Photosynthesis: Oxygenic and Anoxygenic Energy Pathways

The Mechanism of Photosynthesis and Energy Conversion

Photosynthesis (pronounced FOH-tə-SINTH-ə-sis)^[1] is a system of biological processes by which photopigment-bearing autotrophic organisms, such as most plants, algae, and cyanobacteria, convert light energy—typically from sunlight—into the chemical energy necessary to fuel their metabolism. The term photosynthesis usually refers to oxygenic photosynthesis, a process that releases oxygen as a byproduct of water splitting.

Storing Chemical Energy and Maintaining the Atmosphere

Photosynthetic organisms store the converted chemical energy within the bonds of intracellular organic compounds (complex compounds containing carbon), typically carbohydrates like:

• Sugars (mainly glucose, fructose, and sucrose)
• Starches
• Phytoglycogen
• Cellulose

When needing to use this stored energy, an organism's cells then metabolize the organic compounds through cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.^[2]

Anoxygenic Photosynthesis: Non-Oxygen Producing Pathways

Some organisms also perform anoxygenic photosynthesis, which does not produce oxygen. This process is observed in various microbial life forms.

Bacterial Anoxygenic Processes

Some bacteria (e.g., purple bacteria) use bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, releasing sulfur instead of oxygen. This was a dominant form of photosynthesis in the euxinic Canfield oceans during the Boring Billion.^[3][4]

Archaeal ATP Synthesis and Early Life

Archaea such as Halobacterium also perform a type of non-carbon-fixing anoxygenic photosynthesis. Here, the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and produce a proton (hydron) gradient across the cell membrane. The subsequent ion movement powers transmembrane proton pumps to directly synthesize adenosine triphosphate (ATP), the "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, as far back as the Paleoarchean, preceding that of cyanobacteria (see Purple Earth hypothesis).^[5]