Anoxygenic Phototrophic Bacteria

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.