Photoadaptive strategies in sea-ice microalgae

Sea-ice microalgae photosynthesize and grow under conditions of very low in situ photon fluence rates and temperature.We sought to determine the mechanisms whereby natural sea-ice algal cells in Hudson Bay adapt to changing photon fluence rates at the ice-water interface at the end of the Arctic winter.It was found that the cells positively adapted to the seasonal increase in photon fluence rates by increasing their maximal photosynthetic rate (P,,,,) and the saturating photon fluence rate (I,) Under low photon fluence rates (7.6 to 9.0 pEinst S' l ) , photosynthesis was optimized by increasing the number of photosynthetic units, and at higher photon fluence rates, the algal cells decreased the number of units as well as the concentration of plastoqulnone.An effective absorption cross section was maintained by increasing the size of photosynthetic units through a reduction in the number of reaction centres while the concentration of the light-harvesting pigments chlorophyll a, chlorophyll c and fucoxanthin remained essentially constant.Estimates of the turnover time of photosystem I reaction centres revealed that there was a faster flow of electrons through the electron transport chain in response to the increase in under-ice photon fluence rates.