Ocean Acidification

BACKGROUND

The production and dissolution of calcium carbonate (CaCO₃) is a key component of the ocean carbon cycle. In the open ocean, nearly all CaCO₃ is produced by three groups of calcifying plankton: coccolithophores, foraminifers, and pteropods. These taxonomically and functionally diverse organisms play a major role in ocean biogeochemistry by modulating air-sea CO₂ exchange, and facilitating the export of carbon and alkalinity to depth.

Despite their biogeochemical importance, these groups are typically considered separately, precluding an integrated understanding. Yet the pathways by which CaCO₃ is produced and cycled through the ocean have important consequences for the carbon cycle and ecosystem functioning. Notably, none of the Earth system models included in the current Coupled Model Intercomparison Project (CMIP6) explicitly represents these groups of organisms. Here, we review the distinct functional traits of coccolithophores, foraminifers, and pteropods to elucidate how these traits shape their global distributions, vulnerabilities to climate change and acidification, and their role in modulating ocean chemistry and the Earth system.

ADVANCES

Recent advances in data compilation at multiple levels offer a comprehensive but still incomplete view of the CaCO₃ cycle, from biomineralization up to the global ocean, with different traits leading to differing vulnerabilities to environmental change. For example, coccolithophores, as primary producers, are relatively less affected by changes in oxygen concentration compared with heterotrophs, but are particularly sensitive to ocean acidification because of the proton load generated during intracellular calcification, which requires effective pH regulation and proton expulsion. Differing resource requirements contribute to the geographic distributions of each group, while traits such as body size and turnover rate are fundamentally linked to global production, export, dissolution, and burial. Compiling these data allows us to compare the markedly different fates of the CaCO₃ produced by each group, from surface production through export to eventual sediment burial. A major imbalance exists in the global CaCO₃ cycling related to each calcifying plankton group, with key uncertainties, especially in rates of group-specific production and shallow biologically mediated dissolution. Current best estimates indicate that a large fraction of coccolithophore-derived CaCO₃—the dominant source of CaCO₃ in the ocean—is dissolved and recycled in the upper ocean. This underscores the central role of ecological processes such as predation, particle aggregation, and microbial respiration in shaping ocean carbonate chemistry.

We suggest that the overlooked process of shallow dissolution, mainly of coccolithophores, is also likely at play within the geological record of this group.

OUTLOOK

The three major groups of calcifying plankton play essential but distinct roles within ocean ecosystems and the marine carbon cycle. Their diverse traits govern global distributions, production, export, and their differing response to environmental change. The magnitude of biologically mediated CaCO₃ dissolution in the upper ocean remains broadly unrecognized, with implications for both the global alkalinity budget and interpretations of the fossil record. Sediment cores provide a fossil record going back 65 million years, revealing large variation in organism size and diversity likely linked to changes in seawater carbonate chemistry (acidification) and warming. The extent to which shallow, selective dissolution has biased this record remains an important unresolved question. Addressing discrepancies between CaCO₃ production and export from the upper ocean will require renewed focus on both quantifying and understanding the individual and combined contribution of these groups, as well as the biological processes driving shallow dissolution. These efforts are also critical for incorporating a mechanistically resolved CaCO₃ cycle into future climate models, thereby supporting a more integrated view of ocean biogeochemistry under climate change.