Abstract:

 Light availability controls the photosynthesis and calcification of the coccolithophores in the global ocean, thereby affecting their crucial roles in primary production and the global carbon cycle. How coccoliths impact the light acclimation of coccolithophores remains unclear. Here we investigated the impacts of light on the growth, photosynthesis, calcification, stoichiometry, and carbon isotope fractionation of phytoplankton with different degrees of calcification, to understand the roles of coccoliths in their photo acclimation responses, with the implications for niche partitioning of coccolithophore in the modern ocean. 

  We found that Geophyrocapsa huxleyi and Pleurchrysis carterae better maintained the maximum photochemical yields of photosystem II (Fv/Fm) than the Isochrysis galbana under elevated light intensity, highlighting a better ability of coccolithophores in dealing with potential photodamage. The calcified coccolithophores appear to have higher photosynthetic efficiency than the naked coccolithophores, demonstrated by their consistently higher effective quantum yield of photosystem II (Y(Ⅱ)) and maximum electron transport rates (ETRmax). More flexible elemental stoichiometry was observed in G. huxleyi than I. galbana and P. carterae, suggesting their roles in carbon fixation and nutrient cycles are likely impacted by light availability. Additionally, calcified G. huxleyi fixed more organic carbon with relatively higher δ13C in POC than the naked counterpart, Overall, our data support the beneficial roles of coccoliths in facilitating the coccolithophores in photoacclimation. They protect the coccolithophores under excess light condition, and calcification may serve as a carbon concentrating mechanism (CCM) to supply additional CO2 to promote photosynthesis.
 

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