Effects of carbon dioxide on the coccolithophorid Pleurochrysis carterae in incubation experiments

Production (abundance and biomass) and net calcification rates of the coccolithophorid Pleurochrysis carterae under different partial pressures of CO 2 (pCO 2 ) were examined using short (15, 24 and 39 h), long (7 d) and dark (7 d) incubation experiments. Short incubations were conducted at ambient, 500 and 820 ppm pCO 2 levels in natural seawater that was enriched with nutrients and inoculated with P. carterae. Long incubations were conducted at ambient and 1200 ppm pCO 2 levels in natural seawater (0.2 m filtered as well as unfiltered) that was enriched with nutrients and inoculated with P. carterae. Dark incubations were conducted at ambient and 1200 ppm pCO 2 in unfiltered seawater that was inoculated with P. carterae. The abundance and biomass of coccolithophorids increased with pCO 2 and time. The abundance and biomass of most noncalcifying phytoplankton also increased, and were hardly affected by CO 2 inputs. Net calcification rates were negative in short incubations during the pre-bloom phase regardless of pCO 2 levels, indicating dissolution of calcium carbonate. Further, the negative values of net calcification in short incubations became less negative with time. Net calcification rates were positive in long incubations during blooms regardless of pCO 2 level, and the rate of calcification increased with pCO 2 . Our results show that P. carterae may adapt to increased (~1200 ppm) pCO 2 level with time, and such increase has little effect on the ecology of noncalcifying groups and hence in ecosystem dynamics. In dark incubations, net calcification rates were negative, with the magnitude being dependent on pCO 2 levels.