Anoxygenic Photosynthesis

Anoxygenic photosynthesis is a light-driven process that converts carbon dioxide into organic compounds without producing oxygen, using alternative electron donors like hydrogen sulfide instead of water. Unlike oxygenic photosynthesis, which uses two photosystems and releases oxygen, anoxygenic phototrophs operate with a single photosystem and use bacteriochlorophylls to absorb infrared light, enabling survival in anaerobic environments where oxygenic phototrophs cannot compete.

Anoxygenic phototrophs use alternative electron donors such as hydrogen sulfide, elemental sulfur, thiosulfate, and organic compounds. These diverse electron donors allow bacteria like purple sulfur bacteria and green sulfur bacteria to thrive in anaerobic or microaerophilic environments where water-splitting oxygenic photosynthesis cannot occur, making them crucial for carbon and sulfur cycling in oxygen-limited ecosystems.

Purple bacteria excite electrons at the P870 reaction center, transferring them through bacteriopheophytin, quinones, iron-sulfur cluster proteins, and cytochromes in a cyclic pathway. This electron flow generates a proton gradient that drives chemiosmosis and ATP synthesis. Additionally, purple bacteria use reverse electron flow powered by the proton motive force to reduce NAD+ to NADH using external electron donors like hydrogen sulfide.

Green sulfur bacteria use a P840 reaction center and transfer electrons through bacteriochlorophylls, iron-sulfur cluster proteins, quinones, and cytochromes. Unlike purple bacteria, green sulfur bacteria employ ferredoxins to directly reduce NAD+ to NADH without requiring reverse electron flow. This efficiency allows them to thrive in low-light environments such as deep-sea hydrothermal vents and stratified lakes.

Bacteriochlorophyll pigments are localized in specialized cellular structures depending on the bacterial type. Purple bacteria contain them in chromatophores, green sulfur bacteria in chlorosomes, and heliobacteria in the cytoplasmic membrane. These structures concentrate pigments to efficiently capture light energy and drive the electron transport chain components necessary for photosynthesis in anaerobic conditions.

Anoxygenic photosynthesis drives carbon and sulfur transformations in anaerobic environments, contributing to primary production in oxygen-limited ecosystems. These bacteria are considered evolutionary precursors to modern oxygenic phototrophs, suggesting early photosynthetic life operated under anoxygenic conditions. Their ability to absorb infrared and low-intensity light enables them to occupy ecological niches unavailable to oxygenic phototrophs, such as deep waters and sediments.

Heliobacteria, belonging to the Firmicutes phylum, use bacteriochlorophyll g and perform photosynthesis in the cytoplasmic membrane without extensive internal membrane structures. Chloroflexi, or green nonsulfur bacteria, employ a mix of phototrophic and heterotrophic metabolism, allowing survival in diverse ecological niches. These diverse bacterial groups demonstrate the evolutionary adaptability of anoxygenic photosynthetic mechanisms across different environments.