Nitrogen Limitation Enhanced Calcification and Sinking Rate in the Coccolithophorid Gephyrocapsa oceanica Along With Its Growth Being Reduced

Phytoplankton are exposed to different concentrations of nutrients in different waters along with changing light levels during diurnal and seasonal cycles. We grew the coccolithophorid Gephyrocapsa oceanica semi-continuously at different nitrate levels under indoor low and outdoor high light conditions, and found that reduced nitrate availability significantly increased its production of particulate inorganic carbon (PIC), with its growth being reduced. High light treatment suppressed the growth of nitrate-limited cells and their efficiency of N assimilation by up to 63% compared to low light treatment. The combination of high light and nitrate limitation increased contents of PIC per cell due to sustained photochemical energy transfer, resulting in faster sinking rates by up to 82% in comparison with nitrate-repleted cells. Additionally, the sinking rates were positively correlated with ratios of PIC to particulate organic carbon (POC). These results imply that coccolithophores distributed in oligotrophic waters could be more effective as the ballast in aggregates, facilitating particulate organic carbon flux to deeper waters.