and chloroplasts in P. tricornutum. (A–C)
Longitudinal sections and (D–G) vertical sections of two daughter
cells just after cell division. (A) Mitochondria (mt), chloroplast (ch),
and nucleus (n) are closely located; (B) the mitochondrion lies
alongside the chloroplast and (C) partly attaches to the chloroplast.
(D, E) The girdle lamella is continuous around the end of
chloroplast just under the chloroplast envelope (D, arrow). Both
daughter cells have mitochondria with clear tubular cristae
(E, arrow). (E) Mitochondrial membrane and chloroplast outer
membrane (chloroplast endoplasmic reticulum; CER) are not
completely attached. (F, G) Longitudinal sections with nucleus and
partial chloroplast. The CER continues to the nuclear membrane
(G, arrow). Scale bars: 0.5 lm (A, B, D, F) and 0.1 lm (C, E, G).
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charged (Vieler et al., 2007a, b; Goss et al., 2009; Lepetit
et al., 2011). In higher plants the most abundant galactoli-
pids are MGDG (monogalactosyldiacylglycerol), while in
diatoms the main lipid is the anionic SQDG (sulphoquino-
vosyldiacylglycerol) (Goss et al., 2009). Another difference
is that diatom thylakoid membranes contain the phospho-
lipid phosphatidylcholine (PC), while in plants this lipid is
not present in thylakoids (Lepetit et al., 2011).
As mentioned above, only about 2% of the original
plastid genome is retained in the chloroplast, while most of
the other genes have either been lost or have become
incorporated into the nucleus of the host. This was pre-
sumably advantageous for avoiding Muller’s ratchet, the
accumulation of deleterious mutations when population
sizes are small (Muller, 1932). Furthermore, the plastid
genome may be subject to higher mutation rates due to
increased oxidat ive stress as a result of the high levels of
reduced oxygen species (ROS) that can be generated from
the photo synthetic apparatus (Allen and Raven, 1996 ). It
has been argued that the large number of plastid genomes
per cell favours net gene transfer from plastid to the
nucleus, simply by a ‘diffusion gradient’ argument, com-
pared with the less probable transfer from a single co py
nuclear genome to multiple copies of the plastid genome
(Martin, 2003). These factors perhaps combined to provide
a selective advantage to moving genes from the plastid to
the nucleus. Although only a small percentage of the
original plastid genome remained, the fact that it did remain
signifies that it has an important role. According to
Puthiyaveetil et al. (2010) plastid-encoded genes are re-
quired for redox regulation within the organelle, while the
genes encoding ribosomal proteins and RNAs support the
primary redox regulatory control of photosynthesis (Allen,
2003; Puthiyaveetil et al., 2010).
Like other eukaryotes, diatom cells contain mitochondria
evolved from a single primary endosymbiotic event involving
an a-proteobacterium. No trace of mitochondria remain in
the secondary endosymbiont, and so it is believed that diatom
mitochondria are derived from the exosymbiont. In support
of this, heterokont, haptophyte, and dinoflagellate mitochon-
dria are all characterized by tubular cristae, as in animals,
whereas green and red algae (as well as cryptophytes) all
contain flattened cristae (
Algae.html’’>Algae-Prokaryotic Algae). The functional sig-
nificance of this difference is unclear (Frey and Mannella,
2000), but given that the cristae contain and organize the
respiratory electron transport chain and the proton-motive
ATP synthase of oxidative phosphorylation, the issue
warrants further investigation.
Following secondary endosymbiosis, the different
genomes of the exosymbiont and endosymbiont are pre-
dicted to have combined to form a novel and unique set of
genes (Falkowski et al., 2004) dispersed within the nuclear,
mitochondrial, and chloroplast genomes (Fig. 1). Consider-
ing that such a fusion is thought to involve two nuclear
genomes, two mitochondrial genomes, and one plastid
genome, the process must have been highly complex and,
because it happened so long ago, reconstructing its history
has proven extremely difficult. The following section will
address how the availability of completed diatom genome
sequences has been able to contribute to addressing this
fundamental question.
Genome sequences and the secondary endosymbiosis
The first diatom species whose genome was fully sequenced,
Thalassiosira pseudonana, belongs to the bi/multipolar
centric family of diatoms known as Mediophyceae.
Armbrust et al. (2004) found that this diatom has a 34 Mb
nuclear genome, a 129 kb plastid genome, and a 44 kb
mitochondrial genome. In total, 11 242 protein-coding
genes are predicted in the nuclear genome, 127 in the
plastid, and 34 genes in the mitochondria.
Following the centric diatom T. pseudonana, a raphid
pennate diatom species (Bacillariophyceae) Phaeodactylum
tricornutum was sequenced, revealing more information
about diatom adaptat ion to the varied aquatic environ-
ments which they inhabit. P. tricornutum has a smaller
nuclear genome, of about 27.4 Mb, a chloroplast genome of
117.4 kb, and a mitochondrial genome of 77.4 kb, predicted
to have 10 402, 130, and 34 genes, respectively (Oudot-Le
Secq et al., 2007; Bowler et al., 2008; Oudot-Le Secq and
Green, 2011).
The two aforementioned groups of diatoms diverged
about 90 million years ago (Sims et al., 2006; Kooistra
et al., 2007), and so significant differences are expected
between their genomes. It is interesting to note that the
divergence is so large between these two types of diatoms
that it can be compared with the difference between Homo
sapiens and Takifugu rubripes (puffer fish), which diverged
around 550 million years ago. This comparison indicates
a high rate of diatom gene modification, losses and gains of
genes and introns, and gene exchanges with other organ-
isms, all of which probably contributed to high diatom
diversification rates . Notwithstanding, about 57% of genes
are shared between T. pseudonana and P. tricornutum.
The secon dary endosymbio nt hypothesis posits that
diatoms acquired genes from both non-photosynthetic
and photosynthetic ancestors, therefore gaining a chimeric
genome from a varied origin. Further studi es of the
T. pseudonana genome provided strong support for this
hypothesis. Of particular interest, in a comparison with
animals (Mus musculus) and green plants (Arabidopsis
thaliana), 806 proteins homologous to sequences found only
in the animal were discove red, whereas 865 protein s
matched sequences found in the plant but not in the mouse.
It was therefore proposed that these two sets of sequences
were derived from the exo- and endosymbionts at the origin
of diatom evolution (and by extrapolation all chromalveo-
lates) (Armbrust et al., 2004). In spite of the large number
of sequences that can be traced with confidence to
Arabidopsis, it was noted with surprise that the T. pseudo-
nana genome contained relatively few genes of red algal
origin. At the time this was attributed to the poor taxon
sampling of the red lineage, because the only whole geno me
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sequence available was that of Cyanidioschyzon merolae
(Matsuzaki et al., 2004), whi ch is not particularly represen-
tative of the red lineage because it is known to be highly
derived due to its extremophile lifestyle. In a subsequent
comparison of the P. tricornutum genome with the red algal
genome, Bowler et al. found just 171 genes of red algal
origin, 108 of which were also found in T. pseudonana
(Bowler et al., 2008). A high number of these, specifically 74
genes (43%), encode plastid-related functions, and 11 genes
are also shared with oomycetes, non-photosynthetic hetero-
konts which are also believed to be derived from the same
secondary endosymbiotic event with a red alga, but which
subsequently lost photosynthesis. By contrast, when the
diatom sequences are compared with those in a cyanobacte-
rium (Nostoc sp. PCC 7120), numerous proteins are found
to be shared with photosynthetic clusters found in both
green and red lineages.
As mentioned earlier, diatom plastids are surrounded by
four membranes, and the outer one is contiguous with the
endoplasmic reticulum (ER) that surrounds the nucleus
(Fig. 2G). This suggests that diatoms must have developed
a special way of transferring proteins from the ER into the
chloroplast. Analysis of nuclear-encoded chloroplast tar-
geted proteins in diatoms indeed shows the presence of ER
signal sequences in addition to the more conventional
chloroplast targeting transit peptides found in plants
(Kilian and Kroth, 2005). The functionality of these dual
targeting sequences has also been verified experimentally to
the extent that it is now possible to predict whether
a nuclear-encoded protein in diatoms is likely to be plastid
targeted (Kroth, 2002).
The incorrectly folded proteins in the ER that cannot be
repaired must be relocated into the cytosol for degradation.
This process occurs via the ER-associated degradation of
lumenal proteins (ERAD-L) system (Nakatsukasa and
Brodsky, 2008). Sommer et al. (2007) suggested that all
chromalveolates with four membranes, including diatoms,
have an ERAD-derived system (called SELMA; symbiomt-
specific ERAD-like machinery) that is located in the plastid/
periplastid thylakoid of the endosymbiont-derived plastid,
and that it coexists with the host ERAD system of the same
cell. They also proposed that this machinery serves as
a transport channel for plastid/periplastid compartment
(PPC) preproteins that are encoded in the nucleus to be
transported from the ER into the periplastid space. The Der1
protein is considered to be the central component of the
ERAD-L system (Knop et al.,1996). In chromalveolates
with four membrane-bound plastids, including P. tricornu-
tum, two host-specific Der1 (hDer1-1 and hDer1-2), and two
symbiont-specific Der1 (sDer1-1 and sDer1-2) proteins were
found (Sommer et al.,2007). Hempel et al. (2009) confirmed
that the symbiont-specific Der1 proteins are indeed located
within the second outermost membrane of the plastid. This
protein complex interacts with the periplastid-targeted
proteins, but not with the stromal-targeted ones, indicating
that the Der1 system could have an important role in
discriminating between the preproteins that are transported
into the PPC across the two outermost membranes.
Components of the ERAD-specific ubiquitination ma-
chinery in diatoms include Der1-1, Der1-2, Cdc48, Ufd1, E1
(Uba1), E2 (Uba2), ubiquitin (Sommer et al., 2007; Hempel
et al., 2009), as well as ptE3P (the likely ubiquitin ligase)
and ptDUP (the de-ubiquitinase) (Hempel et al., 2010).
While Felsner et al. (2010) confirmed that all SELMA
components can be traced back to their red algal origins,
their data indicate that the SELMA system has apparently
not arisen in a single endosymbiotic event, but rather
through serial secondary endosymbioses with rhodophytes,
thus contradicting the chromalveolate hypothesis. On the
other hand, based on sequence analysis of a newly discov-
ered protein related to Toc75 from the red alga C. merolae,
and the discovery that all Omp85-like proteins from
chromalveolates form an individual clade related to Toc75
from C. merolae, Bullmann et al. (2010) found consistency
with the hypothesis that the plastids of chromalveolates
share a common evolutionary origin.
In summary, the collected data suggests multiple gene
transfers from the endo- and exosymbiont, from one
genome to another, such as genes transferred from the algal
plastid to the algal nucleus, and then incorporated into the
host, in this case diatom, nucleus. While the genome
sequences theref ore provided support for the secondary
endosymbiosis theory, it has been difficult to establish using
phylogeny whet her the photosynthetic endosymbiont was of
green or red algal origi n. However, a re-examination of the
gene content of diatom plastid genomes does indicate more
similarity with red plastids than with green plastids (Fig. 3)
(Allen JF et al., 2011). Furthermore, Chloroplast Sensor
Kinase (CSK), a nuclear-encoded bacterial-type sensor
kinase that plays a major role in transc riptional control of
plastid genes, is of clear red algal origin in diatoms
(Puthiyaveetil et al., 2010).
Diatom mitochondria are considered to have been de-
rived from the exosymbiont. The mitoch ondrial genomes
from both diatoms (as well as a third species Synedra acus)
were found to display almost the same gene composition as
in the mitochondrial genomes of haptophytes and crypto-
phytes (Oudot-Le Secq and Green, 2011). While there is
evidence of significant gene transfer from the plastid and
mitochondrial genomes to the nuc leus in higher plants
(Martin, 2003) and dinoflagellates (Hackett et al., 2004), in
the diatoms no proof of such gene transfers from the
mitochondria to the nucleus were found. There are no red
algal mitochondrial genes found in the T. pseudonana or
P. tricornutum nuclear genomes, which strengthens the
conclusion that the red algal mitochondrion was lost and
that only the exosymbiont mitochondria was retained
during the evolutionary process. The general properties of
diatom mitochondrial genomes a re indeed more similar to
animal than to plant mitochondrial genomes (Oudot-Le
Secq and Green, 2011). Furthermore, recent studies have
highlighted the novel metabolisms within diatom mitochon-
dria, that are quite unlike those of green algae and higher
plants, such as a urea cycle (see later).
Besides providing evidence for the ancient secondary
endosymbiotic event at the origin of diatoms (and perhaps
Chloroplast mitochondria cross-talk in diatoms | 5of15
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all chromalveolates), it was found that diatom geno mes also
contain a large number of genes (587 in P. tricornutum)
proposed to be derived from bacteria by horizontal gene
transfer (Bowler et al.,2008). These genes were not found in
any other photosynthetic eukaryotes (other than other hetero-
konts), and it is believed that they have provided diatoms with
increased flexibility in their regulatory and metabolic networks
that may have contributed to their evolutionary success. For
example, diatoms probably acquired genes enabling novel
metabolic offshoots from the urea cycle through such pro-
cesses (see later). Equally striking are a bacterial glutamine
synthetase targeted to the mitochondria, and a bacterial
NAD(P)H nitrite reductase (Allen et al.,2006). Such observa-
tions hint that nitrogen metabolism has been configured in
a unique manner in diatoms.
Evidence for a third partner in the secondary
As mentioned above, althoug h multiple evidence indicates
that diatoms and other chromalveolates gained a plastid of
red algal origin through secondary endosymbiosis, the
initial analysis of whole genome sequences did not provide
incontrovertible evidence about the origin of diatom
plastids (Armbrust et al., 2004; Bowler et al., 2008). The
subject was reinvestigated by Moustafa et al. (2009), who
used a phylogenomic analysis pipeline based on the
wholesale generation of phylogenetic trees from each di-
atom protein sequence encoded by the two diatom nuclear
genomes. Using this approach, in T. pseudonana 2423 genes
were identified as being of red or green algal origin, while in
Fig. 3. Venn diagram of gene content of the plastid genomes of Arabidopsis thaliana, the green alga Ostreococcus tauri, the red alga
Porphyra purpurea, the brown macroalga Ectocarpus siliculosus, and P. tricornutum. Larger overlap between the diatom and
Arabidopsis than between the diatom and Ostreococcus is likely to be an effect of genome reduction in Ostreococcus.
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P. tricornut um 2533 such genes were found. Subsequent
analysis revealed that more than 70% of these sequences
were more sim ilar to green algal proteins than to red algal
proteins, contrasting with the prediction of the chromalveo-
late hypothesis (Cavalier-Smith, 1999). The >1700 identified
‘green’ genes comprise arou nd 16% of the diatom genome,
in con trast to the ‘red’ genes which only represented around
3% of the nuclear genome. Some green proteins known to
be encoded in the nucleus of green algae and higher plants,
such as naphthoate synthase, haem oxygenase, pyruvate
dehydrogenase, and GUN4, can similarly be found in the
diatom nucleus even though these genes are encoded in the
plastids of red algae. Rather than being of red algal origin
and present in the plastid genome, they are therefore absent
in the red algal-derived plastid of diatoms and are instead of
green algal origin and encoded in the nucleus. Another
impressive example of green gene acquisition is phytoene
desaturase, which catalyses one of the early steps in
carotenoid biosynt hesis, as well as many other genes
encoding enzymes of this pathway. This is of particular
significance because diatoms are known to contain a di-
atom-specific xanthophyll cycle involved in photoprotection
(Lohr and Wilhelm, 1999; Coesel et al., 2008). It would
therefore appear that this innovation was derived from
genes originally encoded in green algae.
The results from Frommolt et al. (2008) indicated for the
first time that the cryptic green alga might have been
a Prasinophyte, a primitive group of green algae. The
conclusion from the phylogenomic studies of Moustafa
et al. (2009) was, therefore, that a cryptic green alga was
involved in the secondary endosymbiotic event, in addition
to the red alga (Fig. 1). The fact that diatom plastids have
clear biochemical and genetic affiliations with red
algal plastids was interpreted as meaning that the green
algal endosymbiont was acquired first, and that the red alga
followed in a serial secondary endoymbiotic event. In such
a scenario, the data would imply that the genes derived
from the green algal nucleus were largely retained and that
the majority of red algal nuclear genes were lost because
they were not required due to the availability of green algal
nucleus-derived orthologues already present in the host
nucleus. By contrast, the data in Fig. 3 indicate that the
genes already present within the plastid of the red algal
endosymbiont were largely retained. The same analytical
pipeline, when used to interrogate the genome sequences
available from other heterokonts and alveolates (as well as
the haptophyte Emiliani a huxleyi, whose phylogeny is
uncertain with respect to the heterokonts and alveolates),
indicated similar overall patterns, suggesting that these
same serial secondary endosymbiotic events were at the
origin of all the chromalveolat es (Moustafa et al., 2009).
Such data therefore confirm the phylogenetic affiliation of
the chromalveolates (Steinko
tter et al., 1994; Petersen et al.,
2006; Frommolt et al., 2008; Reyes-Prieto et al., 2008),
although the implication of a third partner in the secondary
endosymbiotic event was a major surprise.
A further question that was addressed by Moustafa et al.
(2009) was what kind of green alga was party to the serial
secondary endosymbiosis. Among the green lineages of the
Viridiplantae, several unicellular members have been studied
at the whole genome level, in particular within the Chlor-
ophyta and Streptophyta clades (Tirichine and Bowler,
2011). The green genes from the two diatoms were found to
share most similarity with those present in prasinophytes,
which lie within the Streptophyta and constitute a primitive
group of green algae most commonly found in marine
environments (Derelle et al.,2006). It was therefore proposed
that a related alga was most likely to have been the source of
the green genes in diatoms.
In summary then, the evolutionary trajectory of diatoms
has brought together a highly unorthodox combination of
genes derived by endosymbiotic gene transfer from two
secondary endosymbionts to the exosymbiont nucleus, as
well as by horizontal gene transfer that permitted numerous
additional acquisitions from bacteria and Archaea. Con-
versely, exosymbiont-derived mitochondria gained the op-
portunity to work alongside a red-algal-derived plastid that
was powered largely by green algal genes. It can therefore
be predicted that diatoms display novel biochemical path-
ways that drive physiological processes that are unknown in
plants, animals, and fungi. The following section reveals
that this is indeed the case.
Biochemical aspects
Interorganellar co-ordination of metabolism has been
studied in green algae and higher plants, for example,
regarding the exchange of metabolites between chloroplasts,
peroxisomes, and mitochondria during photorespiration
(Raghavendra and Padmasree, 2003; Noctor et al., 2007;
Parker et al., 2008). However, the preceding discussion has
presented evidence that diatom chloroplasts and mitochon-
dria both have unusual characteristics with respect to those
in better studied experimental organisms. An additional
curiosity is that these two organelles are often found in very
close proximity within the diatom cell (Fig. 2A–E). This
juxtaposition suggests some important metabolic interac-
tions that perhaps go beyond those in the green lineage.
Some possibilities are discussed below.
Mitochondrial alternative oxidase as a valve for
electrons from the chloroplast
Photosynthesis occurs by the light-dependent generation of
energy in the chloroplast through a process known as linear
electron flow (LEF), where electrons are transferred from
water to NADP via three major complexes: Photosystem II
(PS II), the cytochrome b
f complex (cyt b
f) and Photosys-
tem I (PS I). The produced NADPH is used in the Calvin–
Benson–Bassham cycle, where carbon dioxide is fixed to
produce organic compounds. Optimized functioning of
photosynthesis requires fine-tuning between conversion of
light into chemical energy as NADPH and ATP, and its use
by metabolic reactions such as carbon fixation. The
metabolic demand for ATP and NADPH varies under
Chloroplast mitochondria cross-talk in diatoms | 7of15
at Queen Mary, University of London on March 25, 2012 from
different physiological and environmental conditions, so
their relative abundances have to be finely controlled. In
green algae and higher plants , the water–water cycle
(WWC; also known as chlororespiration or pseudo-cyclic
electron flow) and cyclic electron flow around PS I (CEF)
play physiologically important roles in both the regulation
of photosynthesis and the alleviation of photoinhibition by
consuming excess electrons and maintaining an appropriate
NADPH/ATP ratio (Grossman et al. , 2010; Wilhelm and
Selmar, 2011). In Arabidopsis and Chlamydomonas, CEF
appears to be particularly important for balancing light
absorption with carbon fixation, whereas in cyanobacteria
and the prasinophyte Ostreococcus WWC appears to
dominate, mediated by the plastoquinol terminal oxidase
(PTOX) (Finazzi et al., 2010).
In diatoms, xanthophyll-dependent non-photochemical
quenching appears to have a prominent role in managing
the flow of photosynthetic electrons, whereas alternative
pathways involved in the diversion of electrons to other
reactions such as CEF and WWC do not appear to be
particularly active (Wilhelm et al., 2006; Lepetit et al. ,
2011). Furthermore, oceanic diatoms contain constitutively
low levels of iron-rich PS I complexes and some diatoms
appear to replace the iron-rich cytochrome c
with the
copper-containing plastocyanin in the photosynthetic elec-
tron transport chain during conditions of chronic iron
limitation (Strzepek and Harrison, 2004; Peers and Price,
2006). Some diatoms are therefore well adapted to growing
at the low iron concentrations that persist in offshore
surface waters. Their rates of carbon dioxide fixation per
unit of iron used are much higher than in their coastal
counterparts, thus reducing their demand for iron relative
to the supply (Strzepek and Harrison, 2004).
In line with these studies that indicate physiological adjust-
ments of diatom photosynthesis in response to environmental
conditions, it was proposed recently that in iron-limiting
conditions diatoms employ an additional mechanism that
reroutes photosynthetic electrons towards the respiratory
chain within their mitochondria (Allen et al., 2008). It was
further shown in the same study that mitochondrial alterna-
tive oxidase (AOX) activity and mRNA levels were signifi-
cantly higher under iron-depleted conditions, implying that
the mitochondria are a major sink for reducing equivalents in
diatoms (Allen et al.,2008). AOX has been well studied in
plants, where it acts as an alternative to the cytochrome
pathway to transfer electrons from the respiratory chain to
molecular oxygen (Van Aken et al., 2009a). Although it
cannot generate ATP like the conventional respiratory
terminal oxidase, AOX plays an important role as an electron
sink for excess reductants diverted from photosynthesis during
conditions of stress. It therefore appears to have an equivalent
function in diatoms during iron limitation. However, its role
as an electron sink appears to be more important because
other mechanisms that are operative in plants and green algae,
such as CEF and WWC, appear to be insignificant in diatoms
(Wilhelm et al., 2006; Lepetit et al., 2011).
The rerouting of electrons during iron-limiting conditions
suggests cross-talk between mitochondria and chloroplasts
in diatoms in order to regulate the redox balance of the cell
and an efficient and appropriate allocation of essential
resources that adjusts cellular metabolism. The flow of
electrons might be directed into mitochondria through the
malate–aspartate shuttle or by as yet unidentified trans-
porters (see later). By analogy with higher plants (Yoshida
et al., 2007), through this mechanism the high levels of
AOX would be expected to act as an electron sink for excess
electrons from photosynthetic LEF, thus also decreasing
reactive ox ygen species (ROS) accumulation. In conditions
such as iron limitation where photosynthesis is dramatically
reduced, besides the rerouting of excess reductant to AOX
which evacuates ROS, P. tricornutum appears able to cope
with sustained illumination through the action of special-
ized light-harvesting complex (LHC ) proteins known as
LHCX (Bailleul et al., 2010). Interestingly, one of the
isoforms of LHCX, known as LHCX1, appears to operate
in normal conditions to optimize photosynthesis further
(Bailleul et al., 2010).
AOX is widely represented among most kingdoms of life,
including animals (McDonald, 2009)(Fig. 4). The
P. tricornutum genome encodes one AOX protein whereas
T. pseudonana has two copies. The proteins from both
diatoms contain the redox-sensitive residues that are the
targets of thioredoxin regulation, as in AOX proteins from all
other organisms examined to date (Van Aken et al., 2009b).
In addition, the diatom AOX sequences contain calmodulin-
like calcium-binding motifs known as EF hands at their
amino termini, suggesting a calcium-mediated regulation of
AOX activity. This is further supported by the up-regulation
of calcium-responsive genes during conditions in which AOX
activity is increased, such as genes encoding a heat shock
protein (HSP), and proteins such as myosin, gelsolin, copine,
regulation and modification (Allen et al.,2008). EF hand
domains appear to be specific to the diatom AOX proteins,
and so it will be well worth addressing whether they impart
some diatom-specific roles to AOX function.
The diatom urea cycle
One of the first surprises to emerge from the seq uencing of
diatom genomes was the presence of an apparently fully
functional urea cycle (Fig. 5)(Armbrust et al., 2004).
Diatoms were, in fact, the first photosynthetic organisms to
contain all the genes for such a cycle, and so it was of great
interest to understand its role in the context of a photosyn-
thetic cell. Prior to its discovery in diatoms, it was believed
to have been a metazoan innovation, invented to cope with
the high amino acid diets that accompanied their adaptation
to life on land by removing excess nitrogen from the body
in the form of urea, a relatively innocuous organic
nitrogenous compound. Its discovery in diatoms overturned
this theory and pushed back the origins of the urea cycle by
more than a billion years (Armbrust et al., 2004; Bowler
et al., 2008; Allen AE et al., 2011).
The first committed step of the urea cycle is driven by
a mitochondrially-localized carbamoyl phosphate synthase,
8of15 | Prihoda et al.
at Queen Mary, University of London on March 25, 2012 from
Fig. 4. Phylogenetic tree of AOX proteins. The phylogenetic tree was constructed with the full-length protein using the Neighbor–Joining
method with the Mega 4 software (Tamura et al., 2007). C reinh is Chlamydomonas reinhardtii and C incerta is Chlamydomonas incerta.
Chloroplast mitochondria cross-talk in diatoms | 9of15
at Queen Mary, University of London on March 25, 2012 from
now known as unCPS (Allen AE et al.,2011), which provides
the substrate carbamoyl phosp hate for the cycle (Fig. 5).
Although absent in plants, in metazoans it plays an important
role in vertebrate metabolic adaptations (Anderson, 1980;
Guppy, 1986; Mommsen and Walsh, 1989; Hong et al., 1994;
Lawson et al.,1996; Holden et al.,1999). Phylogenetic
analysis of the unCPS sequences from T. pseudonana and
P. tricornutum revealed their similarity with the metazoan
enzymes and confirmed their absence in red and green algae,
but indicated that other heterokont genomes, as well as the
haptophyte E. huxleyi, all contained a similar sequence (Allen
AE et al.,2011). It was therefore proposed that the enzyme
was most likely to be of exosymbiont origin, but that the gene
had undergone several duplications during its evolutionary
history that can explain its neo-functionalization.
Other components of the diatom urea cycle closely
resemble those found in the metazoan cycle from a func-
tional point of view, although the genes do not all appear to
be of exosym biont origin (Fig. 5). Furthermore, diatom
genomes appear to encode several enzymes acquired by
horizontal gene transfer from bacteria that potentially can
greatly expand the functionality of the cycle with respect to
its animal co unterpart. Of particular interest, both diatom
nuclear genomes encode an ornit hine cyclodeaminase
(OCD), predicted to convert ornithine into proline, as well
as agmatinase (Agm), which has been predicted to link the
urea cycle to polyamine biosynthesis (Allen et al., 2006).
Hints about the role of the urea cycle in diatoms have been
derived from analysis of the expression of integral and
peripheral urea cycle genes (Allen AE et al., 2011). On the
one hand, studies of their expression patterns in cultures
grown in different nitrogen sources indicated that their
expression is co-ordinately regulated. On the other hand,
examination of their expression over an even broader range
of growth conditions (Maheswari et al.,2010) indicated that
they were particularly expressed during conditions that favour
anabolic rather than catabolic metabolism. These findings
were interpreted as suggesting that the urea cycle may be
principally involved in the biosynthesis of organic nitrogenous
compounds rather than their breakdown, as is the case in
animals (Lee et al.,2000; Esteban-Pretel et al.,2010).
To determine the role of the urea cycle in diatoms better,
Allen et al. generated unCPS knockdown mutants in
P. tricornutum (Allen AE et al.,2011). It was found that with
respect to wild-type cells these lines displayed a delayed
recovery following conditions of nitrogen limitation, mir-
rored also in the delayed accumulation of metabolites derived
from the urea cycle. Further analysis of metabolite profiles in
these lines provided clues as to the metabolic roles of the
cycle in diatoms, and it has been proposed to be of particular
Fig. 5. The urea cycle in diatoms. Scheme showing the principal components of the diatom urea cycle. Genes encoding enzymes of
bacterial origin are indicated in purple, metazoan origin in black, and red algal origin in red. Those with uncertain affiliation are shaded in
blue. Abbreviations: Agm, agmatinase; Arg, arginase; ASL, argininosuccinate lyase; AsuS, argininosuccinate synthase; CK, carbamate
kinase; OCD, ornithine cyclodeaminase; OdC, ornithine decarboxylase; OTC, ornithine transcarbamylase; unCPS, mitochondrial
carbamoyl phosphate synthase III; Ure, urease.
10 of 15 | Prihoda et al.
at Queen Mary, University of London on March 25, 2012 from
importance as a recycling hub for anaplerotic carbon fixation
into organic nitrogen compounds. Particularly important
offshoots of the pathway appear to go to proline and
polyamines. Proline is the major osmolyte in diatoms (Krell
et al.,2007), and proline-rich proteins are common cell wall
structural components. By contrast, arginine and ornithine
are precursors for polyamines such as spermidine and
putrescine (Kro
ger and Poulsen, 2008), which are, in turn,
converted into highly modified long-chain polyamines that
are used for building the diatom-specific siliceous cell walls
ger and Poulsen, 2008). Both these offshoots of the urea
cycle are probably enabled by bacterial genes (Fig. 5). Hence,
the diatom urea cycle appears to play an important role in
building organic nitrogen compounds that are particularly
critical for building the unique features of diatom cells. In
addition, it was proposed that the urea cycle is connected to
glutamine synthetase/glutamate synthase (GS/GOGAT) and
to the tricarboxylic acid (TCA) cycle through the aspartate–
argininosuccinate shunt (Fig. 5). Such interactions are not
unique to diatoms, but it is of interest that the shunt provides
an additional point of integration for the urea cycle into
central metabolism. In summary, the urea cycle appears to
function in the recycling and biosynthesis of organic nitrogen
compounds, rather than for their breakdown as in animals.
Due to its positioning between the mitochondria and
cytoplasm, it is likely to be of special importance for the
exchange of nutrients between these two cellular compart-
ments. It remains to be determined whether it also exchanges
nutrients with plastids, but given their close juxtaposition
with diatom mitochondria such a possibility certainly war-
rants investigation. One hypothesis that has been proposed is
that the urea cycle may interact with photorespiratory
pathways, another is that C
-type photosynthesis may
operate in some diatoms by metabolite exchange between
chloroplasts and mitochondria (Kroth et al.,2008; Parker
et al.,2008).
Diatom nutrient transporters
For endosymbiosis to be successful, it was of the utmost
importance to the endosymbiotic organelles (both mito-
chondria and chloroplasts) to control the exchange of
metabolites between the host and themselves. Connections
between the host and the endosymbiont metabolism were
shown by Linka and Weber (2010), Weber an d Linka
(2011), and Palmieri et al. (2011), who found that the
diatom plastid triose-phosphate antiporter likely evolved
from an existing ER/Golgi metabolite translocator protein
family. This suggests that the plastid endosymbiont had
already developed an endomembrane system before it was
incorporated into the host cell (Bhattacharya et al., 2007).
Furthermore, the proposed involvement in diatoms of the
mitochondrial AOX as an electron sink for excess reductant
generated by the photosynthetic electron transport chain
(Allen et al.,2008; Finazzi et al.,2010) implies that electrons
generated in the chloroplasts must be exported to the
mitochondria using metabolic carriers. Such co-operation
between chloroplasts and mitochondria implies translocators
such as the Arabidopsis oxoglutarate malate transporter
(OMT/Dit1) and dicarboxylate transporter (DCT/Dit2)
(Linka and Weber, 2010; Weber and Linka, 2011; Palmieri
et al.,2011). Examination of the P. tricornutum genome
revealed the presence of two Dit1 proteins (Kroth et al.,
2008,Prihodaet al., unpublished results), and the Dit1 gene
was also found in other diatoms such as T. pseudonana and
Fragilariopsis cylindrus. However, neither Dit2 nor DCT
could be detected in any of the three diatom genomes
(Prihoda et al., unpublished results). This suggests that
diatom Dit1 may either replace Dit2 and DCT or that
another gene product fulfils their function. Obvious candi-
dates are the numerous triose-phosphate transporters identi-
fied in P. tricornutum (Weber and Linka, 2011). However,
the fact that diatom chloroplasts are surrounded by four
rather than two membranes means that analogies with plant
and green algal systems are only of limited value, and that
fundamental studies of putative diatom transporters are
required (Hempel et al.,2010). For each candidate trans-
porter, its localization needs to be determined to the level of
a specific membrane, and its substrate specificity needs to be
Mitochondrial transporters may be easier to define by
analogy with transporter s in mitochondria of other eukar-
yotes because of their likely orthodoxy. But although
studies in organisms from the green lineage are quite
advanced (reviewed in Palmieri et al., 2011), almost no
studies have been performed in diatoms. These transporters
are expected to work toget her with the mitochondrial inner
membrane uncoupling protein (UCP) to maintain redox
balance (Noctor et al., 2007; Linka and Weber, 2010).
Another component to consider is thioredoxin. Sequence
analysis and localization studies in P. tricornutum have
identified eight thioredoxins, three of which are localized to
the chloropl ast and one to the mitochon dria (Kroth et al.,
2008; Weber et al., 2009). By an alogy with higher plants,
the mitochondria-localized thioredoxin is likely to be
important for the redox regulation of the diatom AOX.
Diatoms have a remarkably dynamic genome derived from
multiple sources, the exosymbiont, a green and red alga, as
well as multiple bacterial genes acquired through HGT.
Although a major challenge to unravel, it is clear that this
unique combination of genes has endowed them with novel
metabolisms that may have contributed to their ecological
success in contemporary oceans. Although our knowledge
of diatom biology has expanded enormously following the
availability of two sequenced genomes, it is sobering to
recall that more than half of their genes have no known
function nor clear phylogenetic affiliation. The availability
of a wide range of tools to interrogate gene function is
a major asset to anyone interested to explore their unique
properties further, and it is truly an exciting time to open
up these organisms to molecular examination. Regarding
chloroplast–mitochondria interacti ons, key questions that
Chloroplast mitochondria cross-talk in diatoms | 11 of 15
at Queen Mary, University of London on March 25, 2012 from
need to be addressed relate to how interactions between the
photosynthetic and respiratory electron transport chains are
mediated and how they are regulated. An impor tant
component of such studies will be the analysis of nutrient
transporters on the chloroplast and mitochondrial mem-
branes. Investigation of such fundamental processes is likely
to lead to major new insights that will alter our rather
green-centric view of how photosynthetic metabolism is
integrated into a eukaryotic cell.
Research in the author’s laboratories is supported by the
Agence Nationale de la Recherche (CB) and The Lever-
hulme Trust (JFA).
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... These include a complete plastid-targeted ornithine cycle, which interacts with a complete mitochondria-targeted urea cycle in diatom amino acid metabolism and recycling (Allen et al., 2011;Nonoyama et al., 2019); a complex suite of plastid-targeted proteins involved in iron storage, acquisition, and stress tolerance (Gao et al., 2021); and elaborate CO 2 concentration and carbon metabolism systems (e.g., a complete glycolysis-gluconeogenesis pathway) not known in plant plastids (Marchand et al., 2018;Nonoyama et al., 2019). Moreover, many of these diatom-associated plastid metabolic innovations, including but not limited to central nitrogen and carbon metabolism, depend on intricate crosstalk between diatom plastids and mitochondria, which typically show close proximity to one another in diatom cells (Prihoda et al., 2012;Bailleul et al., 2015;Uwizeye et al., 2020). These plastid-related novel metabolic activities may depend on transporter innovations across the four membranes, or metabolite exchanges between organelles (Shai et al., 2016;Dorrell et al., 2017;Mix et al., 2018). ...
... The intricate metabolic connections between diatom plastids and mitochondria (Prihoda et al., 2012;Bailleul et al., 2015;Uwizeye et al., 2020) likely depend on transporters that transfer metabolites either between the two organelles, or with adjacent compartments such as the peroxisome (Shai et al., 2016;Dorrell et al., 2017;Mix et al., 2018). In order to find potential plastid transporters that may be related to this cross-talk, the average Pearson correlation coefficient values were calculated between 66 plastid transporters and 34 genes from the mitochondria genome, for which equivalent relative fold-expression change data were present in published microarray data (Oudot-Le Secq et al.; Ashworth et al., 2016). ...
... The transporter architecture of the diatom plastid may underpin its unique metabolic potential (Prihoda et al., 2012;Marchand et al., 2018;Nonoyama et al., 2019). Previously, for example, Bailleul et al. have proposed that import of mitochondrial ATP and export of plastid NADPH facilitates diatom photoacclimation to high light and post-illumination conditions (Bailleul et al., 2015). ...
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Diatoms are an important group of algae, contributing nearly 40% of total marine photosynthetic activity. However, the specific molecular agents and transporters underpinning the metabolic efficiency of the diatom plastid remain to be revealed. We performed in silico analyses of 70 predicted plastid transporters identified by genome-wide searches of Phaeodactylum tricornutum. We considered similarity with Arabidopsis thaliana plastid transporters, transcriptional co-regulation with genes encoding core plastid metabolic pathways and with genes encoded in the mitochondrial genomes, inferred evolutionary histories using single-gene phylogeny, and environmental expression trends using Tara Oceans meta-transcriptomics and meta-genomes data. Our data reveal diatoms conserve some of the ion, nucleotide and sugar plastid transporters associated with plants, such as non-specific triose phosphate transporters implicated in the transport of phosphorylated sugars, NTP/NDP and cation exchange transporters. However, our data also highlight the presence of diatom-specific transporter functions, such as carbon and amino acid transporters implicated in intricate plastid-mitochondria crosstalk events. These confirm previous observations that substrate non-specific triose phosphate transporters (TPT) may exist as principal transporters of phosphorylated sugars into and out of the diatom plastid, alongside suggesting probable agents of NTP exchange. Carbon and amino acid transport may be related to intricate metabolic plastid-mitochondria crosstalk. We additionally provide evidence from environmental meta-transcriptomic/meta- genomic data that plastid transporters may underpin diatom sensitivity to ocean warming, and identify a diatom plastid transporter (J43171) whose expression may be positively correlated with temperature.
... Diatoms are ancient organisms that appeared due to secondary endosymbiosis involving green and red algae and heterotrophic eukaryotes (Prihoda et al., 2012). As a result of endosymbiosis involving several organisms and their genomes, diatoms have inherited many of the metabolic features of their ancestors (Gruber and Haferkamp, 2019). ...
A new strain of benthic diatom Cylindrotheca fusiformis producing fucoxanthin (Fx) and polyunsaturated fatty acids (PUFAs) proved to be promising for biotechnology. Identification of Fx was carried out from the analysis of the ultraviolet and visible (UV-VIS) absorption spectra, electrospray ionization mass spectra (ESI MS) and proton magnetic resonance spectra (¹H NMR). The specific growth rate of the culture was 0.52 1/day, and the maximum productivity 1.56 g/(L × day). The accumulation rates of eicosapentaenoic and arachidonic acids in the stationary growth phase were 4.5 and 3.2 mg/(L × day), respectively, and that of Fx was 6 mg/(L × day). The accumulation of PUFAs in the biomass reached 4.68% of dry weight. The Fx content in the biomass reached 21.4 mg/g. The strain settles to the photobioreactor bottom in the absence of mixing, which property simplifies the biomass harvesting.
... The "just-in-time" strategy requires a higher FtsH:PsbA ratio than the 'warehouse' strategy. Furthermore, PSII repair continues in the dark in diatoms (Li et al. 2016), which we hypothesize is driven by tight integration of chloroplastic and mitochondrial metabolism to fuel the chloroplastic ATP requirements for FtsH, RNA transcription, and translation (Prihoda et al. 2012;Bailleul et al. 2015). This capacity for dark repair of previously photoinactivated PSII allows for temporal offsets between episodes of net photoinactivation under high light with transient accumulation of [PSII] inactive and subsequent counteracting repair in low light or darkness, and could thereby contribute to the success of diatoms in environments of fluctuating light. ...
... Flori et al. (2017) observed physical contact between mitochondria and chloroplasts in P. tricornutum using focused ion beam scanning electron microscopy (FIB-SEM). This and the existence of genes encoding malate shuttle transporters (Prihoda et al., 2012), may allow the energetic exchange between these two organelles (Bailleul et al., 2015). Thus, we propose that malate produced in the mitochondria might be transported into the plastid via malate shuttle transporters or through physical interactions between the two organelles. ...
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Photosynthetic carbon fixation is often limited by CO2 availability, which led to the evolution of CO2 concentrating mechanisms (CCMs). Some diatoms possess CCMs that employ biochemical fixation of bicarbonate, similar to C4 plants, but it is controversially discussed whether biochemical CCMs are a commonly found in diatoms. In the diatom Phaeodactylum tricornutum, Phosphoenolpyruvate Carboxylase (PEPC) is present in two isoforms, PEPC1 in the plastids and PEPC2 in the mitochondria. We used real‐time quantitative PCR, western blots, and enzymatic assays to examine PEPC expression and PEPC activities, under low and high concentrations of dissolved inorganic carbon (DIC). We generated and analyzed individual knockout cell lines of PEPC1 and PEPC2, as well as a PEPC1/2 double‐knockout strain. While we could not detect an altered phenotype in the PEPC1 knockout strains at ambient, low or high DIC concentrations, PEPC2 and the double‐knockout strains grown under ambient air or lower DIC availability, showed reduced growth and photosynthetic affinity to DIC, while behaving similarly as WT cells at high DIC concentrations. These mutants furthermore exhibited significantly lower 13C/12C ratios compared to WT. Our data implies that in P. tricornutum at least parts of the CCM relies on biochemical bicarbonate fixation catalyzed by the mitochondrial PEPC2.
... The "just-in-time" strategy requires a higher FtsH:PsbA ratio than the 'warehouse' strategy. Furthermore, PSII repair continues in the dark in diatoms (Li et al. 2016), which we hypothesize is driven by tight integration of chloroplastic and mitochondrial metabolism to fuel the chloroplastic ATP requirements for FtsH, RNA transcription, and translation (Prihoda et al. 2012;Bailleul et al. 2015). This capacity for dark repair of previously photoinactivated PSII allows for temporal offsets between episodes of net photoinactivation under high light with transient accumulation of [PSII] inactive and subsequent counteracting repair in low light or darkness, and could thereby contribute to the success of diatoms in environments of fluctuating light. ...
... MBE Ochrophyta homologs have been replaced by plastidtargeted ones. This observation can be rationalized by metabolic and genetic redundancy kept in the last common ancestor of Actinophryidae and Ochrophyta, redundancy which has been eliminated in the current plastids of Ochrophyta (Kroth et al. 2008;Prihoda et al. 2012). A similar assumption was previously made for the plastid evolution in Archaeplastida and Picozoa (Schön et al. 2021). ...
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Ochrophyta is an algal group belonging to the Stramenopiles and comprises diverse lineages of algae which contribute significantly to the oceanic ecosystems as primary producers. However, early evolution of the plastid organelle in Ochrophyta is not fully understood. In this study, we provide a well-supported tree of the Stramenopiles inferred by the large-scale phylogenomic analysis that unveils the eukaryvorous (non-photosynthetic) protist Actinophrys sol (Actinophryidae) is closely related to Ochrophyta. We used genomic and transcriptomic data generated from A. sol to detect molecular traits of its plastid and we found no evidence of plastid genome and plastid-mediated biosynthesis, consistent with previous ultrastructural studies that did not identify any plastids in Actinophryidae. Moreover, our phylogenetic analyses of particular biosynthetic pathways provide no evidence of a current and past plastid in A. sol. However, we found more than a dozen organellar aminoacyl-tRNA synthases (aaRS) that are of algal origin. Close relationships between aaRS from A. sol and their ochrophyte homologs document gene transfer of algal genes that happened prior to the divergence of Actinophryidae and Ochrophyta lineages. We further showed experimentally that organellar aaRSs of A. sol are targeted exclusively to mitochondria, although organellar aaRSs in Ochrophyta are dually-targeted to mitochondria and plastids. Together, our findings suggested that the last common ancestor of Actinophryidae and Ochrophyta had not yet completed the establishment of host-plastid partnership as seen in the current Ochrophyta species, but acquired at least certain nuclear-encoded genes for the plastid functions.
... Overall, these data raise the question of how the function of this protein is regulated in diatoms. The KEA3 protein is predicted to contain a peculiar EF hand domain in P. tricornutum, similarly to other proteins involved in cell energetic metabolism (Prihoda et al., 2012). This Ca 2+ binding domain is located close to the RCK domain (Fig. S6), i.e. in a position where it could modulate the activity of the antiporter (Wang et al., 2017;Galvis et al., 2020). ...
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Diatoms are successful phytoplankton clades able to acclimate to changing environmental conditions, including e.g. variable light intensity. Diatoms are outstanding at dissipating light energy exceeding the maximum photosynthetic electron transfer (PET) capacity via the Non Photochemical Quenching (NPQ) process. While the molecular effectors of NPQ as well as the involvement of the Proton Motive Force (PMF) in its regulation are known, the regulators of the PET/PMF relationship remain unidentified in diatoms. We generated mutants of the H+/K+ antiporter KEA3 in the model diatom Phaeodactylum tricornutum. Loss of KEA3 activity affects the PET/PMF coupling and NPQ responses at the onset of illumination, during transients and in steady‐state conditions. Thus, this antiporter is a main regulator of the PET/PMF coupling. Consistent with this conclusion, a parsimonious model including only two free components, KEA3 and the diadinoxanthin de‐epoxidase, describes most of the feedback loops between PET and NPQ. This simple regulatory system allows for efficient responses to fast (minutes) or slow (e.g. diel) changes in light environment, thanks to the presence of a regulatory Ca2+‐binding domain in KEA3 modulating its activity. This circuit is likely tuned by the NPQ‐effector proteins, LHCXs, providing diatoms with the required flexibility to thrive in different ocean provinces.
The neurotoxin β-N-methylamino-L-alanine (BMAA) has been presumed as an environmental cause of human neurodegenerative disorders, such as Alzheimer’s disease. Marine diatoms Thalassiosira minima are demonstrated here to produce BMAA-containing proteins in axenic culture while the isomer diaminobutyric acid was bacterially produced. In the co-culture with Cyanobacterium aponinum, diatom growth was inhibited but the biosynthesis of BMAA-containing proteins was stimulated up to seven times higher than that of the control group by cell-cell interactions. The stimulation effect was not caused by the cyanobacterial filtrate. Nitrogen deprivation also doubled the BMAA content of T. minima cells. Transcriptome analysis of the diatom in mixed culture revealed that pathways involved in T. minima metabolism and cellular functions were mainly influenced, including KEGG pathways valine and leucine/isoleucine degradation, Endocytosis, pantothenate and CoA biosynthesis, and SNARE interactions in vesicular transport. Based on the expression changes of genes related to protein biosynthesis, it was hypothesized that ubiquitination and autophagy suppression, and limited COPII vesicles transport accuracy and efficiency were responsible for biosynthesis of BMAA-containing proteins in T. minima. This study represents a first application of transcriptomics to investigate the biological processes associated with BMAA biosynthesis in diatoms.
The greenhouse gas emission from fossil fuel and higher economic cost in its transportation are stimulating scientists to explore biomass energy production at the local level. In the present review, the authors have explored the prospects of commercial-scale biofuels production from the microalgal group, diatoms. Insights on suitability of mass cultivation systems for large-scale production of diatoms have been deliberated based on published literature. Diatoms can proliferate extracting nutrients from the wastewater and the same biomass can be harvested for biofuel production. Residues can be further utilized for the formation of other bioproducts and biofertilizers. The residual applications of diatoms from mass culture are estimated to compensate for the additional costs incurred in the removal of impurities. Well-planned research is required to optimize the commercial-scale production of biofuels from diatoms. The aim of this review is therefore, to demonstrate the economically feasible, hygienically safe cultivation of diatoms on nutrients from wastewater, limitations in using diatoms for biofuel production, and how these limitations can be shorted out for optimum utilization of diatom for biofuel production.
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This chapter reviews the evolution of the diatoms and of various groups within the diatoms and evaluates the significance of particular shared derived traits. The chapter also covers the question of why the diatoms dominate the modem marine phytoplankton, why they as members of the chromist lineages win over red and green unicellular phytoplankton, why heterokontophytes win over dinoflagellates and haptophytes, and why diatoms win over other heterokontophytes. Such insights would be valuable to climatologists trying to discover if there are any historical analogues for the current Earth system, with atmospheric CO2 now at levels unknown since the mid-Tertiary and an ocean pH that within a few centuries may be lower than at any time since the end of the Carboniferous. The study of molecular evolution among closely related organisms or groups of organisms helps us to provide the foundations for the application of the use of genes and their products for understanding ecosystem resilience, to understand the key mechanisms that have given rise to the similarities and differences within and between species' genomes, to understand the consequences of variation in genome sizes, gene families, and the exploitation of sequence polymorphisms in organisms, to appreciate how combinations of a diversity of kinds of mutations, genetic drift and selection have given rise to today's organisms, to see how the comparative biological approaches within and between species give additional insight into genome structure, evolution, and function.
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Phytoplankton in the Antarctic deplete silicic acid (Si(OH)4) to a far greater extent than they do nitrate (NO3-). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH)4:NO3- uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO3- depletion with excess Si(OH)4 remaining in surface waters. New δ30Si and δ15N records from Antarctic sediments confirm diminished Si(OH)4 use and enhanced NO3- depletion during the last three glaciations. The present low-Si(OH)4 water is transported northward to at least the subtropics. We postulate that the glacial high-Si(OH)4 water similarly may have been transported to the subtropics and beyond. This input of Si(OH)4 may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO2 by as much as 60 ppm.
This chapter provides a brief history of the conceptual framework of plankton evolutionary ecology to explain why the role of defense in the evolution of unicellular plankton has been neglected so far. The aim of this historical overview is also to point out what can be learned from studying the arms race: the other side of the coin. Given the range of attack techniques to which protists are exposed—from viruses to zooplankton—armor is defined as all forms of mechanical defense against pathogens, parasites, and ingestors. These in turn will range from slimy or tough cell walls that hamper purchase, bar entry, or withstand puncturing to long spines that deter ingestion. Following a brief comparison with terrestrial systems the chapter expands on the argument, that evolution of eukaryotes in the plankton is driven by the arms race. To this end, the range of attack systems evolved by pathogens, parasites, and predators in the plankton is surveyed, align them with their respective defense systems, and speculate on their evolutionary history. In the final section, the chapter examines how the various types of armor, that is, cell walls, scales, frustules, and colony skin of selected groups provide protection against specific forms of attack. In order to link structural features of an organism to a function, knowledge of the physical and ecological context, andthus,of the factors that cause significant selection pressure, is crucial.
This chapter is concerned with the peripheral antennas and is restricted to those of cyanobacteria and chloroplasts. Different groups of photosynthetic organisms have evolved different adaptations, and these are reflected in striking differences in their membrane structure. The first part of the chapter covers how cyanobacteria changed the world. The most important factor in preventing damage to the photosynthetic reaction centers is to get rid of excess excitation energy. The next section of the chapter covers a discussion on the light-harvesting antennas and the evolution of the algae. The major light-harvesting antennas of cyanobacteria and chloroplasts involve members of just three protein families: the phycobiliproteins, the IsiA. -Pcb Chl. -proteins (part of the Core Complex family), and the light-harvesting complex (LHC) superfamily. The phycobilisomes and IsiA protein are also discussed in detail. The chapter also discusses in detail about chlorophylls (Chls). The Chl c family of pigments is found only in the four groups of algae that are the result of secondary endosymbiosis involving a red algal endosymbiont. An in depth discussion is presented on LHC superfamily. The chapter touches upon the light-harvesting antennas, the stress-response connection and prokaryotic ancestry of the LHC superfamily.
Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of ∼45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.
Chloroplasts are organelles of endosymbiotic origin, and they transferred most of their genetic information to the host nucleus during this process. They therefore have to import more than 95% of their protein complement post-translationally from the cytosol. In vivo results from the model plant Arabidopsis thaliana — together with biochemical, biophysical and structural data from other plants — now allow us to outline the mechanistic details of the molecular machines that facilitate this translocation. It has become clear that chloroplasts evolved a unique translocation system, which is inherited, in part, from their bacterial ancestors.
Dinoflagellate algae are important primary producers and of significant ecological and economic impact because of their ability to form “red tides” [1]. They are also models for evolutionary research because of an unparalleled ability to capture photosynthetic organelles (plastids) through endosymbiosis [2]. The nature and extent of the plastid genome in the dominant perdinin-containing dinoflagellates remain, however, two of the most intriguing issues in plastid evolution. The plastid genome in these taxa is reduced to single-gene minicircles 3. and 4. encoding an incomplete (until now 15) set of plastid proteins. The location of the remaining photosynthetic genes is unknown. We generated a data set of 6,480 unique expressed sequence tags (ESTs) from the toxic dinoflagellate Alexandrium tamarense (for details, see the Experimental Procedures in the Supplemental Data) to find the missing plastid genes and to understand the impact of endosymbiosis on genome evolution. Here we identify 48 of the non-minicircle-encoded photosynthetic genes in the nuclear genome of A. tamarense, accounting for the majority of the photosystem. Fifteen genes that are always found on the plastid genome of other algae and plants have been transferred to the nucleus in A. tamarense. The plastid-targeted genes have red and green algal origins. These results highlight the unique position of dinoflagellates as the champions of plastid gene transfer to the nucleus among photosynthetic eukaryotes.