3.17: Anoxygenic Photosynthesis

Anoxygenic photosynthesis is a phototrophic process that captures light energy to drive carbon fixation without producing molecular oxygen. Unlike oxygenic photosynthesis, which utilizes water as an electron donor and releases oxygen, anoxygenic phototrophs use alternative electron donors such as hydrogen sulfide (H₂S), elemental sulfur (S⁰), or thiosulfate (S₂O₃²⁻). This process is carried out by diverse groups of bacteria, including purple bacteria, green sulfur bacteria, heliobacteria, and other anoxygenic phototrophs, which have evolved specialized photosynthetic mechanisms using a single photosystem.

Differences Between Oxygenic and Anoxygenic Photosynthesis

Oxygenic and anoxygenic photosynthesis differ primarily in electron donors, oxygen production, and photosystem composition. Oxygenic photosynthesis, carried out by cyanobacteria, algae, and plants, uses water (H₂O) as an electron donor, leading to oxygen (O₂) generation as a byproduct. It operates with two photosystems (PSI and PSII) arranged in a Z-scheme, allowing the direct reduction of NADP⁺ to NADPH. In contrast, anoxygenic photosynthesis relies on alternative electron donors such as hydrogen sulfide or organic compounds and does not release oxygen. It functions with a single photosystem, primarily utilizing cyclic electron flow for ATP synthesis, with some bacteria requiring reverse electron transport to generate reducing power. Additionally, anoxygenic phototrophs use bacteriochlorophylls instead of chlorophylls, enabling them to absorb infrared light and thrive in anaerobic or microaerophilic environments.

Photosystems and Electron Transport

Anoxygenic phototrophs rely on bacteriochlorophyll pigments to absorb light energy. These pigments are localized in distinct cellular structures: chromatophores in purple bacteria, chlorosomes in green sulfur bacteria, and other specialized membrane-associated structures in heliobacteria. The energy absorbed by these pigments excites electrons in specialized reaction centers—P870 in purple nonsulfur bacteria, P840 in green sulfur bacteria, and variations in other anoxygenic phototrophs.

In purple bacteria, the excited electrons from the P870 reaction center are transferred sequentially through bacteriopheophytin, a quinone pool, iron-sulfur (FeS) cluster proteins, and cytochromes before returning to the reaction center. This cyclic electron flow generates a proton gradient across the membrane, leading to ATP synthesis via chemiosmosis. Additionally, purple bacteria require an external electron donor, such as hydrogen sulfide, to reduce NAD⁺ to NADH. This reduction is achieved through reverse electron flow, which requires additional energy input from the proton motive force.

Green sulfur bacteria operate with a different electron transport mechanism. The P840 reaction center donates high-energy electrons to a series of acceptors, including bacteriochlorophylls, FeS cluster proteins, quinones, and cytochromes. However, unlike purple bacteria, green sulfur bacteria utilize ferredoxins, which directly transfer electrons to NAD⁺, forming NADH without requiring reverse electron flow. This efficiency allows green sulfur bacteria to thrive in low-light environments, such as deep-sea hydrothermal vents and stratified lakes.

Diversity of Anoxygenic Phototrophs

Besides purple and green sulfur bacteria, other bacterial groups perform anoxygenic photosynthesis. Heliobacteria, belonging to the Firmicutes phylum, use a single photosystem with bacteriochlorophyll g. Unlike other anoxygenic phototrophs, they lack extensive internal membranes and perform photosynthesis in the cytoplasmic membrane. Chloroflexi, also known as green nonsulfur bacteria, employ a mix of phototrophic and heterotrophic metabolism, allowing them to survive in diverse ecological niches.

Ecological and Evolutionary Significance

Anoxygenic photosynthesis is crucial in global biogeochemical cycles by driving carbon and sulfur transformations in anaerobic environments. These bacteria contribute to primary production in oxygen-limited ecosystems and are considered evolutionary precursors to modern oxygenic phototrophs. The ability of different bacterial groups to utilize a variety of electron donors suggests that early photosynthetic life on Earth may have operated under anoxygenic conditions before the advent of water-splitting oxygenic photosynthesis. Furthermore, adapting bacteriochlorophylls to absorb infrared and low-intensity light enables these bacteria to thrive in environments where oxygenic phototrophs cannot compete, such as deep waters, sediments, and hydrothermal systems.