Hydrogen sulfide can inhibit and enhance oxygenic photosynthesis in a cyanobacterium from sulfidic springs

Abstract

Summary We used microsensors to investigate the combinatory effect of hydrogen sulfide ( H 2 S ) and light on oxygenic photosynthesis in biofilms formed by a cyanobacterium from sulfidic springs. We found that photosynthesis was both positively and negatively affected by H 2 S : (i) H 2 S accelerated the recovery of photosynthesis after prolonged exposure to darkness and anoxia. We suggest that this is possibly due to regulatory effects of H 2 S on photosystem I components and/or on the C alvin cycle. (ii) H 2 S concentrations of up to 210 μM temporarily enhanced the photosynthetic rates at low irradiance. Modelling showed that this enhancement is plausibly based on changes in the light‐harvesting efficiency. (iii) Above a certain light‐dependent concentration threshold H 2 S also acted as an inhibitor. Intriguingly, this inhibition was not instant but occurred only after a specific time interval that decreased with increasing light intensity. That photosynthesis is most sensitive to inhibition at high light intensities suggests that H 2 S inactivates an intermediate of the oxygen evolving complex that accumulates with increasing light intensity. We discuss the implications of these three effects of H 2 S in the context of cyanobacterial photosynthesis under conditions with diurnally fluctuating light and H 2 S concentrations, such as those occurring in microbial mats and biofilms.

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