CHROMATIC REGULATION IN CHLAMYDOMONAS-REINHARDTII ALTERS PHOTOSYSTEM STOICHIOMETRY AND IMPROVES THE QUANTUM EFFICIENCY OF PHOTOSYNTHESIS

The work addressed the adjustment of the photosystem ratio in the gree n alga Chlamydomonas reinhardtii. It is shown that green algae, much l ike cyanophytes and higher plants, adjust and optimize the ratio of th e two photosystems in chloroplasts in response to the quality of irrad iance during growth. Such adjustments are compensation reactions and h elp C. reinhardtii to retain a quantum efficiency of oxygen evolution near the theoretical maximum. Results show variable amounts of PS I an d a fairly constant amount of PS II in chloroplasts and suggest that p hotosystem stoichiometry adjustments, occurring in response to the qua lity of irradiance during plant growth, are mainly an adjustment in th e concentration of PS I. The work delineates chromatic effects on chlo rophyll accumulation in the chloroplast of C reinhardtii from those pe rtaining to the regulation of the PS I/PS II ratio. The detection of t he operation of a molecular feedback mechanism for the PS VPS II ratio adjustment in green algae strengthens the notion of the highly conser ved nature of this mechanism among probably all oxygen evolving photos ynthetic organisms. Findings in this work are expected to serve as the basis of future biochemical and mutagenesis experiments for the eluci dation of the photosystem ratio adjustment in oxygenic photosynthesis.