Diurnally fluctuating pCO2 enhances growth of a coastal strain of Emiliania huxleyi under future-projected ocean acidification conditions

The carbonate chemistry in coastal waters is more variable compared with that of open oceans, both in magnitude and time scale of its fluctuations. However, knowledge of the responses of coastal phytoplankton to dynamic changes in pH/pCO(2) has been scarcely documented. Hence, we investigated the physiological performance of a coastal isolate of the coccolithophore Emiliania huxleyi (PML B92/11) under fluctuating and stable pCO(2) regimes (steady ambient pCO(2), 400 mu atm; steady elevated pCO(2), 1200 mu atm; diurnally fluctuating elevated pCO(2), 600-1800 mu atm). Elevated pCO(2) inhibited the calcification rate in both the steady and fluctuating regimes. However, higher specific growth rates and lower ratios of calcification to photosynthesis were detected in the cells grown under diurnally fluctuating elevated pCO(2) conditions. The fluctuating pCO(2) regime alleviated the negative effects of elevated pCO(2) on effective photochemical quantum yield and relative photosynthetic electron transport rate compared with the steady elevated pCO(2) treatment. Our results suggest that growth of E. huxleyi could benefit from diel fluctuations of pH/pCO(2) under future-projected ocean acidification, but its calcification was reduced by the fluctuation and the increased concentration of CO2, reflecting a necessity to consider the influences of dynamic pH fluctuations on coastal carbon cycles associated with ocean global changes.