Biochemical and morphological characterization of sulfur-deprived and hydrogen-producing Chlamydomonas reinhardtii (green algae)

Abstract

Sulfur-deprivation in green algae causes a reversible inhibition in the activity of the H2O-oxidizing PSII complex. Within 24 h of S-deprivation, rates of photosynthetic O2-evolution drop below those of O2-consumption by respiration. In consequence, sealed cultures of Chlamydomonas reinhardtii become anaerobic in the light, enzymes of the hydrogenase pathway are induced, followed by photosynthetic H2 gas production. Concomitant with such H2-production, cells consume significant amounts of internal starch and protein. Such catabolic activity may sustain, directly or indirectly, the H2-production process. Profile analysis of selected photosynthetic proteins showed a precipitous decline in the amount of Rubisco as a function of time in S-deprivation, a more gradual decline in the level of PSII and PSI proteins, and change in the composition of the LHC-II. It is concluded that, in the absence of Rubisco (t>48 h in S-deprivation), electrons derived upon a residual H2O-oxidation activity of PSII feed into the hydrogenase pathway, thereby contributing to the H2-production process. Increase in the level of the enzyme Fe-hydrogenase was noted during the initial stages of S-deprivation (0-80 h) followed by a decline in the level of this enzyme during longer (t>80 h) S-deprivation times. Microscopic observations showed distinct morphological changes in C. reinhardtii. Ellipsoid-shaped cells (normal photosynthesis) gave way to spherical and larger cell shapes during the initial stages of S-deprivation (0-24 h), followed by cell mass reductions at longer S-deprivation and H2-production times. These biochemical and morphological changes are discussed in terms of cellular physiology in the S-deprived and H2-producing Chlamydomonas reinhardtii.