Autotrophic bacteria are microorganisms that build their own organic molecules from carbon dioxide, requiring no food from other living things. They are, in a real sense, self-feeders, and they occupy a role in microbiology roughly analogous to plants in the visible world. But the comparison sells them short. While plants rely on a single carbon-fixing enzyme and a single energy source (sunlight), autotrophic bacteria have evolved at least six distinct biochemical pathways for turning COâ‚‚ into cell material, and they draw energy from sources as varied as sunlight, hydrogen gas, iron, sulfur, and ammonia. Some thrive kilometers underground, in places that have never seen a photon.
Two Ways to Power a Self-Feeding Cell
All autotrophic bacteria need two things: a source of carbon (COâ‚‚) and a source of energy to drive the chemistry that converts that carbon into useful molecules. The energy source is what splits them into two broad camps. Photoautotrophic bacteria harvest light. Chemoautotrophic bacteria extract energy from chemical reactions, usually by oxidizing an inorganic substance.
Photoautotrophic bacteria include cyanobacteria, the group responsible for much of the oxygen in Earth’s atmosphere, and a set of older, oxygen-free lineages. Green sulfur bacteria and purple sulfur bacteria carry out a form of photosynthesis that predates the oxygen-producing kind. Instead of splitting water molecules to grab electrons the way plants do, these bacteria use hydrogen sulfide as their electron donor, oxidizing it to elemental sulfur in the process.1PubMed Central. Anoxygenic photosynthesis with emphasis on green sulfur bacteria and a perspective for hydrogen sulfide detoxification of anoxic environments The result is photosynthesis without oxygen as a byproduct, a reminder that the version we learn about in school is actually the newer, modified edition.
The molecular machinery inside purple and green sulfur bacteria centers on sulfur metabolism. Both families oxidize hydrogen sulfide to elementary sulfur as part of the energy-harvesting process, using reaction centers and pigments tuned to wavelengths of light that other photosynthesizers largely ignore.2PubMed Central. Molecular Physiology of Anaerobic Phototrophic Purple and Green Sulfur Bacteria This lets them operate in murky, sulfide-rich environments like hot springs and stratified lakes where light penetrates poorly but the chemical buffet is rich.
Chemoautotrophic bacteria are the ones that truly challenge our intuitions. They need no light at all. Instead, they oxidize inorganic chemicals and use the released energy to fix COâ‚‚. Sulfur-oxidizing species like those in the genus Acidithiobacillus break down sulfur compounds through a battery of specialized enzymes, extracting energy at each step.3PubMed Central. Sulfur Oxidation in the Acidophilic Autotrophic Acidithiobacillus spp. Nitrifying bacteria get their energy by oxidizing nitrogen compounds. A species from the genus Nitrospira, for instance, can carry out complete nitrification on its own, oxidizing ammonia all the way to nitrate and using that energy to grow autotrophically.4PubMed Central. Complete nitrification by Nitrospira bacteria Iron-oxidizing bacteria represent yet another strategy, pulling electrons from dissolved iron to power their metabolism. The marine species Mariprofundus ferrooxydans is an obligate iron-oxidizing autotroph, meaning iron is all it eats, and researchers have even grown it using a poised electrode as a stand-in for dissolved iron.5PubMed Central. Cultivation of an obligate Fe(II)-oxidizing lithoautotrophic bacterium using electrodes
Hydrogen-oxidizing bacteria, sometimes called “Knallgas” bacteria, use molecular hydrogen as their energy source. These organisms are widespread in soil, and researchers have detected genes for high-affinity hydrogen-grabbing enzymes in rhizosphere communities around plant roots.6PubMed Central. Cultivation-independent detection of autotrophic hydrogen-oxidizing bacteria by DNA stable-isotope probing Some are remarkably versatile. The thermoacidophilic bacterium Methylacidiphilum fumariolicum SolV carries two different hydrogen-uptake enzymes, one oxygen-sensitive and one not, allowing it to grow on hydrogen alone across a range of oxygen levels.7The ISME Journal. Methylacidiphilum fumariolicum SolV, a thermoacidophilic ‘Knallgas’ methanotroph with both an oxygen-sensitive and -insensitive hydrogenase
Not Just One Way to Fix Carbon
When most people hear “carbon fixation,” they think of the Calvin cycle, the pathway plants and cyanobacteria use to turn COâ‚‚ into sugar. Many autotrophic bacteria do use a version of the Calvin cycle, built around the enzyme RuBisCO. But the bacterial world has produced several alternative approaches, and understanding this diversity matters because it explains how autotrophic bacteria manage to colonize such wildly different habitats.
RuBisCO itself comes in multiple forms. Forms I and II are the ones most relevant to bacterial carbon fixation, and they have meaningfully different properties. Form I enzymes tend to work better when oxygen is present, while Form II enzymes are better suited to high-COâ‚‚, low-oxygen conditions.8Journal of Experimental Botany. Multiple Rubisco forms in proteobacteria: their functional significance in relation to CO2 acquisition by the CBB cycle In the deep ocean, organisms carrying these enzymes show up in the vast majority of water samples and can make up around a fifth of the microbial community below 150 meters. There, autotrophic carbon fixation via RuBisCO is dominated by just a few lineages of gammaproteobacteria and SAR324, and gene expression data suggest these bacteria flip their carbon-fixing machinery on and off rather than running it all the time, consistent with flexible, opportunistic metabolisms.9PubMed Central. Abundant and metabolically flexible bacterial lineages underlie a vast potential for rubisco-mediated carbon fixation in the dark ocean
Beyond the Calvin cycle, at least five other carbon fixation pathways have been documented in autotrophic microorganisms. The reductive tricarboxylic acid (rTCA) cycle runs the familiar TCA cycle in reverse, pulling in COâ‚‚ instead of releasing it. Researchers found the first direct evidence for this pathway operating in epsilon-proteobacteria by detecting all the key enzyme activities and amplifying genes for the hallmark enzyme, ATP citrate lyase.10PubMed Central. Evidence for autotrophic CO2 fixation via the reductive tricarboxylic acid cycle by members of the epsilon subdivision of proteobacteria This pathway is common in bacteria that live around hydrothermal vents and other hot, sulfur-rich habitats.
The Wood-Ljungdahl pathway takes a different approach entirely, stitching two one-carbon units together to form the two-carbon molecule acetate.11PubMed Central. Acetogenesis and the Wood-Ljungdahl pathway of CO(2) fixation It is used by acetogenic bacteria and is considered one of the most ancient carbon fixation strategies. Some researchers view it as a window into the kind of chemistry that may have supported early life before more elaborate pathways evolved.
Then there is the 3-hydroxypropionate cycle, first described in the phototrophic bacterium Chloroflexus aurantiacus. This organism can grow autotrophically but does not use the Calvin cycle, the rTCA cycle, or the Wood-Ljungdahl pathway. Instead, it carboxylates acetyl-CoA and runs a cyclic series of reactions through 3-hydroxypropionate and propionyl-CoA, ultimately regenerating its starting molecule and releasing glyoxylate as the net product of carbon fixation.12PubMed. Enzymes of a novel autotrophic CO2 fixation pathway in the phototrophic bacterium Chloroflexus aurantiacus, the 3-hydroxypropionate cycle Later work identified the missing enzymatic steps for how the glyoxylate product is actually assimilated into cell material.13PubMed Central. Identifying the missing steps of the autotrophic 3-hydroxypropionate CO2 fixation cycle in Chloroflexus aurantiacus
Certain archaea, close evolutionary relatives of bacteria, have their own variants. Among the Crenarchaeota, two cycles have been identified: the dicarboxylate/4-hydroxybutyrate cycle in anaerobic or microaerobic lineages, and the 3-hydroxypropionate/4-hydroxybutyrate cycle in aerobic ones.14PubMed Central. Identification of missing genes and enzymes for autotrophic carbon fixation in crenarchaeota Which cycle an organism uses correlates with its evolutionary lineage rather than simply with whether it lives in the presence of oxygen.15PubMed. Study of the distribution of autotrophic CO2 fixation cycles in Crenarchaeota The sheer number of independently evolved solutions to the same problem, fixing COâ‚‚, suggests this challenge has been solved multiple times throughout the history of life.
Life Without Sunlight
The places where chemoautotrophic bacteria truly shine (figuratively, since there is no light) are the deep ocean floor and the subsurface crust. At hydrothermal vents, superheated fluid mixing with cold seawater creates a chemical disequilibrium loaded with reduced compounds like hydrogen sulfide, hydrogen, and dissolved metals. Because the oxidation reactions that would normally neutralize this disequilibrium happen slowly in the absence of biological catalysts, chemoautotrophic bacteria step in and harvest the energy. These microorganisms are the primary producers of an entire submarine ecosystem that is essentially independent of photosynthesis.16Geochimica et Cosmochimica Acta. Geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems Giant tube worms, shrimp, and vent crabs all depend, directly or indirectly, on the carbon that bacteria fix from COâ‚‚ using nothing more than vent chemistry.
Even more isolated communities exist deep inside continental rock. In the Witwatersrand Basin of South Africa, fracture water collected from deep siliciite and volcanic formations contains dissolved hydrogen at concentrations far higher than anything seen in shallow groundwater or marine sediments. The hydrogen there is not biological in origin. It comes from the radiolytic splitting of water molecules by the natural radioactive decay of uranium, thorium, and potassium in the surrounding rock. This radiolytic hydrogen fuels methanogens and other autotrophic microbes that have been cut off from the surface for potentially millions of years.17Geochemistry, Geophysics, Geosystems. Radiolytic H2 in continental crust: Nuclear power for deep subsurface microbial communities The researchers who measured these systems describe radiolytic hydrogen production as a virtually limitless energy source for deep crustal ecosystems, which reframes how we think about the boundaries of the biosphere. Life does not stop where sunlight stops. It does not even stop where geothermal energy stops.
Partnerships With Animals
Some of the most striking examples of autotrophic bacteria at work involve symbioses with marine invertebrates. Vesicomyid clams, found near hydrothermal vents and cold seeps, depend entirely on sulfur-oxidizing endosymbiotic bacteria housed inside their gill cells for nutrition.18PubMed Central. Cospeciation of chemoautotrophic bacteria and deep sea clams The clams have reduced digestive systems. They do not filter-feed in the usual sense. Instead, they position their feet in sulfide-rich sediment and channel that sulfide to their gills, where their resident bacteria oxidize it and fix carbon to feed the host.
A similar arrangement exists in Solemya clams. In these bivalves, sulfur-oxidizing chemoautotrophic bacteria live directly within host cells and serve a dual function: they detoxify the hydrogen sulfide that would otherwise poison the clam, and they fix carbon to support the animal’s metabolic needs.19PubMed. Bacterial endosymbioses in Solemya (Mollusca: Bivalvia)–model systems for studies of symbiont-host adaptation These partnerships are so deeply embedded that the bacteria are transmitted from parent to offspring, and some lineages have been coevolving with their hosts for millions of years. The giant tube worm Riftia pachyptila, the poster organism of hydrothermal vent biology, relies on a similar arrangement with sulfur-oxidizing endosymbionts. It has no mouth and no gut. Its entire nutrition comes from bacterial carbon fixation happening inside a specialized organ called the trophosome.
Surviving Extremes
Autotrophic bacteria turn up in environments that seem incompatible with life as we usually imagine it. The Antarctic Dry Valleys are among the coldest, driest, and most UV-blasted habitats on Earth, yet metagenomic surveys there have identified major metabolic pathways for autotrophic, heterotrophic, and nitrogen-fixing strategies, indicating that microbial communities maintain significant metabolic flexibility even under these conditions.20PubMed Central. Functional ecology of an Antarctic Dry Valley Bacteria in these soils are not simply dormant; they have active metabolisms tuned to an environment where liquid water is rare and temperatures can plunge far below freezing.
At the opposite thermal extreme, autotrophic bacteria and archaea thrive in volcanic hot springs and deep-sea black smokers where temperatures exceed 80°C. Acidithiobacillus species tolerate intensely acidic conditions, oxidizing sulfur and iron compounds to generate sulfuric acid and ferric ions. Their acid tolerance and metabolic flexibility also make them effective agents for extracting metals from rock.21PubMed Central. Urban Biomining of Rare Earth Elements: Current Status and Future Opportunities This ability did not evolve for our benefit, but humans have found ways to put it to use.
Industrial and Biotechnology Applications
The fact that autotrophic bacteria can build complex organic molecules from COâ‚‚ and simple inorganic energy sources makes them attractive for several industrial processes. One of the most developed applications is gas fermentation, in which bacteria convert waste gases and greenhouse gases, including carbon monoxide and COâ‚‚, into commodity chemicals and fuels.22PubMed Central. Gas fermentation for commodity chemicals and fuels Companies are already operating commercial-scale gas fermentation plants, using acetogenic bacteria running the Wood-Ljungdahl pathway to produce ethanol from industrial off-gases that would otherwise be burned or vented.
Hydrogen-oxidizing bacteria are being developed as a source of single-cell protein for food and animal feed. The appeal is straightforward: these organisms fix COâ‚‚ into protein-rich biomass without needing arable land, sunlight, or large quantities of fresh water. Compared to conventional agriculture, the land and water footprint is drastically smaller, and the growth rates are higher.23PubMed Central. Hydrogen oxidising bacteria for production of single-cell protein and other food and feed ingredients Several startups are producing protein powders from hydrogen-oxidizing bacteria that are already approved for sale in some markets. Beyond protein, the bacterium Cupriavidus necator has been engineered to use its hydrogen-driven metabolism for producing specialty chemicals. One proof-of-concept study used the organism’s hydrogenase enzyme for hydrogen-driven conversion of xylose into xylitol, a sugar alcohol used as a sweetener, achieving near-complete conversion of 30 grams per liter of the starting sugar.24Aalto University publication series Doctoral Theses. Advancing hydrogen-oxidizing ‘Knallgas’ bacteria as production hosts for biotechnology
Biomining is another area where autotrophic bacteria earn their keep. Acidithiobacillus species naturally produce sulfuric acid and ferric ions that dissolve metals out of rock, and the mining industry has used this capability for decades to extract copper and gold from low-grade ores. More recently, attention has turned to rare earth elements, which are critical for electronics, wind turbines, and electric vehicles. These bacteria can solubilize rare earth elements from ores and electronic waste under acidic conditions, offering a lower-cost and less chemically intensive alternative to conventional extraction methods.25PubMed Central. Urban Biomining of Rare Earth Elements: Current Status and Future Opportunities Meanwhile, research on iron-oxidizing autotrophs has taken an unusual turn. Acidithiobacillus ferrooxidans can switch its electron source from dissolved iron to an electrode when electrical current is the only available energy, and it fixes COâ‚‚ while doing so.26PubMed Central. From chemolithoautotrophs to electrolithoautotrophs: CO2 fixation by Fe(II)-oxidizing bacteria coupled with direct uptake of electrons from solid electron sources This “electrolithoautotrophy” is early-stage science, but it raises the possibility of microbial production systems powered directly by renewable electricity rather than hydrogen or chemical feedstocks.
Why They Matter for the Planet’s Carbon Budget
Autotrophic bacteria are not minor players in global carbon cycling. In the ocean alone, the dark water column below the sunlit zone teems with bacteria carrying RuBisCO genes, and these organisms can represent up to roughly a fifth of the microbial community in deep samples.27PubMed Central. Abundant and metabolically flexible bacterial lineages underlie a vast potential for rubisco-mediated carbon fixation in the dark ocean How much carbon they actually fix is harder to pin down, because their gene expression appears to fluctuate rather than run constantly, but the potential is large. In soil, hydrogen-oxidizing autotrophs fix COâ‚‚ in the rhizosphere, where their activity interacts with plant root chemistry in ways that are still being mapped. In subsurface rock, autotrophic communities operate on geological timescales, slowly converting inorganic carbon into biomass sustained by radioactive decay. These are not ecosystems on the margin. They represent a parallel biosphere with its own energy economy, one that was running long before photosynthesis evolved and will continue long after any given surface ecosystem changes.
Climate models and carbon budgets have historically focused on photosynthesis and its reverse, respiration. Chemoautotrophic fixation in the deep ocean, in soils, and in subsurface rock is harder to measure and has been underrepresented in these estimates. As genomic and metagenomic tools improve, the true scale of non-photosynthetic carbon fixation is becoming clearer, and it is larger than earlier generations of microbiologists expected. Whether that changes the math on atmospheric COâ‚‚ in a policy-relevant way remains uncertain, but it changes the biology substantially. The textbook image of carbon fixation as something that happens in green leaves and sunlit surface water misses an entire dimension of the process happening in the dark.