Coccolithophores are unicellular photosynthetic plankton that perform extraordinary feats in ionic homeostasis to fabricate intricate nano-patterned plates made of calcium carbonate (CaCO3) crystals called coccoliths (1). Outside marine science communities, coccolithophores are less known than animal calcifiers such as shellfish or the cnidarians that form coral reefs. However, coccolithophores are one of Earth’s greatest biological producers of CaCO3. The production and sinking of coccoliths play complex roles in ocean carbon cycles, helping carry organic carbon to the deep sea as well serving on a geological scale to help the ocean buffer CO2 fluctuations (2, 3). Unlike other calcifying organisms, where precipitation of CaCO3 is extracellular, coccolithophores calcify in special intracellular Golgi-derived coccolith vesicles. To do this, they maintain among the greatest fluxes of Ca2+ and H+ known for any cell ions which would be toxic if allowed to accumulate in the cytoplasm (1). In PNAS, Kottmeier et al. (4) demonstrate how they rely on voltage-gated proton channels to expel H+ released by CaCO3 precipitation, which also offers a way forward to resolving disparate results from two decades of research on coccolithophore sensitivity to ocean acidification.
Approximately a third of human CO2 emissions are absorbed by the ocean, resulting in ocean acidification. As CO2 dissolves in the sea it reacts with water to form carbonic acid, generating H+ (decreasing pH) and perturbing a set of interlocked equilibria involving CO2, HCO3−, CO32−, H+, and Ca2+ by increasing [HCO3−], decreasing [CO32−], and lowering the saturation states of alternative forms of CaCO3 (5). The inorganic chemistry is complex but comparatively well known. The response of calcifying organisms should be simple to predict if it depended only on the tendency of CaCO3 to precipitate or dissolve in seawater: Organisms such as coccolithophores which produce calcite, the more stable form of CaCO3, should be less sensitive to ocean acidification compared to organisms like many corals which produce less-stable forms such as aragonite.
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von Dassow P., 2022. Voltage-gated proton channels explain coccolithophore sensitivity to ocean acidification. Proceedings of the National Academy of Sciences 119(25): e2206426119. doi: 10.1073/pnas.2206426119. Article.
