Anoxygenic phototrophic bacteria are a diverse group of microorganisms that perform photosynthesis without producing oxygen. They primarily include purple sulfur bacteria, purple nonsulfur bacteria, green sulfur bacteria, and green nonsulfur bacteria. These bacteria are classified into the Gammaproteobacteria, Alphaproteobacteria, Betaproteobacteria, Chlorobi, and Chloroflexi lineages, each with distinct physiological and ecological adaptations.
Purple sulfur bacteria belong to the Gammaproteobacteria and thrive in sulfur-rich, anoxic environments such as meromictic lakes, marine sediments, and salt marshes. They oxidize hydrogen sulfide (H₂S) as an electron donor for photosynthesis, initially producing elemental sulfur (S₀), which is stored inside the cells, and later oxidizing it to sulfate (SO₄²⁻). Representative genera include Chromatium and Thiocapsa. These bacteria contain bacteriochlorophyll a or b and utilize a Q-type photosystem for light capture.
Purple nonsulfur bacteria, found in Alphaproteobacteria and Betaproteobacteria, are metabolically versatile and thrive in environments with low H₂S levels. They typically function as photoheterotrophs, using light for energy and organic compounds for carbon, but some species can grow photoautotrophically using H₂, low H₂S, or Fe²⁺. They also possess Q-type photosystems and contain bacteriochlorophyll a or b. Notable genera include Rhodobacter and Rhodospirillum.
Green sulfur bacteria, classified in Chlorobi, are strictly anaerobic, nonmotile phototrophs that use H₂S as an electron donor. Unlike purple sulfur bacteria, they store S₀ outside the cells and rely on the reverse citric acid cycle for CO₂ fixation. They contain bacteriochlorophyll c, d, or e, housed in chlorosomes—unique light-harvesting structures. Chlorobaculum tepidum is a well-studied species that thrives in sulfidic environments with minimal light.
Green nonsulfur bacteria, or filamentous anoxygenic phototrophs, belong to Chloroflexi. They primarily grow as photoheterotrophs but can also perform photoautotrophy using H₂ or H₂S. Their photosynthetic system is a hybrid between those of purple and green sulfur bacteria, containing both a Q-type photosystem and chlorosomes. Chloroflexus aurantiacus is a representative species, commonly found in hot springs. Thermomicrobium, a related nonphototrophic genus, is a thermophile with unusual membrane lipids.
Heliobacteria are anoxygenic phototrophic gram-positive bacteria belonging to the phylum Firmicutes. They possess an FeS-type photosystem and uniquely produce bacteriochlorophyll g. These bacteria grow photoheterotrophically, utilizing organic compounds such as pyruvate, lactate, acetate, or butyrate. The group includes five genera: Heliobacterium, Heliophilum, Heliorestis, Heliomonas, and Heliobacillus. They exist as rod-shaped or filamentous cells, with Heliophilum forming motile bundles.
Another distinct group of anoxygenic phototrophs, Chloracidobacterium thermophilum, belongs to the phylum Acidobacteria. This thermophilic bacterium was discovered in microbial mats of thermal springs. It shares some traits with green sulfur bacteria, such as producing bacteriochlorophyll a and c within chlorosomes and utilizing an FeS-type photosystem. However, unlike green sulfur bacteria, C. thermophilum is oxygen-tolerant and can grow aerobically. It is a photoheterotroph that uses short-chain fatty acids as a carbon source but lacks autotrophic capabilities.
These bacteria play essential roles in global sulfur and carbon cycling, thriving in diverse ecological niches with low oxygen and high sulfur availability.
Anoxygenic phototrophic bacteria that do not generate oxygen as a product of photosynthesis include purple sulfur, purple nonsulfur, green sulfur, and green nonsulfur bacteria.
Purple sulfur bacteria belong to several genera in the class Gammaproteobacteria, including Chromatium. They are found predominantly in sulfur-rich meromictic lakes and typically have sulfur granules in the cells.
In contrast, purple nonsulfur bacteria, classified into Alpha and Betaproteobacteria, thrive in low-sulfur environments. A key genus is Rhodomicrobium.
Both groups of purple bacteria have lamellar intracellular photosynthetic membrane systems and contain bacteriochlorophyll a or b.
Green sulfur bacteria belong to the phylum Chlorobi. They contain bacteriochlorophyll c, d, or e, housed in chlorosomes, as seen in the representative species Chlorobaculum tepidum. They thrive in sulfur-rich and deep-sea environments.
Green nonsulfur bacteria are classified into the phylum Chloroflexi. Most species, including Chloroflexus aurantiacus, are filamentous and contain bacteriochlorophylls a or c for photosynthesis.
繁體中文
