Green Bacteria: Insights into Green Bacterial Evolution through Genomic Analyses

Green bacteria are physiologically and metabolically heterogeneous assemblage of organisms that belong to three phyla of the Domain Bacteria: Chlorobi, Chloroflex, and Acidobacteria. Green bacteria are defined by their ability to synthesize one of three bacteriochlorophylls (BChls) c, d, or e and to assemble these BChls into large antenna structures known as chlorosomes. Over the past decade, the genome sequences of many green bacteria have been completed and analysed. This chapter mainly focuses on key aspects of the chlorophotoautotrophic physiology of members of the phylum Chlorobi. Until recently, this phylum was generally considered to be synonymous with the green sulfur bacteria (Order Chlorobiales). However, genome and metagenome sequencing has shown that the members of this phylum are more diverse than previously imagined. Surprisingly, early-diverging members of this phylum are aerobic chemoorganoheterotrophs and aerobic anoxygenic photoheterotrophs. The evolution of members of the phylum Chlorobi is discussed. Pathways for the synthesis of (bacterio)chlorophylls ((B) Chls) are described and discussed in the context of the Granick hypothesis, which states that biosynthetic pathways recapitulate their evolution. Finally, based upon information obtained from genome sequences and the discovery of the first phototrophic member of the phylum Acidobacteria,"Candidatus Chloracidobacterium thermophilum"("Ca. Cab. thermophilum"), some thoughts on the evolution of photosynthesis are discussed.