Biochemical and morphological characterization of sulfur-deprived and H2-producing Chlamydomonas reinhardtii (green alga).

Sulfur deprivation in green algae causes reversible inhibition of photosynthetic activity. In the absence of S, rates of photosynthetic O2 evolution drop below those of O2 consumption by respiration. As a consequence, sealed cultures of the green alga Chlamydomonas reinhardtii become anaerobic in the light, induce the "Fe-hydrogenase" pathway of electron transport and photosynthetically produce H2 gas. In the course of such H2-gas production cells consume substantial amounts of internal starch and protein. Such catabolic reactions may sustain, directly or in directly, the H2-production process. Profile analysis of selected photosynthetic proteins showed a precipitous decline in the amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) as a function of time in S deprivation, a more gradual decline in the level of photosystem (PS) II and PSI proteins, and a change in the composition of the PSII light-harvesting complex (LHC-II). An increase in the level of the enzyme Fe-hydrogenase was noted during the initial stages of S deprivation (0-72 h) followed by a decline in the level of this enzyme during longer (t >72 h) S-deprivation times. Microscopic observations showed distinct morphological changes in C. reinhardtii during S deprivation and H2 production. Ellipsoid-shaped cells (normal photosynthesis) gave way to larger and spherical cell shapes in the initial stages of S deprivation and H2 production, followed by cell mass reductions after longer S-deprivation and H2-production times. It is suggested that, under S-deprivation conditions, electrons derived from a residual PSII H2O-oxidation activity feed into the hydrogenase pathway, thereby contributing to the H2-production process in Chlamydomonas reinhardtii. Interplay between oxygenic photosynthesis, mitochondrial respiration, catabolism of endogenous substrate, and electron transport via the hydrogenase pathway is essential for this light-mediated H2-production process.