Evolutionary Divergence of Marine Aerobic Anoxygenic Phototrophic Bacteria as Seen from Diverse Organisations of Their Photosynthesis Gene Clusters

Aerobic anoxygenic phototrophic bacteria (AAPB) represent an important group of microorganisms inhabiting euphotic zones of the oceans, freshwater or saline lakes. They harvest light using bacterial reaction centres (RCs) containing bacteriochlorophyll (BChl). Their photosynthetic apparatus is encoded by a number of genes organised in a so-called photosynthesis gene cluster (PGC). In this chapter, the organisation of PGCs in 10 AAPB strains belonging to the Roseobacter, Eryth Citro and NOR5/OM60 clades is summarised. The distribution of four types of gene arrangements, based on permutation and combination of the two conserved regions bchFNBHLM-Iha-puhABC and crtF-bchCXYZ does not correspond to the phylogenetic affiliation of individual AAPB. While PGCs of all analysed species contain the same set of genes for BChl synthesis and assembly of photosynthetic centres, they differ greatly in the carotenoid biosynthetic genes. Spheroidenone, spirilloxanthin, and zeaxanthin biosynthetic pathways were found separately in each clade. The AAPB in Roseobacter and the NOR5/OM60 clades are closely related to purple non-sulfur bacteria. To date, only the AAPB in the Eryth Citro clade are obligately aerobic phototrophs with unique carotenoid biosynthetic pathways and pigments, compared to the other clades. Interestingly, members of the Eryth Citro clade possess the shortest and simplest PGC structure of the known AAPB, and the light-harvesting complex 2 genes are completely absent; perhaps this indicates that the AAPB in this clade arose more recently than their counterparts in other clades. Comparison of two closely related Citromicrobial genomes (98.1% sequence identity of complete 16S rRNA genes), Citromicrobium sp. JL354, which contains two copies of RC genes, and Citromicrobial strain JLT1363, which is chemotrophic, revealed evidence for the loss of photosynthesis genes. The incomplete PGC (pufLMC-puhCBA) in strain JL354 was located within a putative integrating conjugative element, which indicates a potential mechanism for the horizontal transfer of genes for phototrophy.