Hydrogen sulfide chemosynthesis is a process where bacteria oxidize hydrogen sulfide (H₂S) to extract energy, then use that energy to convert carbon dioxide into organic molecules. It’s the biological equivalent of photosynthesis, but powered by chemical energy from a toxic gas instead of sunlight. The process happens in two linked stages: an energy-harvesting stage that strips electrons from H₂S, and a carbon-fixing stage that builds sugars from CO₂.
How Bacteria Extract Energy From H₂S
The core chemistry starts when bacteria strip electrons from hydrogen sulfide. These electrons carry energy, and the bacteria funnel them through a chain of protein carriers embedded in their cell membranes to ultimately produce ATP, the universal energy currency of life. Two main enzymatic routes handle this first critical step. One uses a protein called sulfide quinone reductase (SQR), which sits in the cell membrane and passes electrons to a carrier molecule called quinone. The other route uses a different enzyme that passes electrons to a small protein called cytochrome c. Either way, the electrons flow “downhill” through a series of carriers until they reach a final acceptor, usually oxygen, which combines with the electrons and hydrogen ions to form water.
This electron flow does the same thing as a hydroelectric dam: the movement of electrons through the chain pumps protons across the membrane, creating a gradient that drives ATP production. The process also generates the reducing power (essentially, stored electrons) needed to run the carbon-fixing reactions that come next.
What makes this different from how your own cells generate energy is the starting fuel. Your mitochondria strip electrons from sugars. These bacteria strip electrons from hydrogen sulfide. The downstream machinery is remarkably similar.
What Happens to the Sulfur
When bacteria pull electrons from H₂S, the sulfur atom left behind has to go somewhere. The first product is typically elemental sulfur, a solid yellow substance. Many sulfur-oxidizing bacteria can then oxidize this elemental sulfur further, all the way to sulfate (SO₄²⁻), squeezing out additional energy at each step. The simplified overall reaction, when sulfide is fully oxidized using oxygen, looks like this:
H₂S + 2O₂ → SO₄²⁻ + 2H⁺
But many organisms stop at elemental sulfur and stockpile it. A 2024 study in Science described a widespread system in bacteria where tiny protein shells called encapsulins act as storage containers for crystalline elemental sulfur. These protein compartments are selectively permeable: they keep out the cell’s normal reducing chemicals, which would otherwise break down the stored sulfur. This lets bacteria bank sulfur as an energy reserve they can tap later when fresh H₂S runs low.
Some bacteria deposit sulfur granules inside their cells, while others excrete elemental sulfur externally. The strategy depends on the species and the environment.
How CO₂ Becomes Organic Carbon
Energy harvesting is only half the story. The ATP and reducing power generated from sulfide oxidation fuel the second stage: building organic molecules from carbon dioxide. Most sulfur-oxidizing bacteria use the Calvin cycle for this, the same set of reactions that plants use during photosynthesis. The key enzyme, RuBisCO, grabs CO₂ and attaches it to an existing five-carbon sugar, eventually producing three-carbon molecules that the cell can assemble into sugars, amino acids, and everything else it needs.
Some chemosynthetic organisms use alternative carbon-fixing pathways. Certain heat-loving archaea that oxidize sulfur compounds use the reverse TCA cycle or the 3-hydroxypropionate/4-hydroxybutyrate cycle instead. These pathways achieve the same end result, converting inorganic carbon into organic molecules, but through different chemical intermediates. The Calvin cycle remains the dominant route among the sulfur-oxidizing bacteria found at hydrothermal vents and in other sulfide-rich environments.
The Key Enzymes That Drive the Process
Several specialized enzymes coordinate the oxidation of sulfur compounds. The Sox (sulfur-oxidizing) system is a set of proteins in the periplasm, the space between a bacterium’s inner and outer membranes, that can oxidize sulfide, elemental sulfur, thiosulfate, and sulfite all the way to sulfate. Electrons released by Sox proteins are passed to cytochrome carriers that feed into the main electron transport chain.
Another important enzyme, sulfur oxygenase reductase (SOR), catalyzes an oxygen-dependent splitting of elemental sulfur into sulfite, thiosulfate, and hydrogen sulfide. This is especially common in heat-loving archaea. The hydrogen sulfide produced as a byproduct gets recycled back through membrane-bound SQR, which re-oxidizes it to elemental sulfur and feeds more electrons into the transport chain. The result is a tightly integrated loop where sulfur intermediates are continuously recycled to maximize energy extraction.
Not every organism has every enzyme. Some species have incomplete Sox systems, missing certain components. Others rely heavily on SQR or on different combinations of these pathways. The specific enzyme toolkit varies by species and habitat.
Chemosynthesis Without Oxygen
Oxygen is the most common final electron acceptor in this process, but it’s not the only option. In oxygen-free sediments, bacteria can oxidize hydrogen sulfide using manganese oxides or iron compounds as electron acceptors instead. The chemistry changes significantly. When H₂S reacts with manganese dioxide (MnO₂), the products are elemental sulfur, reduced manganese, and hydroxide ions:
3H₂S + 3MnO₂ → 3S⁰ + 3Mn²⁺ + 6OH⁻
The elemental sulfur can then be further oxidized by manganese dioxide to sulfate. Some bacteria carry out a process called sulfur disproportionation, splitting elemental sulfur into both hydrogen sulfide and sulfate simultaneously. Under standard conditions this reaction is thermodynamically unfavorable, but it becomes viable when metal oxides in the sediment react with and remove the H₂S product, pulling the reaction forward.
These anaerobic pathways are important in deep ocean sediments and other environments where oxygen never penetrates. They link the sulfur cycle directly to the manganese and iron cycles in marine sediments.
Giant Tube Worms and Symbiotic Chemosynthesis
The most dramatic example of H₂S chemosynthesis in action is the giant tube worm, Riftia pachyptila, which lives at hydrothermal vents on the ocean floor. These worms have no mouth, no gut, and no anus. Instead, they rely entirely on chemosynthetic bacteria living inside a specialized organ called the trophosome.
The worm’s blood contains a special hemoglobin that binds both oxygen and hydrogen sulfide simultaneously, delivering both to its bacterial symbionts. The bacteria are sulfide specialists. They oxidize sulfide rapidly across a wide range of concentrations, from 5 micromolar up to 2 millimolar, with peak activity above 1 mM. Remarkably, they show no signs of being poisoned even at high sulfide levels, unlike many other sulfur-oxidizing bacteria.
When researchers tracked radioactively labeled sulfide through the system, the H₂S disappeared almost completely within one minute. The major product was elemental sulfur stored in insoluble form, with sulfate and polysulfides appearing as soluble products. No thiosulfate was produced at any stage. The bacteria are true sulfide specialists: they cannot use thiosulfate or sulfite as alternative energy sources. This extreme specialization reflects the consistent supply of H₂S streaming from hydrothermal vents, which can reach concentrations of several millimolar in the vent fluid.
The bacteria use the energy from sulfide oxidation to fix CO₂ through the Calvin cycle, producing organic carbon that feeds the worm. In this partnership, the worm is essentially a delivery system for raw materials, and the bacteria are the chemical factory that turns volcanic gas into food.