Understanding phytoplankton and the ocean carbon sink
Looking at the role of marine microorganisms in the regulation of atmospheric carbon.
With two-thirds of Earth's surface covered by water, the oceans play a critical role in the sequestration of atmospheric carbon, and therefore climate change. Around a third of the carbon dioxide released by human activity during industrial age has been removed by oceans. Along with the world's forests, oceans are one of the largest sinks of anthropogenic CO2.
At the heart of this carbon sink are a common group of marine phytoplankton which turn carbon from the air into calcium carbonates. Being an alkaline material, calcium carbonate regulates ocean acidity, and the chemical reaction involved in carbon absorption is highly sensitive to the relative acidity of surface seawater.
Oceanographers at the Universitat Autònoma de Barcelona in Catalonia have found that the exchange of carbon between atmosphere and ocean is highly modulated by the photosynthesizing phytoplankton known as coccolithophores. These micro-organisms live in the upper, sunlit layer of the sea, where they form plates of calcium carbonate that can be seen on the sea surface around chalk cliffs.
Researcher Patrizia Ziveri and her colleagues report1 that coccolithophores dominate the production of calcium carbonate, contributing 90% in the surface layer. The other two main calcifying groups - zooplankton (pteropods) and foraminifera - play a secondary role in atmospheric CO2 modulation.
The Ziveri-led study also reveals that, rather sinking into the deep ocean, a large proportion of the carbonate plates dissolve close to the surface in the sunlight-penetrating photic zone. "This extensive shallow dissolution explains the apparent discrepancy between previous estimates of CaCO3 production derived from satellite observations/biogeochemical modelling versus sinking particle estimates from shallow sediment traps", explains Ziveri.
Ziveri’s collaborator William Gray, based at the Laboratoire des Sciences du Climat et de l’Environnement in Paris, says that the dissolving of so much calcium carbonate close to the surface shows the exchange of carbon between ocean and atmosphere is far more complex than previously understood. Gray adds: “Until we better understand the processes driving this shallow dissolution, it will be difficult to predict how the ocean will uptake carbon in the future.”
Ziveri et al., “Pelagic calcium carbonate production and shallow dissolution in the North Pacific Ocean”, Nature Communications 14, 805 (2023); doi:10.1038/s41467-023-36177-w.