**Phylogeny of Anoxygenic Photosynthesis Based on Sequences of Photosynthetic Reaction Center Proteins and a Key Enzyme in Bacteriochlorophyll Biosynthesis, the Chlorophyllide Reductase**

#### **Johannes F. Imho**ff **1,\*, Tanja Rahn 1, Sven Künzel 2 and Sven C. Neulinger 3**


Received: 29 October 2019; Accepted: 15 November 2019; Published: 19 November 2019

**Abstract:** Photosynthesis is a key process for the establishment and maintenance of life on earth, and it is manifested in several major lineages of the prokaryote tree of life. The evolution of photosynthesis in anoxygenic photosynthetic bacteria is of major interest as these have the most ancient roots of photosynthetic systems. The phylogenetic relations between anoxygenic phototrophic bacteria were compared on the basis of sequences of key proteins of the type-II photosynthetic reaction center, including PufLM and PufH (PuhA), and a key enzyme of bacteriochlorophyll biosynthesis, the light-independent chlorophyllide reductase BchXYZ. The latter was common to all anoxygenic phototrophic bacteria, including those with a type-I and those with a type-II photosynthetic reaction center. The phylogenetic considerations included cultured phototrophic bacteria from several phyla, including *Proteobacteria* (138 species), *Chloroflexi* (five species), *Chlorobi* (six species), as well as *Heliobacterium modesticaldum (Firmicutes)*, *Chloracidobacterium acidophilum (Acidobacteria),* and *Gemmatimonas phototrophica (Gemmatimonadetes)*. Whenever available, type strains were studied. Phylogenetic relationships based on a photosynthesis tree (PS tree, including sequences of PufHLM-BchXYZ) were compared with those of 16S rRNA gene sequences (RNS tree). Despite some significant differences, large parts were congruen<sup>t</sup> between the 16S rRNA phylogeny and photosynthesis proteins. The phylogenetic relations demonstrated that bacteriochlorophyll biosynthesis had evolved in ancestors of phototrophic green bacteria much earlier as compared to phototrophic purple bacteria and that multiple events independently formed different lineages of aerobic phototrophic purple bacteria, many of which have very ancient roots. The *Rhodobacterales* clearly represented the younges<sup>t</sup> group, which was separated from other *Proteobacteria* by a large evolutionary gap.

**Keywords:** phylogeny; photosynthetic reaction center proteins; bacteriochlorophyll biosynthesis; phototrophic purple bacteria; evolution of anoxygenic photosynthesis
