Photosynthesis is a process by which organisms convert light energy into chemical energy, fueling life on Earth. While most people are familiar with the type of photosynthesis performed by plants, which releases oxygen, there is another ancient and widespread form known as anoxygenic photosynthesis. This process represents a diverse strategy for energy production, carried out by various bacterial groups that have adapted to environments without oxygen. It highlights the flexibility of life’s energy-harvesting mechanisms.
Understanding Anoxygenic Photosynthesis
Anoxygenic photosynthesis involves the capture of light energy to generate chemical energy in the form of adenosine triphosphate (ATP) and reducing power, such as nicotinamide adenine dinucleotide phosphate (NADPH) or nicotinamide adenine dinucleotide (NADH). Unlike its oxygen-producing counterpart, this process does not use water as an electron source and does not release oxygen. Instead, it relies on different electron donors for energy conversion.
The mechanism involves light absorption by specialized pigments, which excite electrons. These excited electrons transfer through an electron transport chain embedded within bacterial membranes. This movement creates a proton motive force across the membrane, used to synthesize ATP. The electrons return to the reaction center, in a cyclic pathway, making the process self-sustaining.
Organisms performing anoxygenic photosynthesis utilize light-absorbing pigments called bacteriochlorophylls. These pigments are structurally similar to chlorophyll but absorb light at different wavelengths, particularly in the near-infrared region (800-1040 nanometers). The type of bacteriochlorophyll and its location within the cell membrane influence the wavelengths of light absorbed, allowing these bacteria to thrive in environments where visible light is limited or already utilized by other photosynthetic organisms.
How It Differs from Plant Photosynthesis
A primary distinction between anoxygenic photosynthesis and oxygenic photosynthesis is the absence of oxygen production. Plant photosynthesis uses water (H₂O) as the electron donor, splitting it to release electrons, protons, and oxygen. Anoxygenic photosynthesis, however, uses alternative compounds as electron sources, so water is not consumed and oxygen is not a byproduct.
These alternative electron donors vary among anoxygenic photosynthetic bacteria. Examples include hydrogen sulfide (H₂S), oxidized to elemental sulfur, and ferrous iron (Fe²⁺). Some groups also utilize organic molecules like succinate or malate. This flexibility allows these organisms to inhabit diverse environments where water is not readily available, or where other reduced compounds are abundant.
Another difference lies in the photosynthetic pigments. While oxygenic phototrophs primarily use chlorophyll a, anoxygenic bacteria employ various bacteriochlorophylls. These bacteriochlorophylls absorb light at longer wavelengths, particularly in the near-infrared region, compared to chlorophyll a, which absorbs in the red and blue parts of the visible spectrum. This difference in absorption spectra allows anoxygenic phototrophs to capture light energy that other organisms cannot.
Many anoxygenic photosynthetic pathways involve cyclic electron flow. In this pathway, electrons excited by light return to the reaction center after passing through an electron transport chain, generating ATP. While ATP is produced, reducing power (NADPH) for carbon fixation may require additional mechanisms. This contrasts with oxygenic photosynthesis, which employs a Z-scheme electron flow involving two photosystems to produce both ATP and NADPH.
Organisms and Their Environments
Anoxygenic photosynthesis is performed by several groups of bacteria, each adapted to anoxic or low-oxygen environments. These include purple non-sulfur bacteria, purple sulfur bacteria, green sulfur bacteria, and heliobacteria. Each group has characteristics enabling survival and energy production in its habitat.
Purple sulfur bacteria use hydrogen sulfide (H₂S) as an electron donor, oxidizing it to elemental sulfur. They thrive in sunlit, anoxic zones of stratified lakes, sulfur springs, and stagnant waters where hydrogen sulfide is abundant. Green sulfur bacteria also utilize hydrogen sulfide, but they deposit the elemental sulfur externally. These bacteria are found in deeper, anoxic waters or sediments, where light penetration is limited and hydrogen sulfide is abundant.
Purple non-sulfur bacteria are a versatile group. While they can use hydrogen sulfide, they prefer organic compounds as electron donors and can tolerate a wider range of oxygen concentrations, performing anoxygenic photosynthesis anaerobically. They are found in aquatic sediments, sewage ponds, and anoxic soils. Heliobacteria are Gram-positive organisms found in soils, where they contribute to nitrogen fixation. These diverse adaptations highlight how anoxygenic phototrophs occupy specific niches, in environments inhospitable to oxygenic life.
Its Role in Earth’s History and Today
Anoxygenic photosynthesis was one of the earliest forms of photosynthesis, predating the oxygenic type that transformed Earth’s atmosphere. Evidence suggests these early anoxygenic phototrophs played a role in shaping the biosphere of the early Earth, before oxygen accumulated in the atmosphere. Their activity contributed to the initial cycling of carbon and other elements in an anoxic world.
In modern ecosystems, anoxygenic photosynthetic bacteria continue to play ecological roles, particularly in anaerobic environments. They are contributors to primary production in habitats like the anoxic layers of stratified lakes, deep-sea hydrothermal vents, and anoxic sediments. These bacteria are integral to biogeochemical cycles, including the sulfur cycle (by oxidizing hydrogen sulfide) and the iron cycle (by utilizing ferrous iron).
Their activity also contributes to the carbon cycle by fixing carbon dioxide into organic matter, even in oxygen-depleted zones where other photosynthetic organisms cannot survive. This makes them players in maintaining the balance of elements in these environments. The study of anoxygenic photosynthesis offers insights into how life harnesses energy, and its enduring presence underscores its ecological significance throughout Earth’s history and today.