Calcification moderates the biochemical responses of Gephyrocapsa oceanica to ocean acidification

Abstract

ABSTRACT We compared the physiological performance of two sub-strains of coccolithophore Gephyrocapsa oceanica NIES-1318, the originally well-calcified strain, and its low-calcified counterpart that significantly decreased the capacity to calcify under present (400 µatm, LC) and elevated pCO2 (1000 µatm, HC) conditions. There were no significant differences in the growth rates between the two sub-strains under LC condition. The growth rates of both sub-strains reduced under HC conditions and the decrease was significantly higher in the high-calcified cells than the low-calcified ones. The low-calcified cultures exhibited reduced production rates of particulate organic carbon (POC) and particulate organic nitrogen (PON), irrespective of CO2 conditions. The decrease in the PON production rates was considerably higher than that in the POC production rates. Compared with the high-calcified cells, a slight decrease was observed in cellular POC contents in the low-calcified cells, whereas cellular PON contents decreased more prominently. The POC/PON ratio showed no significant difference between the two pCO2 treatments in the high-calcified cultures, whereas elevated CO2 increased the rates in the low-calcified cells. We believe that the nitrogen acquisition by low-calcified cells was more susceptible to ocean acidification, probably due to a lack of stabilized microenvironment provided by coccoliths. Our results demonstrate that the calcification level can affect the biochemical responses of G. oceanica to ocean acidification, with likely consequences for marine biogeochemical cycling.