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

Purple aerobic anoxygenic phototrophs (AAPs) are the only organisms known to capture light energy to enhance growth only in the presence of oxygen but do not produce oxygen. The highly adaptive AAPs compose more than 10% of the microbial community in some euphotic upper ocean waters and are potentially major contributors to the fixation of the greenhouse gas CO 2 . We present the complete genomic sequence and feature analysis of the AAP Roseobacter denitrificans, which reveal clues to its physiology. The genome lacks genes that code for known photosynthetic carbon fixation pathways, and most notably missing are genes for the Calvin cycle enzymes ribulose bisphosphate carboxylase (RuBisCO) and phosphoribulokinase. Phylogenetic evidence implies that this absence could be due to a gene loss from a RuBisCO-containing ␣-proteobacterial ancestor. We describe the potential importance of mixotrophic rather than autotrophic CO 2 fixation pathways in these organisms and suggest that these pathways function to fix CO 2 for the formation of cellular components but do not permit autotrophic growth. While some genes that code for the redox-dependent regulation of photosynthetic machinery are present, many light sensors and transcriptional regulatory motifs found in purple photosynthetic bacteria are absent.Among the five major phyla containing phototrophic prokaryotes, the purple proteobacteria are the most metabolically diverse. The anaerobic purple photosynthetic bacteria grow photoautotrophically only at low oxygen levels, while at higher oxygen levels, the photosynthetic apparatus is down-regulated, resulting in chemotrophic growth using organic carbon (3). In contrast, some related marine ␣-proteobacteria produce bacteriochlorophyll (BChl) a only in the presence of oxygen in the dark (39). These aerobic anoxygenic phototrophs (AAPs) (also known as AAnP or APB for aerobic phototrophic bacteria) have long been overlooked in oceanic studies, but recent data indicate that their contribution to the global carbon cycle could be significant (22).Typically, AAPs grow photoheterotrophically by respiration of organic substrates (39), resulting in the release of CO 2 , counter to the traditional "CO 2 sink" of the upper ocean. However, light-stimulated CO 2 uptake has been seen in some AAP species (22, 39), and several members of the Roseobacter clade were shown to oxidize CO to CO 2 (6).The marine AAP species Roseobacter denitrificans grows not only photoheterotrophically in the presence of oxygen and light but also anaerobically in the dark using nitrate or trimethylamine N-oxide as an electron acceptor (39). This adaptability has made R. denitrificans the most studied AAP and a model organism for this group.Our study of the R. denitrificans genome reveals a variety of metabolic options available to make this species successful in a competitive oligotrophic marine environment. We also investigated the complement of transcriptional regulators in R. denitrificans that are thought to be responsible for aerobic regulation of photosyn...