Physiological Responses Of Calcifying And Non-calcifying Coccolithophore Strains To Seawater Acidification And UV Radiation

Coccolithophores belong to the phylum Haptophyta and are unicellular calcifying microalgae that not only fix carbon into biomass through photosynthesis but also assimilate carbon into calcite through calcification.Therefore they play a particularly important role in the global carbon cycle.In their natural environment,coccolithophores are subjected to aspects of global change such as increasing ocean acidification(OA)and ultraviolet radiation(UVR;280-400 nm).Recent works had shown that both ocean acidification and UV radiation affected the photosysthesis and calcification processes of coccolithophores.However uncertainties still remain due to variable and partly contradictory results.Therefore,studies investigating the underlying mechanisms are still needed.In this study,the physiological responses of calcified and naked cells of Gephyrocapsa oceanica(NIES-1318)were compared for cultures grown under single or combined OA and UVR conditions.The main results are as follows:The specific growth rates of calcified cells were similar to those of naked cells under indoor conditions(without UVR),indicating that the loss of calcification in the naked cell did not lead to a competitive disadvantage under such conditions.The growth rates of both cell types were lower when transferred to outdoor conditions(with UVR),with the growth rate of naked cells decreasing more than that of calcified cells,which might reflect a protecting role of coccoliths by scattering electro-magnetic radiation(especially UVR).When exposed to short-term high UV radiation,outdoor acclimated cells showed a better photochemical performance compared with indoor acclimated cells,which might reflect an acclimation-persistent influence on the cells physiology.Similar growth rates and Fv/Fm values were observed in both types of cell under LC conditions.Detrimental effects of elevated pCO2 were observed in both cell types,with the growth rate of naked cells decreasing more markedly,which might reflect a negative impact of higher cytoplasmic[H+].When exposed to short-term high UV radiation,similar trends in effective quantum yield were observed in both cell types acclimated to LC conditions.However a higher NPQ was found in naked cells,indicating a greater degree of light stress which might be caused by the loss of coccoliths.Elevated pCO2 strongly reduced effective quantum yield in naked cells but no significant influence was observed in calcified cells.Based on these facts and comparisons with previous studies,we suggest that the negative impact of elevated cytoplasmic[H+]would exacerbate the detrimental effects of UV radiation while the possess of coccoliths attenuated this influence.