Anoxygenic Phototrophic Bacteria

Anoxygenic phototrophic bacteria include four main groups: purple sulfur bacteria, purple nonsulfur bacteria, green sulfur bacteria, and green nonsulfur bacteria. Each group belongs to distinct bacterial phyla and occupies different ecological niches. Purple bacteria contain bacteriochlorophyll a or b, while green bacteria contain bacteriochlorophyll c, d, or e. These bacteria perform photosynthesis without producing oxygen as a byproduct.

Purple sulfur bacteria, belonging to Gammaproteobacteria, thrive in sulfur-rich, anoxic environments like meromictic lakes and marine sediments. They oxidize hydrogen sulfide (H₂S) as an electron donor for photosynthesis, initially producing elemental sulfur (S₀) stored inside their cells. Representative genera include Chromatium. These bacteria contain lamellar intracellular photosynthetic membrane systems with bacteriochlorophyll a or b.

Purple nonsulfur bacteria, classified in Alphaproteobacteria and Betaproteobacteria, thrive in low-sulfur environments unlike their sulfur-rich counterparts. They are metabolically versatile photoheterotrophs, using light for energy and organic compounds for carbon. Some species can grow photoautotrophically using H₂, low H₂S, or Fe²⁺. Both groups share lamellar photosynthetic membranes and bacteriochlorophyll a or b.

Green sulfur bacteria, belonging to phylum Chlorobi, contain bacteriochlorophyll c, d, or e housed in chlorosomes—unique light-harvesting structures absent in purple bacteria. They are strictly anaerobic and use H₂S as an electron donor, storing elemental sulfur outside their cells. Representative species Chlorobaculum tepidum thrives in sulfidic environments with minimal light and uses the reverse citric acid cycle for carbon fixation.

Heliobacteria are gram-positive anoxygenic phototrophic bacteria belonging to phylum Firmicutes with an unusual FeS-type photosystem. They uniquely produce bacteriochlorophyll g, distinct from other anoxygenic phototrophs. These bacteria grow photoheterotrophically, utilizing organic compounds like pyruvate, lactate, acetate, or butyrate. The group includes five genera with rod-shaped or filamentous cell morphologies.

Chloracidobacterium thermophilum, from phylum Acidobacteria, shares chlorosome structures and bacteriochlorophyll a and c with green sulfur bacteria but differs fundamentally in oxygen tolerance. Unlike green sulfur bacteria, C. thermophilum is oxygen-tolerant and grows aerobically. This thermophilic bacterium discovered in thermal springs is a photoheterotroph using short-chain fatty acids but lacks autotrophic capabilities.

Anoxygenic phototrophic bacteria play essential roles in global sulfur and carbon cycling, thriving in diverse ecological niches with low oxygen and high sulfur availability. Purple sulfur bacteria oxidize H₂S to sulfate, while green sulfur bacteria store sulfur externally. These bacteria inhabit meromictic lakes, deep-sea environments, hot springs, and marine sediments, contributing significantly to nutrient transformation in anoxic ecosystems.