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The Vast Majority of the Carbon That Makes Up Life is Released Back into the Water and Indirectly Into the Atmosphere in the Form of CO2 Gas

The carbon cycle within the euphotic layer of the ocean, where sunlight penetrates and supports photosynthesis, is both intricate and dynamic. This region of the ocean teems with life and biological activity, significantly influencing the movement and transformation of carbon through various processes. Predominantly, respiration and decomposition facilitate this cycle, supplemented by other mechanisms that are essential to maintaining the marine ecosystem. This article explores these processes in detail, shedding light on the roles they play in the broader context of the ocean-atmosphere carbon exchange and the implications for climate change.

Phytoplankton, the tiny green powerhouses of the ocean, form the backbone of photosynthetic activity in the euphotic layer. These microscopic plants harness sunlight to convert carbon dioxide (CO2) and water into organic compounds like glucose, a process vital for their growth and for forming the base of the marine food web. However, as soon as these organic compounds are created, they begin to be transformed and recycled through respiration. Marine organisms, including phytoplankton themselves, zooplankton, and various fish, respire by consuming oxygen and organic compounds and releasing CO2 back into the water. This released CO2 can either be reused for photosynthesis or diffuse into the atmosphere.[^1][^2][^3][^4][^5][^6]

Decomposition​

The cycle continues even after marine organisms die. As their bodies descend through the water column, decomposers such as bacteria and other microorganisms start breaking them down. In the euphotic layer, this decomposition process transforms complex organic materials back into simpler molecules, including CO2, which reenters the water, perpetuating the cycle.[^7][^8]

Ocean-Atmosphere Exchange​

The boundary between the ocean and atmosphere is a dynamic interface where gases are exchanged continuously. The CO2 produced by respiration and decomposition within the euphotic layer can diffuse from the ocean into the atmosphere. At the same time, atmospheric CO2 can dissolve into the ocean water, maintaining an essential balance within the carbon cycle. [^9][^10]

Biological Carbon Pump​

Despite the constant release of CO2, some carbon in the euphotic layer becomes part of larger particles, such as fecal pellets and dead organisms, which sink into deeper ocean layers, thereby contributing to the biological carbon pump. This mechanism sequesters carbon in the deep ocean for extended periods, thus playing a critical role in the long-term carbon cycle. Moreover, some organic carbon dissolves in seawater and is transported by ocean currents to distant regions. While most of this dissolved organic carbon is eventually decomposed, a fraction remains in the water column for extended periods, aiding in long-term carbon storage.[^11]

Sources​

[^1] Falkowski, P. G., & Raven, J. A. (2007). Aquatic photosynthesis. Princeton University Press.

[^2] Sabine, C. L., & Feely, R. A. (2007). The oceanic sink for carbon dioxide. In Greenhouse gas sinks (pp. 31-49). Wallingford UK: CABI. https://doi.org/10.1079/9781845931896.0031

[^3] Intergovernmental Panel on Climate Change (IPCC). (2007). Climate change: The physical science basis. IPCC Reports.

[^4] National Oceanic and Atmospheric Administration (NOAA). (n.d.). Ocean acidification. NOAA Ocean Service Education. https://oceanservice.noaa.gov/education/pd/oceans_concepts/acidification.html

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[^6] Falkowski, P. G. (1994). The role of phytoplankton photosynthesis in global biogeochemical cycles. Photosynthesis Research, 39(3), 235-258. https://doi.org/10.1007/BF00014586

[^7] Kirchman, D. L. (1994). The microbial loop in Antarctic waters. Antarctic Science, 6(3), 235-242. https://doi.org/10.1017/S0954102094000454

[^8] Azam, F., et al. (1983). The ecological role of water-column microbes in the sea. Marine Ecology Progress Series, 9, 257-263. https://doi.org/10.3354/meps010257

[^9] Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., Lee, K., Bullister, J. L., Wanninkhof, R., Wong, C. S., Wallace, D. W. R., Tilbrook, B., Millero, F. J., Peng, T.-H., Kozyr, A., Ono, T., & Rios, A. F. (2004). The oceanic sink for anthropogenic CO2. Science, 305(5682), 367-371. https://doi.org/10.1126/science.1097403

[^10] Jiao, N., Luo, T., Chen, Q., Zhao, Z., Xiao, X., Liu, J., Jian, Z., Xie, S., Thomas, H., Herndl, G. J., et al. (2024). The microbial carbon pump and climate change. Nature Reviews Microbiology, 1-12. https://doi.org/10.1038/s41579-024-00817-2

[^11] Honjo, S., Manganini, S. J., Krishfield, R. A., & Francois, R. (2008). Particulate organic carbon fluxes to the ocean interior and factors controlling the biological pump: A synthesis of global sediment trap programs since 1983. Progress in Oceanography, 76(3), 217-285. https://doi.org/10.1016/j.pocean.2007.11.003