Spaceflight Microgravity Reduced Photosynthetical Electron Transport and Altered Energy Distribution in Euglena gracilis

The effects of 17-day spaceflight microgravity on the photosynthetic activity of Euglena gracilis were investigated during the SIMBOX mission on board the Chinese Shenzhou-8 spacecraft. We found that microgravity decreased Fv/Fm, while chlorophyll a and carotenoid contents were increased. The fluorescence yields were clearly reduced by microgravity, but the shape of fluorescence transients (O-J-I-P) was not changed. The quantum yield of primary photochemistry (cpPo), the quantum yield of electron transport (cpEo) and the performance index of PS II (PIABS and PICS) in microgravity group were decreased, while ABS/RC and DIo/RC were increased. 77K fluorescence emission spectra indicated that microgravity altered energy distribution between PS II and PS I and there are redshifts under microgravity. These results suggested that microgravity may impair photosynthesis by inhibition of acceptor sides of PS II in electron transfer pathway and alter PS I structure to cause a reduction of energy transfer to PS I in Euglena gracilis.