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 O-2 evolution drop below those Of O-2 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 H-2 gas. In the course of such H-2-gas production cells consume substantial amounts of internal starch and protein. Such catabolic reactions may sustain, directly or indirectly, the H-2-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 Fehydrogenase 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 H-2 production, Ellipsoid-shaped cells (normal photosynthesis) gave way to larger and spherical cell shapes in the initial stages of S deprivation and H-2 production, followed by cell- mass reductions after longer S-deprivation and H-2-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 H-2-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 H-2-production process.