*Article* **Genome Sequence and Characterization of a Xanthorhodopsin-Containing, Aerobic Anoxygenic Phototrophic** *Rhodobacter* **Species, Isolated from Mesophilic Conditions at Yellowstone National Park**

**John A. Kyndt 1,\* , Sydney Robertson <sup>1</sup> , Isabella B. Shoffstall <sup>2</sup> , Robert F. Ramaley <sup>3</sup> and Terrance E. Meyer 4,†**


**Abstract:** The genus *Rhodobacter* consists of purple nonsulfur photosynthetic alphaproteobacteria known for their diverse metabolic capabilities. Here, we report the genome sequence and initial characterization of a novel *Rhodobacter* species, strain M37P, isolated from Mushroom hot spring runoff in Yellowstone National Park at 37 ◦C. Genome-based analyses and initial growth characteristics helped to define the differentiating characteristics of this species and identified it as an aerobic anoxygenic phototroph (AAP). This is the first AAP identified in the genus *Rhodobacter*. Strain M37P has a pinkish-red pigmentation that is present under aerobic dark conditions but disappears under light incubation. Whole genome-based analysis and average nucleotide identity (ANI) comparison indicate that strain M37P belongs to a specific clade of recently identified species that are genetically and physiologically unique from other representative *Rhodobacter* species. The genome encodes a unique xanthorhodopsin, not found in any other *Rhodobacter* species, which may be responsible for the pinkish-red pigmentation. These analyses indicates that strain M37P is a unique species that is well-adapted to optimized growth in the Yellowstone hot spring runoff, for which we propose the name *Rhodobacter calidifons* sp. nov.

**Keywords:** aerobic anoxygenic phototrophs; AAP; *Rhodobacter*; purple nonsulfur bacteria; hot spring; Yellowstone; xanthorhodopsin
