Anaerobic photosynthesis is a metabolic process used by certain bacteria to convert light energy into chemical energy in oxygen-free environments. Unlike the more familiar photosynthesis in plants, this ancient method does not produce oxygen as a byproduct. This process represents one of the diverse metabolic strategies life has adapted for survival.
The Defining Difference from Oxygenic Photosynthesis
The primary distinction between anaerobic and oxygenic photosynthesis is the electron donor. In plants, algae, and cyanobacteria, water (H₂O) serves as the electron donor, and its splitting releases molecular oxygen (O₂). Anaerobic photosynthesis relies on other reduced substances for electrons, which is why oxygen is not a byproduct and the chemical outputs are different.
To capture light energy, these organisms use pigments called bacteriochlorophylls. These are similar to the chlorophylls in plants but are structured to absorb light at different wavelengths, often in the infrared spectrum. This allows anaerobic photosynthetic bacteria to thrive where the available light is outside the visible spectrum that plants use.
The energy conversion mechanism follows similar principles. Light energy is captured by antenna systems and channeled to a reaction center. This initiates a flow of electrons through carriers, creating a proton gradient that drives the synthesis of adenosine triphosphate (ATP), the cell’s energy currency.
The Microbes Performing the Process
A diverse group of bacteria performs anaerobic photosynthesis, each adapted to specific anoxic habitats. These microorganisms are aquatic, found in places like sulfur springs, stagnant ponds, and the deep, oxygen-poor layers of lakes and marine environments. They are classified into several major groups based on their pigments and metabolism.
Purple bacteria are one major group, divided into sulfur and non-sulfur types. Purple sulfur bacteria are found in environments rich in hydrogen sulfide, such as sulfidic springs. Purple non-sulfur bacteria can utilize a wider range of organic compounds and are metabolically versatile.
Green bacteria are another group, also divided into sulfur and non-sulfur varieties. Green sulfur bacteria are strict anaerobes and efficient at photosynthesis in low-light conditions, often living deeper in aquatic sediments. Green non-sulfur bacteria exhibit broader metabolic flexibility.
A distinct group known as Heliobacteria resides in soils, particularly in waterlogged rice paddies. They are the only photosynthetic bacteria classified within the Firmicutes phylum. Their cellular machinery for capturing light energy is simpler than that of other anaerobic photosynthesizers.
Alternative Electron Donors and Byproducts
The specific electron donor used varies among anaerobic photosynthetic bacteria and directly determines the reaction’s byproduct. This metabolic flexibility allows these microbes to colonize a wide array of anoxic niches.
A common electron donor is hydrogen sulfide (H₂S), which is oxidized by bacteria like purple and green sulfur bacteria. Instead of oxygen, this process forms elemental sulfur (S) as a waste product. This sulfur can accumulate as granules inside or outside the bacterial cell.
Molecular hydrogen (H₂) is another frequent electron donor. Some bacteria can also use ferrous iron (Fe²⁺) or certain organic compounds as their source of electrons. The use of these varied donors highlights the adaptability of these organisms.
Evolutionary and Ecological Importance
Anaerobic photosynthesis is considered one of the earliest forms of photosynthesis to evolve, predating the oxygen-producing version. It likely originated when Earth’s atmosphere was devoid of oxygen. The first photosynthetic organisms used reducing agents like hydrogen or hydrogen sulfide as their electron sources.
This ancient metabolic pathway was a dominant form of energy production for life on early Earth. The subsequent evolution of oxygenic photosynthesis in cyanobacteria altered the planet’s atmosphere, leading to the Great Oxidation Event. This event paved the way for the evolution of aerobic respiration and more complex life forms.
Anaerobic photosynthetic organisms continue to occupy ecological roles in their environments. They are contributors to biogeochemical cycles, particularly the carbon and sulfur cycles in anoxic zones. By converting light into chemical energy and cycling elements, these microbes sustain productivity where oxygenic photosynthesis is not possible.