Effects of varying growth irradiance and nitrogen sources on calcification and physiological performance of the coccolithophore Gephyrocapsa oceanica grown under nitrogen limitation

Gephyrocapsa oceanica is a widespread species of coccolithophore that has a significant impact on the global carbon cycle through photosynthesis and calcium carbonate precipitation. We investigated combined effects of light (50 μmol m−2 s−1, 190 μmol m−2 s−1, and 400 μmol m−2 s−1) and the nitrogen sources NO3− and NH4+ on its physiological performance under nitrogen-limited conditions. The specific growth rate was highest at the mid-range light level of 190 μmol m−2 s−1, where it was further accelerated by NH4+ relative to NO3−. There were no significant growth rate differences between NO3−- and NH4+-grown cells at the two light levels either above or below this optimum irradiance. Cellular particulate organic carbon (POC) and nitrogen (PON) content were not significantly affected by different light intensities and nitrogen sources. However, both the cellular particulate inorganic carbon (PIC) content and the PIC to POC ratio were greatly decreased by increased light levels, and were further decreased by NH4+ only at the highest light level. Non-photochemical quenching (NPQ) increased with increasing light intensity, and was higher in NO3− rather than in NH4+-grown cells at medium and high light intensities. Our results demonstrate that under low, relatively realistic oceanic nitrogen concentrations, increasing light intensity and the replacement of NO3− by NH4+ would have a significant negative effect on the calcification of the coccolithophore G. oceanica. If these findings are also applicable to other coccolithophore species, the future ocean carbon cycle may be greatly affected.