The vertical distribution of bacteriochlorophyll a, the numbers of infrared fluorescent cells, and the variable fluorescence signal at 880 nanometers wavelength, all indicate that photosynthetically competent anoxygenic phototrophic bacteria are abundant in the upper open ocean and comprise at least 11% of the total microbial community. These organisms are facultative photoheterotrophs, metabolizing organic carbon when available, but are capable of photosynthetic light utilization when organic carbon is scarce. They are globally distributed in the euphotic zone and represent a hitherto unrecognized component of the marine microbial community that appears to be critical to the cycling of both organic and inorganic carbon in the ocean.

Horizontal gene transfer is important in the evolution of bacterial and archaeal genomes. An interesting genetic exchange process is carried out by diverse phage-like gene transfer agents (GTAs) that are found in a wide range of prokaryotes. Although GTAs resemble phages, they lack the hallmark capabilities that define typical phages, and they package random pieces of the producing cell's genome. In this Review, we discuss the defining characteristics of the GTAs that have been identified to date, along with potential functions for these agents and the possible evolutionary forces that act on the genes involved in their production.The prevalence of horizontal gene transfer (HGT), in which DNA is exchanged between closely or distantly related lineages, has altered the way we think about evolution 1,2 , having profound effects on our views of the evolutionary relationships among living organisms. It has caused a shift from a bifurcating 'tree of life' view to a 'net of life' or 'web of life' view [3][4][5] , in which "highways of gene sharing" (REF. 6) represent major connections, with genes (or DNA segments) viewed as "public goods, available for all organisms to integrate into their genomes" (REF. 7). HGT occurs within and between all three domains of life, and it has been estimated that 0.05-80% of genes in bacterial and archaeal genomes have been affected by HGT, depending on the analytical method used and the genome analysed [8][9][10] . Although HGT is widespread, it is not random: patterns of preferential gene exchange among specific groups of organisms that share ancestry or habitat have been * Present address. aslang@mun.ca; olgazh@dartmouth.edu; j.beatty@mail.ubc.ca Competing interests statementThe authors declare no competing financial interests. 6,9,11,12 . Such patterns are probably a consequence of existing barriers to HGT, which have been reviewed in detail elsewhere 13,14 . FURTHER INFORMATIONAlthough we have known about some HGT mechanisms for decades, novel processes that expand the existing repertoire of gene exchange methods are continually being identified. Transformation was the first mechanism of gene exchange to be discovered 15 , wherein DNA is taken up directly from the environment (reviewed elsewhere 16,17 ). In transduction, DNA transfer is mediated by viral particles (BOX 1). In conjugation, which was discovered in the 1940s 18 , DNA is transferred from a donor to a recipient cell during cell-to-cell contact; conjugation is used in the transfer of plasmids, transposons, integrons, and integrative and conjugative elements (ICEs) [19][20][21] . There are other modes of gene exchange that do not fit well into any of these three canonical mechanisms, including temporary cell fusion followed by chromosomal recombination and plasmid exchange 22 , intercellular connection through nanotubes 23 , and the release of membrane vesicles containing chromosomal, plasmid and phage DNA that can then merge with nearby cells 24 . In addition, a novel mechanism of gene transfer that has feature...

SUMMARY The aerobic anoxygenic phototrophic bacteria are a relatively recently discovered bacterial group. Although taxonomically and phylogenetically heterogeneous, these bacteria share the following distinguishing features: the presence of bacteriochlorophyll a incorporated into reaction center and light-harvesting complexes, low levels of the photosynthetic unit in cells, an abundance of carotenoids, a strong inhibition by light of bacteriochlorophyll synthesis, and the inability to grow photosynthetically under anaerobic conditions. Aerobic anoxygenic phototrophic bacteria are classified in two marine (Erythrobacter and Roseobacter) and six freshwater (Acidiphilium, Erythromicrobium, Erythromonas, Porphyrobacter, Roseococcus, and Sandaracinobacter) genera, which phylogenetically belong to the α-1, α-3, and α-4 subclasses of the class Proteobacteria. Despite this phylogenetic information, the evolution and ancestry of their photosynthetic properties are unclear. We discuss several current proposals for the evolutionary origin of aerobic phototrophic bacteria. The closest phylogenetic relatives of aerobic phototrophic bacteria include facultatively anaerobic purple nonsulfur phototrophic bacteria. Since these two bacterial groups share many properties, yet have significant differences, we compare and contrast their physiology, with an emphasis on morphology and photosynthetic and other metabolic processes.

The abundance of life on Earth is almost entirely due to biological photosynthesis, which depends on light energy. The source of light in natural habitats has heretofore been thought to be the sun, thus restricting photosynthesis to solar photic environments on the surface of the Earth. If photosynthesis could take place in geothermally illuminated environments, it would increase the diversity of photosynthetic habitats both on Earth and on other worlds that have been proposed to possibly harbor life. Green sulfur bacteria are anaerobes that require light for growth by the oxidation of sulfur compounds to reduce CO 2 to organic carbon, and are capable of photosynthetic growth at extremely low light intensities. We describe the isolation and cultivation of a previously unknown green sulfur bacterial species from a deep-sea hydrothermal vent, where the only source of light is geothermal radiation that includes wavelengths absorbed by photosynthetic pigments of this organism.photosynthesis ͉ anoxygenic ͉ green sulfur bacterium ͉ evolution ͉ habitat