Anoxygenic photosynthesis and photooxidative stress: a particular challenge for Roseobacter

P>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 (1O(2)) 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 1O(2) compared with Rhb. sphaeroides. While photosynthetic membranes of Rsb. denitrificans generated more 1O(2) during light exposure, key regulator genes rpoE and rpoH(II) were more strongly induced in response to 1O(2) 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 1O(2) 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 1O(2) in AAnPs. Our data reveal similarities but also significant differences in the response of Rsb. denitrificans and Rhb. sphaeroides to 1O(2), most likely a consequence of their different lifestyles.