MEPS 261:111-122 (2003)  -  doi:10.3354/meps261111

ABSTRACT: Precipitation of calcium carbonate by phytoplankton in the photic oceanic layer is an important process regulating the carbon cycling and the exchange of CO₂ at the ocean-atmosphere interface. Previous experiments have demonstrated that, under nutrient-sufficient conditions, doubling the partial pressure of CO₂ (pCO₂) in seawater‹a likely scenario for the end of the century‹can significantly decrease both the rate of calcification by coccolithophorids and the ratio of inorganic to organic carbon production. The present work investigates the effects of high pCO₂ on calcification by the coccolithophore Emiliania huxleyi (Strain TW1) grown under nitrogen-limiting conditions, a situation that can also prevail in the ocean. Nitrogen limitation was achieved in NO₃-limited continuous cultures renewed at the rate of 0.5 d^-1 and exposed to a saturating light level. pCO₂ was increased from 400 to 700 ppm and controlled by bubbling CO₂-rich or CO₂-free air into the cultures. The pCO₂ shift has a rapid effect on cell physiology that occurs within 2 cell divisions subsequent to the perturbation. Net calcification rate (C) decreased by 25% and, in contrast to previous studies with N-replete cultures, gross community production (GCP) and dark community respiration (DCR) also decreased. These results suggest that increasing pCO₂ has no noticeable effect on the calcification/photosynthesis ratio (C/P) when cells of E. huxleyi are NO₃-limited.

KEY WORDS: Calcification · Carbon fixation · Coccolith · Emiliania huxleyi · Nitrate · Alkalinity · CO₂