How does plankton regulate the climate?

The climate is regulated by many parameters, and one of the major ones is the quantity of carbon dioxide (CO2) in the atmosphere, also called carbon dioxide concentration, written pCO2. The pCO2 increase is causing an increase of the greenhouse gas effect, and so, an increase of the temperature at the Earth surface. With the increase of greenhouse gases concentration, more of the sun's radiation remains on the Earth's surface, causing an increase of the energy and so, of the temperature (Goosse, 2015). The pCO2 changed and is changing over time, following the equilibrium between the inputs and the outputs of carbon dioxide in the atmosphere: it is the carbon cycle. The carbon is located in different reservoirs (e.g. atmosphere reservoir, geological reservoir, biomass reservoir, ocean reservoir) and moving from one reservoir to the other, with different residence time depending on the reservoir.

With the increase of human activities and industry (and fossil fuels), more and more carbon dioxide is released in the atmosphere, causing the pCO2 to increase (Intergovernmental Panel On Climate Change, 2021). A part of this CO2 is absorbed by the vegetation on land, doing photosynthesis, but a bigger part is absorbed by tiny marine organisms living at the surface of the ocean: the plankton! There are gas exchanges at the ocean-atmosphere interface, and the concentration of carbon dioxide dissolved in the surface ocean depends on the atmospheric pCO2 and the temperature (Feely et al., 2004; Gao et al., 2017; Honjo et al., 2014; Landschützer et al., 2014; Takahashi et al., 2009). We will here focus on the marine carbon cycle, zooming on what is occurring in the ocean.

The plankton, living in the first metres to hundreds of metres of water, is removing carbon from the surface ocean, helping to regulate and counter-balance the CO2 emissions (Intergovernmental Panel On Climate Change, 2021; Marilaure Grégoire et al., 2023).

There are two major processes involved: the photosynthesis (by phytoplankton, nano-algae and cyanobacteria) and the calcification processes (by the organisms building a calcite shell).

The photosynthesis produces oxygen, as shown in the following equation:

Sunlight Energy + Water (H2O) + Carbon dioxide (CO2) → Organic matter + Dioxygen (O2)

Thanks to this process, these organisms, are growing and constituting organic matter as well as releasing oxygen.

Regarding the calcification, it uses the carbonate ions (CO32-) resulting from the dissolution of the CO2 in the surface ocean and calcium ions (Ca2+) to form calcite (CaCO3) constituting the shell, as shown in the following equation:

Ca2+ + CO32- ↔ CaCO3

This is the carbonate uptake or carbonate biological compensation (by opposition to the dissolution or chemical compensation).

At their death, the plankton is sinking from the surface to the deep ocean. It is called marine snow. they are deposited on the ocean floor, and slowly accumulates, the most recent ones above the older. With the diagenesis processes, the sediments become rocks and the carbon contained in these organisms is stocked in the long-term geological reservoir.

Via these processes, the plankton contributes significantly to the carbon removal from the atmosphere, it export from the surface to the deep ocean, and to store this carbon in the form of rocks. This way, they buffer the increase of atmospheric carbon dioxide concentration and so, regulate climate (Boudreau et al., 2018; Ridgwell and Zeebe, 2005).

Because of their importance in the ocean capacity to absorb CO2, scientists are currently working on the possible response of these organisms with climate change and increasing temperatures. For example, recent studies have shown that in previous warming events from the recent history, the number of planktonic organisms building a carbonate shell was higher during the warmer intervals, but composed of smaller specimens. Knowing that models are predicting climate conditions 1.5 to 5 degrees warmer than present with a pCO2 up to 1200 ppm and a decrease of the atmosphere to ocean carbon flux from now to 2100 (Intergovernmental Panel On Climate Change, 2021), we expect an increasing carbonate production and a lower capacity of the ocean to absorb CO2 (Cornuault, 2023).

Attention: never forget that plants and algae do respiration too, and marine organisms breath too! Respiration in consuming oxygen and releasing CO2.

Furthermore, there is another mechanism involved in calcification: the carbonate counter-pump. This one is involving the bicarbonate ions (HCO3-) and is releasing CO2 as shown in the following equation:

Ca2+ + 2HCO3- ↔ CaCO3 + H2O +CO2

To find out even more about ocean chemistry, take a look at the figure at the very end of this article (after the bibliography). It takes into account even more of the ocean chemistry parameters that researchers take into account when studying changes in the ocean environment. You will become an expert! 

Bibliography

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Cornuault, P. M. L.: Natural variation of pelagic carbonate production during Cenozoic warm periods, Universität Bremen (PhD dissertation), https://doi.org/10.26092/elib/2651., 2023.

Feely, R. A., Sabine, C. L., Lee, K., Berelson, W., Kleypas, J., Fabry, V. J., and Millero, F. J.: Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans, Science, 305, 362–366, https://doi.org/10.1126/science.1097329, 2004.

Gao, K., Zhang, Y., and Häder, D.-P.: Individual and interactive effects of ocean acidification, global warming, and UV radiation on phytoplankton, Journal of Applied Phycology, 30, 743–759, https://doi.org/10.1007/s10811-017-1329-6, 2017.

Goosse, H.: Climate System Dynamics and Modelling:, 1st ed., Cambridge University Press, https://doi.org/10.1017/CBO9781316018682, 2015.

Honjo, S., Eglinton, T., Taylor, C., Ulmer, K., Sievert, S., Bracher, A., German, C., Edgcomb, V., Francois, R., Iglesias-Rodriguez, M. D., Van Mooy, B., and Rapeta, D.: Understanding the Role of the Biological Pump in the Global Carbon Cycle: An Imperative for Ocean Science, oceanog, 27, 10–16, https://doi.org/10.5670/oceanog.2014.78, 2014.

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Landschützer, P., Gruber, N., Bakker, D. C. E., and Schuster, U.: Recent variability of the global ocean carbon sink, Global Biogeochem. Cycles, 28, 927–949, https://doi.org/10.1002/2014GB004853, 2014.

Marilaure Grégoire, Oschlies, A., Canfield, D. E., Castro, C., Ciglenečki, I., Croot, P., Salin, K., Schneider, B., Serret, P., Slomp, C., Tesi, T., and Yucel, M.: Ocean Oxygen: the role of the Ocean in the oxygen we breathe and the threat of deoxygenation, Zenodo, https://doi.org/10.5281/ZENODO.7941157, 2023.

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September 2024

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