Comparative analyses of H 2 photoproduction in magnesium- and sulfur-starved Chlamydomonas reinhardtii cultures.

Magnesium (Mg)-deprived Chlamydomonas reinhardtii cells are capable to sustain hydrogen (H-2) photoproduction at relatively high photosystem II (PSII) activity levels for an extended time period as compared with sulfur (S)-deprived cells. Herein, we present a comparative study of H-2 photoproduction induced by Mg and S shortage to unravel the specific rearrangements of the photosynthetic machinery and cell metabolism occurring under the two deprivation protocols. The exhaustive analysis of photosynthetic activity and regulatory pathways, respiration and starch metabolism revealed the specific rearrangements of the photosynthetic machinery and cellular metabolism, which occur under the two deprivation conditions. The obtained results allowed us to conclude that the expanded time period of H-2 production upon Mg-deprivation is due to the less harmful effects that Mg-depletion has on viability and metabolic performance of the cells. Unlike S-deprivation, the photosynthetic light and dark reactions in Mg-deprived cells remained active over the whole H-2 production period. However, the elevated PSII activity in Mg-deprived cells was counteracted by the operation of pathways for O-2 consumption that maintain anaerobic conditions in the presence of active water splitting.