Light-Harvesting Strategy during CO2-Dependent Photosynthesis in the Green Alga Chlamydomonas reinhardtii.

To maximize the efficiency of photosynthesis, photosynthetic organisms must properly balance their light-harvesting ability and CO2 utilization. However, the molecular mechanisms of light harvesting under various CO2 conditions remain unclear. To reveal these mechanisms, we performed new analysis on cells of the green alga Chlamydomonas reinhardtii under different CO2 conditions. The analysis combines three kinds of fluorometries: pulse-amplitude modulated fluorescence, steady-state fluorescence with absolute intensity, and time-resolved fluorescence. Under low CO2 conditions, the main regulatory mechanism was migration of a light-harvesting chlorophyll-protein complex (LHC) II from photosystem (PS) II to PSI. However, under CO2-deficient conditions with carbon supplementation, some of the LHCII separated from the PSI and aggregated with quenching. These different light-harvesting abilities of LHCII may play an important role in the regulation of light harvesting in C. reinhardtii under various CO2 conditions.