Anoxygenic photosynthesis and photooxidative stress: a particular challenge for Roseobacter.

Roseobacter clade aerobic anoxygenic phototrophic bacteria (AAnP) are abundant in photic zone environments of marine ecosystems. These bacteria form a photosynthetic apparatus at oxygen saturation, a situation expected to generate high levels of singlet oxygen (¹O₂) when light is present. Rhodobacter sphaeroides, an anaerobic anoxygenic phototroph, represses photosynthesis genes at high oxygen tension. Here we report that Roseobacter denitrificans showed higher sensitivity to ¹O₂ compared with Rhb. sphaeroides. While photosynthetic membranes of Rsb. denitrificans generated more ¹O₂ during light exposure, key regulator genes rpoE and rpoH(II) were more strongly induced in response to ¹O₂ stress compared with Rhb. sphaeroides. The regulon controlled by RpoE was different in Rsb. denitrificans and Rhb. sphaeroides. Patterns of synthesized soluble proteins strongly changed upon high light exposure in Rsb. denitrificans but not in Rhb. sphaeroides, and most changes were not further promoted by artificial ¹O₂ generation. The strong increase of small RNA RDs2461 levels by photooxidative stress implies a role for sRNAs in post-transcriptional regulation of the response to ¹O₂ in AAnPs. Our data reveal similarities but also significant differences in the response of Rsb. denitrificans and Rhb. sphaeroides to ¹O₂, most likely a consequence of their different lifestyles.