Yes, photosynthesis can absolutely occur without oxygen. In fact, entire groups of bacteria carry out photosynthesis that neither requires nor produces oxygen. This process, called anoxygenic photosynthesis, likely predates the oxygen-producing version performed by plants and algae. It relies on different fuel sources, generates different byproducts, and thrives in environments where oxygen is scarce or entirely absent.
How Photosynthesis Works Without Water
The photosynthesis you learned about in school splits water molecules to harvest electrons, releasing oxygen as a byproduct. Anoxygenic photosynthesis skips water entirely. Instead, these bacteria pull electrons from hydrogen sulfide, pure hydrogen gas, or various organic compounds. Because they never split water, they never release oxygen.
When hydrogen sulfide serves as the electron source, the bacteria convert it along with carbon dioxide into organic matter, just as plants do. But instead of releasing oxygen, they produce elemental sulfur. Some bacterial families store these sulfur globules inside their cells, while others deposit them outside. The end result is the same core achievement as plant photosynthesis: light energy is captured and used to build organic molecules from carbon dioxide. The chemistry just takes a different route.
The Bacteria That Do It
At least seven major groups of bacteria perform anoxygenic photosynthesis, each with its own preferred habitat and chemistry. The best studied are purple sulfur bacteria, purple non-sulfur bacteria, and green sulfur bacteria. Beyond those, filamentous anoxygenic phototrophs, heliobacteria, acidobacteria, and a group called gemmatimonadetes round out the list.
One key difference between these groups is the molecular machinery they use to capture light. Plants have two light-harvesting systems that work in tandem. Anoxygenic bacteria typically have just one. Purple bacteria use a system similar to one of the plant versions, while green sulfur bacteria and heliobacteria use a system closer to the other. No single anoxygenic organism combines both, which is part of why none of them can split water the way plants do.
Why Oxygen Is Actually a Problem
These bacteria don’t just tolerate oxygen-free conditions. Many of them need them. Oxygen and its reactive byproducts damage the light-harvesting machinery inside cells, and this damage gets worse when light is also present. Plants and cyanobacteria have evolved elaborate protective systems to cope with this, since they generate oxygen themselves. Anoxygenic phototrophs largely lack those defenses, suggesting they have spent most of their evolutionary history in low-oxygen or oxygen-free environments.
This oxygen sensitivity shapes where these organisms live today. They tend to occupy zones below oxygen-producing algae and cyanobacteria in layered microbial communities, or they colonize environments where oxygen never accumulates in the first place.
Where These Organisms Live
Anoxygenic phototrophs are found across a remarkable range of habitats: sulfur-rich lakes, microbial mats, hot springs, and hypersaline lagoons. Researchers have documented them in geothermal features across Yellowstone National Park, as well as hot springs in Iceland, Japan, New Zealand, Russia, the Arctic, China, the Tibetan Plateau, and several sites across the western United States.
Their tolerance for extreme conditions is striking. In Yellowstone alone, they’ve been found in waters ranging from pH 2 to pH 9 (highly acidic to mildly alkaline) and temperatures from 31°C to 71°C. At 71°C, researchers detected active gene transcripts from anoxygenic phototrophs, placing them near the upper temperature limit of any photosynthesis on Earth, roughly 72 to 73°C. In acidic environments, though, photosynthesis drops off above about 56°C.
An Ancient Form of Energy Capture
Anoxygenic photosynthesis is older than the oxygen-producing kind. The exact timeline remains debated. Molecular clock estimates place the ancestor of cyanobacteria (the first organisms to produce oxygen through photosynthesis) somewhere between 1.5 and 3.5 billion years ago, a wide range that reflects genuine uncertainty. What researchers do agree on is that anoxygenic photosynthesis came first.
That said, the gap between the two may have been shorter than once assumed. Some analyses suggest the evolutionary distance from the origin of photosynthesis to the emergence of the oxygen-producing version could be less than 200 million years. Earth’s atmosphere was oxygen-free for its first couple billion years, and when oxygen-producing photosynthesis finally appeared, the resulting oxygen would have been toxic to the anaerobic life that dominated the planet. Anoxygenic phototrophs, with their limited defenses against oxygen, are living relics of that earlier world.
Practical Uses Today
Because anoxygenic phototrophs consume hydrogen sulfide, a toxic and foul-smelling gas, they have real potential for cleaning up wastewater and biogas. Hydrogen sulfide is a common contaminant in both industrial and municipal wastewater, and in the biogas produced by anaerobic digesters. Conventional removal methods are chemical and energy-intensive. These bacteria offer a biological alternative: they convert hydrogen sulfide into elemental sulfur, which is insoluble in water and easy to separate out.
The results in laboratory settings have been impressive. In one study, green sulfur bacteria grown in tube-based reactors achieved 100% removal of hydrogen sulfide at concentrations up to 286 milligrams per liter per hour, converting 92 to 95% of it into elemental sulfur. Another experiment using synthetic biogas (a mix of methane, carbon dioxide, and 0.5% hydrogen sulfide) reached complete desulfurization within seven days. Anoxygenic phototrophs can handle hydrogen sulfide concentrations of 100 to 150 milligrams per liter, far higher than the 16 milligrams per liter that competing biological approaches using cyanobacteria can tolerate.
Researchers are also exploring whether the metabolic activity of these bacteria can be harnessed in microbial electrochemical cells, essentially connecting their internal chemistry to an electrical circuit to generate current. The technology is still in early stages, but it points to a future where oxygen-free photosynthesis contributes not just to waste treatment but to renewable energy production.