Ocean warming modulates the effects of acidification on Emiliania huxleyi calcification and sinking

Ongoing ocean warming and acidification are tied to the rapid accumulation of human-induced carbon dioxide (CO₂) in the atmosphere and subsequent uptake of heat and CO₂ by the surface ocean. These processes are expected to drive large changes in marine ecosystems. While numerous studies have examined the effects of ocean acidification on coccolithophores, less is known on their combined effect. In this study, we investigate temperature modulation of the carbonate chemistry sensitivity of the coccolithophore Emiliania huxleyi (RCC1827 from the Western Mediterranean) in a culture experiment. We analyzed the responses of coccolith morphology, particulate inorganic and organic carbon production, and sinking rate of individual cells. E. huxleyi was exposed to three CO₂ levels (ca. 400 μatm, 900 μatm, and 1400 μatm) at 15°C and 20°C. Temperature adds to the negative effect of increasing pCO₂ on coccolith morphology, suggesting that a significant number of E. huxleyi strains might suffer from a temperature increase, hampering their evolutionary success. Temperature amplified the positive effect of increasing pCO₂ on organic carbon production, while modulating the response of calcification rates, indicating that the response to increasing pCO₂ must be taken with caution depending on the temperature range studied. Sinking rates were positively correlated with temperature, whereas pCO₂ did not have any effect. The combined effect of carbonate chemistry and temperature on the E. huxleyi ratio between particulate inorganic carbon and particulate organic carbon (PIC/POC) might also lower the sinking rate of aggregates. In conclusion, in a warmer and more acidified ocean, individual coccolithophore cells might sink faster, while aggregates might sink slower.