Abstract
Like a majority of photosynthetic microorganisms, the green unicellular alga Chlamydomonas reinhardtii may encounter O2 deprived conditions on a regular basis. In response to anaerobiosis or in a respiration defective context, the photosynthetic electron transport chain of Chlamydomonas is remodeled by a state transition process to a conformation that favours the photoproduction of ATP at the expense of reductant synthesis. In some unicellular green algae including Chlamydomonas, anoxia also triggers the induction of a chloroplast-located, oxygen sensitive hydrogenase, which accepts electrons from reduced ferredoxin to convert protons into molecular hydrogen. Although microalgal hydrogen evolution has received much interest for its biotechnological potential, its physiological role remains unclear. By using specific Chlamydomonas mutants, we demonstrate that the state transition ability and the hydrogenase function are both critical for induction of photosynthesis in anoxia. These two processes are thus important for survival of the cells when they are transiently placed in an anaerobic environment.
Highlights
Despite their ability to produce oxygen by photosynthesis, oxygenic autotrophs can encounter hypoxia or even complete anoxia in a number of situations
By analyzing activation of photosynthetic electron transport and O2 evolution in wild-type and in mutants devoid of hydrogenase activity or locked in state 1, or having both defects, we show that hydrogenase activity and state transition ability are both critical for activation of photosynthesis in the absence of O2
Our present observations show that the activation of linear photosynthetic electron transport and O2 evolution in anoxic Chlamydomonas cells is influenced by the electron sink capacity in the chloroplast, independently of the state transition process
Summary
Despite their ability to produce oxygen by photosynthesis, oxygenic autotrophs can encounter hypoxia or even complete anoxia in a number of situations. This is true during night for microalgae, which may be part of dense O2 respiring microbial communities [1]. In response to these situations, a lot of photosynthetic micro-organisms are well adapted to hypoxic or anoxic conditions. Anaerobically incubated plant chloroplasts, light fails to induce carbon assimilation unless small amounts of PSI electron acceptors such as O2, oxaloacetate or nitrite are added to the preparation [11]
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