Low sensitivity of a heavily calcified coccolithophore under increasing CO ₂ : the case study of <i>Helicosphaera carteri</i>

Abstract. Studies on CO2 effects on coccolithophores, unicellular calcifying phytoplankton, show species-specific responses, although only fewer 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. H. carteri being 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 not significantly affected by increasing CO2, in contrast to other heavily calcified species. The protoplast volume and coccosphere shape of Helicosphaera carteri did not vary with changes in CO2, and neither did its particulate inorganic carbon (PIC) and particulate organic carbon (POC) quotas, nor the PIC:POC ratio, indicating unaltered physiological state. 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.