Elevated pCO2 enhances under light but reduces in darkness the growth rate of a diatom, with implications for the fate of phytoplankton below the photic zone

Experimentally elevated pCO(2) and the associated pH drop are known to differentially affect many aspects of the physiology of diatoms under different environmental conditions or in different regions. However, contrasting responses to elevated pCO(2) in the dark and light periods of a diel cycle have not been documented. By growing the model diatom Phaeodactylum tricornutum under 3 light levels and 2 different CO2 concentrations, we found that the elevated pCO(2)/pH drop projected for future ocean acidification reduced the diatom's growth rate by 8-25% during the night period but increased it by up to 9-21% in the light period, resulting in insignificant changes in growth over the diel cycle under the three different light levels. The elevated pCO(2) increased the respiration rates irrespective of growth light levels and light or dark periods and enhanced its photosynthetic performance during daytime. With prolonged exposure to complete darkness, simulating the sinking process in the dark zones of the ocean, the growth rates decreased faster under elevated pCO(2), along with a faster decline in quantum yield and cell size. Our results suggest that elevated pCO(2) enhances the diatom's respiratory energy supplies to cope with acidic stress during the night period but enhances its death rate when the cells sink to dark regions of the oceans below the photic zone, with implications for a possible acidification-induced reduction in vertical transport of organic carbon.