Anoxygenic photosynthesis is a light-driven process utilized by certain microorganisms to convert light energy into chemical energy. Unlike the more commonly known oxygenic photosynthesis performed by plants, algae, and cyanobacteria, this process does not produce oxygen as a byproduct. Instead, it relies on alternative electron donors to power its metabolic reactions, generating ATP and reducing power for various cellular functions.
The Core Process of Anoxygenic Photosynthesis
This form of photosynthesis differs from oxygenic photosynthesis in its electron donor. While oxygenic photosynthesis uses water (H₂O), anoxygenic phototrophs utilize other reduced substances. Common examples include hydrogen sulfide (H₂S), elemental sulfur (S⁰), thiosulfate (S₂O₃²⁻), molecular hydrogen (H₂), ferrous iron (Fe²⁺), or certain organic molecules. Since water is not split, molecular oxygen (O₂) is not released. The overall objective remains the same: to generate adenosine triphosphate (ATP) for energy and reducing power, such as NADH or NADPH, which are then used to fix carbon dioxide and create organic compounds.
Key Organisms and Their Pigments
Several groups of bacteria perform anoxygenic photosynthesis, each adapted to specific environmental conditions. These include Purple Bacteria (sulfur or non-sulfur types), Green Bacteria (sulfur or non-sulfur), and Heliobacteria. These organisms employ specialized pigments called bacteriochlorophylls.
Bacteriochlorophylls are analogous to chlorophylls in plants and cyanobacteria but absorb light at different wavelengths. They primarily absorb electromagnetic radiation in the near-infrared spectrum, between 800 and 1040 nm. For instance, green sulfur bacteria use bacteriochlorophylls c through e, absorbing light in the range of 720 to 755 nm. This ability to utilize longer wavelengths allows these bacteria to thrive in environments where visible light is scarce or absorbed by other photosynthetic organisms.
Environmental Niches
Anoxygenic photosynthetic organisms are found in anaerobic (oxygen-free) environments where light is available. These habitats provide the electron donors these bacteria require.
Such niches include deeper, stagnant layers of lakes and ponds, beneath oxygen-rich surface waters. They also thrive in hydrothermal vents, hot springs, sediments, and microbial mats. Hypersaline environments can also support these bacteria. These locations are characterized by abundant electron donors like hydrogen sulfide and the absence of oxygen, which can be toxic to many anoxygenic phototrophs.
Evolutionary Significance
Anoxygenic photosynthesis is believed to have evolved on Earth before oxygenic photosynthesis. This ancient metabolic pathway played a significant role on early Earth, where the atmosphere and oceans were largely anoxic, lacking free oxygen. It represented an early way life harnessed solar energy.
The planet underwent a profound transformation with the “Great Oxidation Event,” beginning approximately 2.46 to 2.426 billion years ago. This event was driven by the evolution of oxygenic photosynthesis in cyanobacteria, which released molecular oxygen into the atmosphere. Oxygen accumulation fundamentally changed Earth’s environment, paving the way for aerobic life but also forcing anoxygenic organisms into the oxygen-depleted niches they occupy today. This process highlights the diverse strategies life has developed to capture energy throughout Earth’s history.