Low sensitivity of a heavily-calcified coccolithophore under increasing CO₂: the case study of Helicosphaera carteri

Abstract. Studies on CO₂ effects on coccolithophores, unicellular calcifying phytoplankton, show species-specific responses, although only less than 5 % of the ~280 living species have been tested so far. Helicosphaera carteri significantly contributes to carbon fluxes and CaCO₃ storage due to its size and high calcite production. Despite its importance, few studies have examined H. carteri under experimental conditions, and only one has addressed the effects of rising CO₂/decreasing pH. Being H. carteri a large-sized, obligated calcifier species, an important aspect to understand is how changes in seawater carbonate chemistry may affect its morphology. It has already been suggested for other coccolithophores species, that the presence of malformed coccoliths may represent a disadvantage for these organisms. Moreover, an alteration in coccolith morphology may affect their contribution to CaCO₃ sedimentation and ballasting. As for H. carteri, it has also been suggested that due to its high PIC:POC ratio, the species could show a high-sensitivity to CO₂ rise. In this study, we investigate for the first time whether high pCO₂/low pH does affect the morphology of H. carteri coccoliths, by culturing this species under pre-industrial CO₂ levels (~295 µatm) and ~600 µatm, i.e., the SSP 2-4.5 scenario for 2100 (IPCC, 2021). We also analyzed cellular PIC and POC quotas using morphometric data, roundness, and protoplast and coccosphere size to observe the pCO₂ influence on the calcification and photosynthesis ratio.

Our results indicate that H. carteri morphology is only slightly affected by increasing CO₂, in contrast to other heavily calcified species. Helicosphaera carteri protoplast and coccosphere shapes did not vary with changes in CO₂, indicating unaltered general health. The low PIC:POC ratio found in this work for H. carteri compared to ratios previously measured in the same strain under different experimental conditions, and compared to other highly-calcified species, could explain the observed low sensitivity of H. carteri to CO₂. Moreover, the observation of a stable ratio between calcification and photosynthesis in H. carteri under increasing CO₂ might suggest a constant contribution to the rain ratio under climate change. However, further studies comparing experimental and field data from past ocean acidification events will be required to confirm the conclusions drawn here.