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Moore. / Mexican Journal of Biotechnology 2020, 5(1):1-10
A biotechnological expansion of shellfish cultivation could
permanently remove carbon dioxide from the atmosphere
Una ampliación biotecnológica del cultivo de moluscos bivalvos podría
eliminar permanentemente el dióxido de carbono de la atmósfera
David Moore
School of Biological Sciences, Faculty of Biology, Medicine and Health, The
University of Manchester, Manchester, United Kingdom.
*Corresponding author
E-mail address: david@davidmoore.org.uk (D. Moore) (retired from the University
of Manchester).
Article history:
Received: 14 November 2019 / Received in revised form: 15 December 2019 /
Accepted: 16 December 2019 / Published online: 1 January 2020.
https://doi.org/10.29267/mxjb.2020.5.1.1
ABSTRACT
To combat climate change, proposals have been made to develop methods that
would pull carbon dioxide out of Earth’s atmosphere. One recommended approach
is to remove CO2 from the atmosphere with activities such as reforestation and
changing forest management and agricultural practices to enhance soil carbon
storage. However, it is also noted that such activities would limit land for food
production and negatively affect biodiversity. Furthermore, decay of dead wood
and fallen leaves in natural forests releases huge quantities of CO2 and other
greenhouse gases back into the atmosphere. The only other carbon-sequestration
technique that is widely considered is the application of CO2 capture processes to
flue gases of power plants, which are responsible for about 80% of the worldwide
CO2 emission from large stationary sources. Hydrate-based processing is a
promising technology for CO2 capture as it results in high CO2 recovery, but its
high cost prevents this technology having much impact. In this note I suggest that
the ability of marine organisms (shellfish and coccolithophore algae) to remove
permanently CO2 from the atmosphere into solid (crystalline) CaCO3 should be
harnessed. I suggest that if the level of finance and effort that are readily
Mexican Journal of Biotechnology 2020, 5(1):1-10
Journal homepage:www.mexjbiotechnol.com
ISSN:2448-6590
SHORT COMMUNICATION
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anticipated for forest management and flue gas treatments were to be applied to
expansion of shellfish cultivation around the world, significant amounts of carbon
dioxide could be permanently removed from the atmosphere within the timescale
that is currently envisaged for carbon capture by afforestation.
Key Words: aquaculture, atmosphere remediation, carbon capture, carbon
dioxide, shellfish, shell-CaCO3.
RESUMEN
Para combatir el cambio climático, se han hecho propuestas para desarrollar
métodos que eliminen el dióxido de carbono de la atmósfera terrestre. Un enfoque
recomendado es eliminar el CO2 de la atmósfera con actividades como la
reforestación, y el cambio en la gestión forestal y las prácticas agrícolas para
mejorar el almacenamiento de carbono del suelo. Sin embargo, esas actividades
limitarían la tierra para la producción de alimentos y afectarían negativamente a la
biodiversidad. Además, la descomposición de la madera muerta y las hojas caídas
en los bosques naturales libera enormes cantidades de CO2 y otros gases de
efecto invernadero a la atmósfera. La única otra técnica de secuestro de carbono
que es ampliamente considerada es la aplicación de procesos de captura de CO2
de los gases de combustión de las centrales eléctricas, que son responsables de
aproximadamente el 80% de la emisión mundial de CO2 de grandes fuentes
estacionarias. El proceso basado en la producción de hidrógeno con captura de
CO2 es una tecnología prometedora para la captura de CO2, ya que resulta en una
alta recuperación de éste gas, pero su alto costo evita que esta tecnología tenga
mucho impacto. En esta nota sugiero que se aproveche la capacidad de los
organismos marinos (moluscos bivalvos) para eliminar permanentemente el CO2
de la atmósfera en sólido (cristalino) CaCO3. Sugiero que, si el nivel de
financiamiento y esfuerzo que se preveé fácilmente para la gestión forestal y los
tratamientos de gases de combustión se aplicaran a la ampliación del cultivo de
moluscos bivalvos en todo el mundo, se podrían eliminar permanentemente
cantidades significativas de dióxido de carbono de la atmósfera dentro del plazo
previsto actualmente para la captura de carbono por reforestación.
Palabras clave: acuicultura, captura de carbono, CaCO3 de concha de moluscos
bivalvos, dióxido de carbono, moluscos bivalvos, remediación atmosférica.
1. INTRODUCTION
Photosynthetic carbon capture by trees is widely considered to be possibly our
most effective strategy to limit the rise of CO2 concentrations in the atmosphere,
and there are several ambitious targets to promote forest conservation,
afforestation, and restoration on a global scale.
The Intergovernmental Panel on Climate Change Special Report of 2018 (IPCC,
2018) suggested that an increase of 1 billion hectares of forest will be necessary to
limit global warming to 1.5°C by 2050. Bastin et al. (2019) mapped the global
potential tree coverage and estimated that the world’s ecosystems could support
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an additional 0.9 billion hectares of continuous forest (corresponding to more than
25% increase in forested area) and that such a change has the potential to cut the
atmospheric carbon pool by about 25%. I like trees and I am all in favour of
planting more of them, but as a mycologist I have to say that there is a negative
side to these estimations that seems to be escaping notice.
This is that forests don’t only contain trees that can store gigatonnes of carbon in
the wood they make; forests also contain wood-decaying fungi that can (and do)
digest that wood, releasing greenhouse gases, including CO2, in the process.
Chlorinated hydrocarbons also make a normal every-day contribution to the
degradation of timber. The fungal chloromethane contribution to the atmosphere
has been estimated at around 150,000 tonnes per annum (Watling & Harper,
1998), which is about 60% more than was released into the atmosphere by
industrial coal burning furnaces worldwide in the year of publication.
Of course, the ultimate end-product of digestion is CO2. On a global scale,
decomposition of seasonally shed leaves, petals, ripe fruit, and dead wood
releases billions of tons of CO2 to the atmosphere each year, a similar magnitude,
in fact, to the annual CO2 emissions from fossil fuel combustion (Rinne‐Garmston
et al., 2019).
However, Boysen et al. (2017) note that using biomass plantations to sequester
carbon would reduce biodiversity, because they are likely to be monocultures, and
occupy land that might otherwise be used for food production. These authors
conclude: ‘…that this strategy of sequestering carbon is not a viable alternative to
aggressive emission reductions…’
Most current research on ‘aggressive emission reductions’ is focussed on the
integration of new technologies to capture CO2 from flue gasses in power plants,
which are responsible for about 80% of the worldwide CO2 emissions (Romano et
al., 2013). Methods based on exposing flue gas to water under suitable conditions
(‘hydrate-based processing’) is a promising and high efficiency technology for CO2
capture, but the high cost of maintaining suitable conditions for hydrate formation is
preventing wide industrial application of this technology (Li et al., 2019).
So, if the forests and capture from flue gases can’t save us, are we doomed? Well,
no, actually; we just need to change our focus; turn away from trees (but still plant
them; they’re good for us in so many ways) and concentrate on shellfish.
2. CARBON SEQUESTRATION POTENTIAL OF SHELLFISH
About half the mass of shellfish is shell, and shellfish-shell is solidified CO2. The
difference is, it’s permanently solidified (mineralised) CO2. Molluscan shell is a
typical biomineral composed of CaCO3 with a small amount of matrix proteins
included that direct the species-specific crystal growth; arthropod (crab, shrimp,
lobster) exoskeletons are composed largely of chitin hardened with calcium-
magnesium carbonate nanocrystals (Boßelmann et al., 2007).
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Ca2+ is absorbed through specific transporters in the tissues of the animals and is
reacted with HCO32-, which is synthesized from CO2. HCO32- is partly absorbed
directly from the surrounding water (or gaseous atmosphere for terrestrial species).
The rest derives from CO2 generated by the animal’s food through the TCA cycle.
The fractions derived from these two sources differ widely (McConnaughey &
Gillikin, 2008; Filgueira et al., 2019). Knowing when calcification draws mainly on
CO2 from food or depends on inorganic carbon from ambient air or water is a
crucial consideration for studies of nutrition, ecology, conservation and cultivation
but it is not relevant to this discussion. My only interest is mineralization of
atmospheric CO2 in the shell. For animals which are filter feeders; the CO2
generated by their TCA cycles comes from digestion of plankton and is derived
from planktonic photosynthesis (Tassanakajon et al., 2008). For terrestrial species
the source is the photosynthesis of terrestrial plants. This is true even for
predators, scavengers and detritus feeders, aquatic and terrestrial; all depend on
fixation of photosynthetic carbon from the atmosphere at the root of the food chain.
There is no other source of metabolic carbon.
Ultimately, then, the CO2 for the shell comes from the atmosphere and stays out of
the atmosphere. Intact shellfish shells are excavated regularly from the middens
associated with coastal Palaeolithic human communities (old Stone Age; from
around 12,000 years ago). Intact shellfish shells abound in deep-water cores of
ancient coastal sediments of hundreds of thousands of years ago. And remember
the fossils from deep time: brachiopods (550 million years ago), trilobites (520
million years ago) and ammonites (240 - 65 million years ago). Certainly, these
fossil shells are changed considerably in chemistry by now, but the shells survive
over geological time in order to be fossilised; and in vast numbers. How much
more permanent, do we need permanent to be?
The Food and Agriculture Organization of the United Nations Fisheries &
Aquaculture Department maintains a database of Global Aquaculture Production
that contains statistics on production volume. In this respect ‘Aquaculture’ is
understood to mean the farming of aquatic organisms including molluscs and
crustaceans. Farming implies some form of intervention in the rearing process to
enhance production, such as regular stocking, feeding, protection from predators,
etc. Farming also implies individual or corporate ownership of the stock being
cultivated. For statistical purposes aquatic organisms which are exploitable by the
public as a common property resource, with or without appropriate licences, are
the harvest of fisheries, not aquaculture.
3. APPLICATIONS OF BIOTECHNOLOGY IN SHELLFISH CULTIVATION
Data from FAO Fisheries and Aquaculture Information and Statistics Branch (as of
25 May 2019) show that over the years 2010 to 2017 aquaculture harvests across
the globe totalled 53,512,850 metric tonnes of crustaceans and 122,527,372 metric
tonnes of molluscs (a combined total of 176,040,222 metric tonnes in 8 years).
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If we assume that the shell represents 50% of the animal’s mass, then the total
shellfish-shell produced was 88 million tonnes over 8 years; which is an average of
11 million tonnes of shell per year. Filgueira et al. (2019) arrive at a similar value
for bivalve molluscs alone. I quote: “Taking into account the global annual
production of cultured bivalves is ≈14 x106 tons, including clams, cockles, oysters,
mussels and scallops (www.fao.org reporting 2015 data) and assuming an average
contribution of shell to total body weight of 50% (general ballpark figure given that
this varies greatly between species), shell represents a residue (potential by-
product) of ≈7 × 106 tons, of which 95% is calcium carbonate.”
Returning to my calculation, if we further assume that for both crustaceans and
molluscs the shell is made from CaCO3; on a molar mass basis, carbon represents
12% of the mass of calcium carbonate. So, 11 million tonnes of shell per year is
equivalent to 1.32 million tonnes of carbon per year being captured from the
atmosphere by current aquaculture activities.
Global carbon emissions from fossil fuel use were 9.795 billion tonnes in 2014 (or
35.9 billion tonnes of carbon dioxide) [https://www.co2.earth/global-co2-emissions].
So, a thousand-fold increase in aquaculture would permanently remove about 14%
of the global carbon emissions in each year.
Could that be done? Possibly. If we doubled aquaculture production of crustaceans
and molluscs each year then from the 14th year we could be removing 10.7 billion
tonnes of carbon from the atmosphere each year.
Sustained annual doubling may not be realistic; but this simple calculation
indicates that with determined effort (and adequate finance) to vastly increase
aquaculture production we could be permanently extracting significant amounts of
carbon annually from the atmosphere within the timescale that is currently
envisaged for carbon capture by vastly increased afforestation.
The carbon balance of the growth phase of the animals is not important. Nor is
harvesting, though the animals within the shells could be a valuable source of
animal protein (with the profits contributing to finance for further expansion in
cultivation). However, because our emphasis is focused on the animal as shell,
rather than the animal as food, our unharvested shellfish farms could be placed in
waters polluted with toxic wastes or toxic microbes. The most relevant fact being
that when the animal dies (either in the aquaculture farm or in your kitchen) it
leaves behind a shell made of insoluble carbonates constructed using CO2 which is
now permanently removed from the atmosphere. The same considerations apply to
crustacea, freshwater shellfish, and land snails.
4. MISSED OPPORTUNITIES
Unlike the forestry industry and its trees, the shellfish industry does not seem to
appreciate the atmosphere-positive aspects of its animals. Just a few examples will
suffice to illustrate this.
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The Shellfish Growers Climate Coalition website stresses the adverse
effects on production due to ocean acidification, increasing seawater
temperature and disruption caused by superstorms (Global Aquaculture
Alliance website). Although, of course, if enhanced shellfish cultivation
permanently sequestered a significant amount of atmospheric CO2,
acidification would also reduce.
Reduced calcification by marine algae due to ocean acidification in
response to rising atmospheric CO2 is also a concern in research on
coccolithophores (Iglesias-Rodriguez et al. 2008). The relevance of this is
that from the mid-Mesozoic Era in our geological history, coccolithophores
have been major calcium carbonate producers in the world’s oceans, today
accounting for about a third of the total marine CaCO3 production.
Although both calcification and net primary production in these species are
significantly increased by high CO2 partial pressures, the possibility that the
algae could be used to trap atmospheric CO2 does not seem to have been
recognised.
It has been suggested that seaweed aquaculture to upscale offshore kelp
forests could provide sufficient CO2 sequestration to mitigate climate change
(Froehlich et al., 2019). This could certainly provide temporary carbon
capture in the short term that would be a useful contribution, but while kelp
forests solve the ‘land-usage’ issue, they still suffer from the same
limitations as terrestrial forests. Specifically: when the plant material dies it is
digested and the CO2 it has sequestered is returned to the atmosphere. The
only permanently removed carbon would be in the crustaceans and
molluscs that would undoubtedly flourish in the seaweed forest.
The book Goods and Services of Marine Bivalves (Smaal et al., 2019) deals
with a wide range of aquaculture topics including genomics-driven
biotechnological innovations like new pharmaceuticals from molluscs,
habitat and ecosystem-engineering modification in coastal protection by
reef-building bivalves, water clarification services provided by their filter
feeding and even shells as collector’s items, but does not include a chapter
dealing specifically with the potential service of extracting carbon from the
atmosphere.
Filgueira et al. (2019) make the closest approach but they conclude that the
“0.45 g CO2 sequestered by the shell of each cultured mussel in Norway is
hardly significant taking into account that a regular car produces more than
100 g CO2 per km”. Personally, I would expect more than one mussel in a
serving; say, at least 20. So, moules marinière for two persons would
sequester about 20 g CO2; in just one meal. Feeding the rest of the family
and a few friends the same way could easily sequester that 100 g CO2 and
be much more beneficial for the atmosphere than ten meals of prime beef.
Remember, the CO2 is permanently sequestered, but you’ll be hungry again
the next day; and, presumably, so will your neighbours. Filgueira et al.
(2019) conclude “although this is far from solving a global problem,
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everything counts. In addition, it is important to re-emphasize that this
comes at no cost or effort given that bivalves are cultured to produce food.” I
am suggesting a change of focus: culture the bivalves, and those other
shellfish, to sequester permanently CO2 from the atmosphere and accept
the food as the by-product.
5. CONCLUSIONS
My suggestion would be that a realistic plan might feature three prime targets:
A. Fund a development foundation that will invest cash immediately in every
existing aquaculture enterprise with the aim of doubling their production each
season for the next three to five seasons. This is unlikely to be easy because of
perception that expansion of the shellfish industry could have negative
environmental effects on coastal waters by exceeding the population size the
environment can sustain (carrying capacity). There are also concerns about social
issues (aesthetic loss) and a supposed “loss of nature” (Newell, 2007; Newell et
al., 2019; Smaal & van Duren, 2019). Such perceived negative environmental
effects are not unique to shellfish cultivation and certainly have their parallels in
large-scale tree planting (use of scarce agricultural land, loss of biodiversity in
monocultures, as noted above).
B. Fund research programmes to study:
existing aquaculture farming methods to adapt them to wider ranges of sites
and locations (Newell et al., 2019) [imagine a mussel farm on every offshore
wind turbine, every oil and gas rig, every pier, wharf and jetty, every
breakwater or harbour wall]. Again, not easy: other people have rights,
privileges, ownerships and fears and prejudices. But then, try suggesting it
would be a good idea to plant a forest of oak trees in Trafalgar Square, the
Avenue des Champs-Élysées, or National Mall and Memorial Parks in
Washington, D.C.
New aquaculture farming methods to establish new organisms and new
methods to enhance incorporation of atmospheric carbon into shells.
C. Fund developmental research into high-technology programmes.
Biotechnological research on aquaculture is well established (e.g. Rasmussen &
Morrissey, 2007; Xiang, 2015). A more unusual suggestion would be to determine
whether we could grow coccolithophore algae in giant illuminated fermenters
(maybe using the Quorn™ fermenters as a model; see Moore et al., 2020)?
Perhaps we could harvest a sludge of insoluble plates of calcium carbonate from
which we could build our own ‘white cliffs of Dover’, because using this calcium
carbonate as a feedstock for cement production could replace the fossil limestone
that is currently used to make quicklime (in 2014, cement production accounted for
6% of the fossil CO2 emissions from industrial sources). Our way of life uses a lot
of cement.
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We need plenty of funding and the determination to do it. So, if there’s anyone out
there with the odd billion dollars to spare just let me know and I’ll get the
programme rolling … but, for the moment, would anyone like another bowl of
moules marinière; or maybe a crab salad?
CONFLICT OF INTEREST
The author has no conflicts of interest to declare.
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