**Adaptation to Photooxidative Stress: Common and Special Strategies of the Alphaproteobacteria** *Rhodobacter sphaeroides* **and** *Rhodobacter capsulatus*

**Mathieu K. Licht 1, Aaron M. Nuss 2, Marcel Volk 1,3, Anne Konzer 4, Michael Beckstette 5, Bork A. Bergho** ff **1,\* and Gabriele Klug 1,\***


Received: 28 January 2020; Accepted: 13 February 2020; Published: 19 February 2020

**Abstract:** Photosynthetic bacteria have to deal with the risk of photooxidative stress that occurs in presence of light and oxygen due to the photosensitizing activity of (bacterio-) chlorophylls. Facultative phototrophs of the genus *Rhodobacter* adapt the formation of photosynthetic complexes to oxygen and light conditions, but cannot completely avoid this stress if environmental conditions suddenly change. *R. capsulatus* has a stronger pigmentation and faster switches to phototrophic growth than *R. sphaeroides*. However, its photooxidative stress response has not been investigated. Here, we compare both species by transcriptomics and proteomics, revealing that proteins involved in oxidation–reduction processes, DNA, and protein damage repair play pivotal roles. These functions are likely universal to many phototrophs. Furthermore, the alternative sigma factors RpoE and RpoHII are induced in both species, even though the genetic localization of the *rpoE* gene, the RpoE protein itself, and probably its regulon, are di fferent. Despite sharing the same habitats, our findings also sugges<sup>t</sup> individual strategies. The *crtIB-tspO* operon, encoding proteins for biosynthesis of carotenoid precursors and a regulator of photosynthesis, and *cbiX*, encoding a putative ferrochelatase, are induced in *R. capsulatus*. This specific response might support adaptation by maintaining high carotenoid-to-bacteriochlorophyll ratios and preventing the accumulation of porphyrin-derived photosensitizers.

**Keywords:** *Rhodobacter capsulatus*; *Rhodobacter sphaeroides*; photooxidative stress; transcriptomics; proteomics; stress defense
