BackgroundAerobic anoxygenic photototrophic (AAP) bacteria represent an important group of marine microorganisms inhabiting the euphotic zone of the ocean. They harvest light using bacteriochlorophyll (BChl) a and are thought to be important players in carbon cycling in the ocean.Methodology/Principal FindingsAerobic anoxygenic phototrophic (AAP) bacteria represent an important part of marine microbial communities. Their photosynthetic apparatus is encoded by a number of genes organized in a so-called photosynthetic gene cluster (PGC). In this study, the organization of PGCs was analyzed in ten AAP species belonging to the orders Rhodobacterales, Sphingomonadales and the NOR5/OM60 clade. Sphingomonadales contained comparatively smaller PGCs with an approximately size of 39 kb whereas the average size of PGCs in Rhodobacterales and NOR5/OM60 clade was about 45 kb. The distribution of four arrangements, based on the permutation and combination of the two conserved regions bchFNBHLM-LhaA-puhABC and crtF-bchCXYZ, does not correspond to the phylogenetic affiliation of individual AAP bacterial species. While PGCs of all analyzed species contained the same set of genes for bacteriochlorophyll synthesis and assembly of photosynthetic centers, they differed largely in the carotenoid biosynthetic genes. Spheroidenone, spirilloxanthin, and zeaxanthin biosynthetic pathways were found in each clade respectively. All of the carotenoid biosynthetic genes were found in the PGCs of Rhodobacterales, however Sphingomonadales and NOR5/OM60 strains contained some of the carotenoid biosynthetic pathway genes outside of the PGC.Conclusions/SignificanceOur investigations shed light on the evolution and functional implications in PGCs of marine aerobic anoxygenic phototrophs, and support the notion that AAP are a heterogenous physiological group phylogenetically scattered among Proteobacteria.

We studied the distribution of anoxygenic phototrophs in 23 steppe lakes in the Transbaikal region (Russia), in Uzbekistan (Central Asia) and in the Crimean peninsula (Ukraine). The lakes varied in their mineral content and composition (salinities from 0.2 to 300 g L À1 ). The Transbaikal lakes were alkaline (pH 4 9), with high amounts of soda. The Uzbek and Crimean lakes were more pH neutral, frequently with high amounts of sulfates. The presence of anoxygenic phototrophs was registered by infrared epifluorescence microscopy, infrared fluorometry and pigment analyses. In mostly shallow, fully oxic lakes, the anoxygenic phototrophs represented 7-65% of the total prokaryotes, with the maxima observed in Transbaikal soda lakes Gorbunka (32%), Khilganta (65%), Zanday (58%) and Zun-Kholvo (46%). Some of the lakes contained over 1 mg bacteriochlorophyll L À1 .In contrast, only small amounts of anoxygenic phototrophs were present in highly mineralized lakes ( 4 100 g total salts L À1 ); Borzinskoe, Tsagan-Nur (Transbaikal), Staroe (Crimea) and in the residual part of the south-west Aral Sea (Uzbekistan). The oxic environment and the specific diurnal changes of bacteriochlorophyll concentration observed suggest that the phototrophic community was mostly composed of aerobic anoxygenic phototrophs. The high abundances and bacteriochlorophyll concentrations point to an important role of aerobic anoxygenic phototrophs in the habitats studied.

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