Emiliania huxleyi response to Ocean Alkalinity Enhancement in Laboratory Culture

In order to limit global warming and reduce ocean acidification, new and more effective methods of carbon capture and storage have to be explored, including ocean alkalinity enhancement (OAE). In principle, OAE increases the ocean's buffering capacity, which could reduce ocean acidification and promote sequestration of atmospheric carbon dioxide (CO2). However, the biological impacts of OAE are not well characterized. Over the summer of 2022 we conducted laboratory experiments to explore how OAE may impact a common species of coccolithophore, Emiliania huxleyi (E. hux), which acts as an important calcifying primary producer that drives ocean carbon and alkalinity cycling. We cultured E. hux (CCMP 374) in seawater f/2-Si media amended to produce three OAE conditions: One unamended control treatment with no alkalinity modification, one unequilibrated group for which we increased media alkalinity (+500 μmol/kg) without equilibration with atmospheric CO2, and one equilibrated group where alkalinity was increased (+500 μmol/kg), followed by bubbling to restore its pCO2 to atmospheric values. In order to assess the response of E. hux we looked at three main factors: growth rate, calcification rate, and photosynthetic health. Growth rates were determined with flow cytometry; calcification rates were determined using a novel 13C-tracer technique; photosynthetic health was assessed by measuring Fv/Fm. Over four generations, the growth rates of all three treatments were similar, with no treatment group outperforming the others on a consistent basis. Calcification rates, cellular calcite quotas, and Fv/Fm all showed no measurable response to OAE. These results imply that unequilibrated and air-equilibrated alkalinity enhancement may have little to no effect on E. hux growth and calcification rates under laboratory culture conditions. Future work should investigate if the lack of a response to OAE continues over longer time periods and under more extreme alkalinity loadings. Natural open-ocean coccolithophore communities should also be investigated in situ. We suggest that the 13C-calcification approach could be used as a cost-effective way to assess calcification rates, both in the laboratory and in situ.