Chemolithotrophs are microorganisms that conserve energy by oxidizing reduced inorganic substances, such as molecular hydrogen, reduced sulfur compounds, ferrous iron or ammonia. Their metabolism is especially important in environments where light is absent or limited, including sediments, soils, groundwater, hydrothermal systems and engineered wastewater-treatment facilities. Rather than describing one taxonomic group, chemolithotrophy describes a way of obtaining electrons for energy metabolism.

Many chemolithotrophs are also autotrophs and use carbon dioxide (CO₂) or bicarbonate as their main carbon source. These organisms are called chemolithoautotrophs. However, chemolithotrophy and autotrophy are not synonyms: some microorganisms obtain energy from inorganic electron donors while assimilating organic carbon, and others can switch between nutritional strategies. Understanding this distinction helps explain the diversity of microbial metabolism.

What does chemolithotrophy mean?

The term combines chemo- (chemical energy), litho- (inorganic electron donor) and -trophy (nutrition). In a typical chemolithotrophic respiratory pathway, electrons released from an inorganic donor pass through membrane-associated electron carriers toward a terminal electron acceptor. The resulting energy conservation can support ATP formation and, where necessary, the generation of reducing power for biosynthesis.

Oxygen serves as the terminal electron acceptor for many familiar aerobic chemolithotrophs, but some chemolithotrophic metabolisms operate under low-oxygen or anaerobic conditions with other electron acceptors. The electron donor, electron acceptor, carbon source and environmental conditions must therefore be considered separately when describing a microorganism.

Chemolithotrophy, chemoorganotrophy and autotrophy

A chemoorganotroph obtains electrons from organic substances such as sugars or organic acids. A chemolithotroph uses reduced inorganic substances as electron donors. An autotroph assimilates inorganic carbon; a heterotroph relies on organic carbon. A hydrogen-oxidizing bacterium growing on H₂ and CO₂ can therefore be chemolithoautotrophic, while an organism using an inorganic electron donor and organic carbon can be chemolithoheterotrophic or mixotrophic, depending on its physiology.

How do chemolithotrophs conserve energy?

The useful energy from an oxidation reaction depends on the electron donor–acceptor pair and on local chemistry. Electron transport across a membrane can establish an electrochemical gradient that drives ATP synthase. ATP supplies energy for cellular work, whereas NADH or NADPH provides reducing equivalents for biosynthetic reactions. In some chemolithotrophs, electrons from the donor must be driven “uphill” through part of the electron-transport chain to produce NAD(P)H. This reverse electron transport consumes part of the energy conserved during respiration.

Not every inorganic donor provides the same energetic return. For example, the redox chemistry of hydrogen differs substantially from that of nitrite or ferrous iron. Consequently, microbial growth yields, oxygen requirements and ecological niches vary widely across chemolithotrophic groups.

1. Hydrogen-oxidizing microorganisms

Hydrogen-oxidizing bacteria and archaea use molecular hydrogen (H₂) as an electron donor. Hydrogenases catalyse the oxidation of H₂ and transfer the resulting electrons into cellular electron-transport or redox systems. Some organisms couple hydrogen oxidation to oxygen reduction, while others use alternative electron acceptors under anoxic conditions.

Certain hydrogen-oxidizing bacteria, including well-studied strains of Cupriavidus necator, can grow chemolithoautotrophically using hydrogen as an energy source and CO₂ for carbon assimilation. Their carbon fixation commonly involves the Calvin–Benson–Bassham cycle. This does not imply that all hydrogen-oxidizing microorganisms use that cycle: carbon fixation pathways differ across bacterial and archaeal lineages, and some hydrogen oxidizers can also use organic substrates.

Hydrogen oxidation links microbial metabolism to natural geochemical processes and has attracted interest in biomanufacturing using H₂ and CO₂. The practicality of such systems depends on hydrogen production, gas transfer, reactor operation, safety and the overall environmental footprint, not simply on the capacity of the microorganism to fix carbon.

2. Sulfur-oxidizing microorganisms

Sulfur-oxidizing microorganisms obtain electrons from reduced sulfur compounds, including hydrogen sulfide (H₂S), elemental sulfur (S⁰), thiosulfate (S₂O₃²⁻) and, in some systems, sulfite. Under suitable conditions, these compounds can be oxidized toward sulfate (SO₄²⁻). Sulfur oxidation occurs among diverse bacteria and archaea and is central to sulfur cycling in sediments, sulfur-rich waters and geothermal environments.

Examples include sulfur-oxidizing representatives of Beggiatoa and Thiothrix, as well as acidophilic sulfur oxidizers and thermophilic archaea. Some filamentous sulfur bacteria accumulate visible intracellular or extracellular sulfur globules during sulfide oxidation. These organisms differ in oxygen tolerance, carbon assimilation and the exact biochemical pathway used.

Sox and other sulfur-oxidation pathways

The Sox multienzyme system is one route for oxidation of thiosulfate and related sulfur intermediates. In a complete Sox system, sulfur compounds can be processed without obligatory accumulation of stored elemental sulfur. However, not all sulfur oxidizers possess the complete set of Sox proteins; other organisms use partial Sox systems and additional enzymes to process sulfur intermediates. It is therefore inaccurate to describe the Sox complex as a universal single-step pathway for oxidation of H₂S directly to sulfate.

Sulfur oxidation can influence pH and mineral chemistry, with consequences for corrosion, acid mine drainage and sulfide management. Carefully controlled sulfur-oxidizing communities are also studied for wastewater treatment and sulfur recovery. Outcomes depend on the sulfur substrate, electron acceptor and system design.

3. Iron-oxidizing microorganisms

Iron oxidizers obtain electrons from ferrous iron (Fe²⁺), converting it to ferric iron (Fe³⁺). The ferric products commonly hydrolyse and precipitate as iron(III) oxyhydroxides, forming yellow, orange or brown deposits. Microbial iron oxidation is found in acidic systems as well as at near-neutral pH, but the ecological circumstances are quite different.

Acidophilic iron oxidation

In acidic environments, species such as Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans can use Fe²⁺ as an electron donor. These organisms are important in the cycling of iron and sulfur in acid mine drainage and in some mineral bioleaching processes. At low pH, ferrous iron can persist in solution and microbial oxidation may make an important contribution to Fe³⁺ production.

Iron oxidation around neutral pH

At circumneutral pH, Fe²⁺ readily undergoes abiotic oxidation in the presence of sufficient oxygen. Some specialized microaerophilic iron oxidizers, including Gallionella ferruginea and members of Sideroxydans, occupy interfaces where ferrous iron from oxygen-poor water meets a limited oxygen supply. In these microhabitats, they can compete with chemical oxidation and form characteristic iron-rich stalks or deposits.

An important correction to older lecture notes is that Fe²⁺ does not oxidize at the same rate whenever oxygen is present. Its abiotic oxidation rate is strongly affected by pH and oxygen availability. The blanket statement that iron oxidizes spontaneously under all oxygenated conditions above pH 1 is therefore misleading. Nor should organisms found near iron deposits automatically be classified as iron-oxidizing chemolithoautotrophs without evidence of their metabolism.

4. Nitrifying microorganisms

Nitrification is the microbial oxidation of reduced nitrogen toward nitrate. In the classical two-step description, ammonia oxidizers convert ammonia to nitrite, and nitrite oxidizers then convert nitrite to nitrate. These energy-conserving reactions are fundamental to the nitrogen cycle and to the design of biological wastewater-treatment systems.

Ammonia oxidation

Ammonia-oxidizing bacteria such as Nitrosomonas use ammonia oxidation for energy conservation. The process begins with ammonia monooxygenase (AMO), which participates in the conversion of ammonia toward hydroxylamine, followed by further oxidation steps leading to nitrite. The exact pathway and intermediates vary among groups, and ammonia-oxidizing archaea differ biochemically from the better-characterized bacterial systems.

A major advance in nitrification research was the recognition that ammonia oxidation is not limited to bacteria. In 2005, Könneke and colleagues reported an autotrophic ammonia-oxidizing marine archaeon. Ammonia-oxidizing archaea are now recognized as important contributors in many marine and terrestrial environments, although their relative contribution varies with local conditions.

Nitrite oxidation and complete ammonia oxidation

Nitrite-oxidizing bacteria such as Nitrobacter and many Nitrospira use nitrite as an electron donor and form nitrate. Importantly, not every Nitrospira carries out only this second step. In 2015, researchers demonstrated that some Nitrospira can perform complete ammonia oxidation to nitrate within a single organism. This metabolism is called comammox, short for complete ammonia oxidation.

The discovery of comammox changed the long-standing textbook picture of nitrification as necessarily requiring two different microorganisms. Classical two-partner nitrification remains important, but contemporary explanations should include ammonia-oxidizing archaea and comammox bacteria. Many nitrifiers are autotrophic, yet their carbon-fixation pathways differ: the Calvin cycle is not universal across bacterial and archaeal nitrifiers.

Carbon fixation in chemolithoautotrophs

Chemolithoautotrophs use the energy obtained from inorganic electron donors to assimilate inorganic carbon into biomass. The Calvin–Benson–Bassham cycle is common in several bacterial groups, but it is not the only natural route. Depending on the organism, inorganic carbon may be fixed through the reductive tricarboxylic acid cycle, the Wood–Ljungdahl pathway, or other specialized pathways, including the 3-hydroxypropionate/4-hydroxybutyrate cycle described in some archaea.

Consequently, it is scientifically inaccurate to attribute the Calvin cycle to every hydrogen, sulfur, iron or nitrogen oxidizer. The particular pathway must be established for the organism or lineage being discussed, ideally using physiological evidence together with genomic or biochemical data.

Comparison of major chemolithotrophic metabolisms

Why chemolithotrophy matters in nature and technology

Chemolithotrophic microorganisms can support primary production in dark environments where chemical energy is available. They also transform nitrogen, sulfur and iron compounds, shaping nutrient availability, water chemistry, mineral formation and ecosystem functioning. At redox boundaries, even narrow zones of low oxygen can host intense microbial activity.

The same metabolisms are relevant to engineered systems. Nitrification is central to biological treatment of ammonia-containing wastewater; sulfur oxidizers can participate in sulfide management; and iron- and sulfur-oxidizing organisms contribute to mineral bioleaching. Hydrogen-oxidizing autotrophs are being investigated for carbon-based bioproduction. These are process-specific applications, and the presence of chemolithotrophs alone does not guarantee an environmental or economic benefit.

How scientists investigate chemolithotrophy

Cultivation studies can test whether growth is supported by a defined inorganic electron donor, appropriate electron acceptor and carbon source. Chemical measurements of substrates and products help demonstrate the net redox reaction. Molecular approaches identify functional genes, such as those involved in ammonia oxidation, hydrogen metabolism or sulfur oxidation, while transcriptomic, proteomic or isotope-tracing methods can provide additional evidence of pathway activity.

A useful distinction for students is that detection of a functional gene establishes metabolic potential, not necessarily active growth or a measured process rate. Likewise, an organism found in iron-rich or sulfur-rich material is not automatically an iron or sulfur oxidizer. Strong interpretation combines taxonomy, physiology, chemistry and environmental context.

Conclusion

Chemolithotrophy illustrates how microorganisms obtain energy from redox reactions involving inorganic substances instead of relying solely on organic electron donors or sunlight. Hydrogen, sulfur, iron and nitrogen oxidation represent different biochemical strategies rather than one uniform pathway. Their ecological importance lies in coupling chemical transformations to microbial growth and, in many species, to the fixation of inorganic carbon.

Modern microbiology has expanded the classical account: archaeal ammonia oxidizers, complete nitrifiers and diverse carbon-fixation pathways reveal a broader range of metabolic options than older lecture notes suggested. For students, the most reliable way to study a chemolithotroph is to ask four questions: What provides the electrons? What accepts them? Where does cellular carbon come from? And under which conditions does the metabolism operate?

References and further reading

1. OpenStax. (2016). 8.1 Energy, matter, and enzymes. In Microbiology. OpenStax. https://openstax.org/books/microbiology/pages/8-1-energy-matter-and-enzymes

2. OpenStax. (2016). 8.7 Biogeochemical cycles. In Microbiology. OpenStax. https://openstax.org/books/microbiology/pages/8-7-biogeochemical-cycles

3. Könneke, M., Bernhard, A. E., de la Torre, J. R., Walker, C. B., Waterbury, J. B., & Stahl, D. A. (2005). Isolation of an autotrophic ammonia-oxidizing marine archaeon. Nature, 437, 543–546. https://doi.org/10.1038/nature03911

4. Daims, H., Lebedeva, E. V., Pjevac, P., Han, P., Herbold, C., Albertsen, M., Jehmlich, N., Palatinszky, M., Vierheilig, J., Bulaev, A., Kirkegaard, R. H., von Bergen, M., Rattei, T., Bendinger, B., Nielsen, P. H., & Wagner, M. (2015). Complete nitrification by Nitrospira bacteria. Nature, 528, 504–509. https://doi.org/10.1038/nature16461

5. van Kessel, M. A. H. J., Speth, D. R., Albertsen, M., Nielsen, P. H., Op den Camp, H. J. M., Kartal, B., Jetten, M. S. M., & Lücker, S. (2015). Complete nitrification by a single microorganism. Nature, 528, 555–559. https://doi.org/10.1038/nature16459

6. Garritano, A. N., Song, W., & Thomas, T. (2022). Carbon fixation pathways across the bacterial and archaeal tree of life. PNAS Nexus, 1(5), pgac226. https://doi.org/10.1093/pnasnexus/pgac226

7. Ghosh, W., & Dam, B. (2009). Biochemistry and molecular biology of lithotrophic sulfur oxidation by taxonomically and ecologically diverse bacteria and archaea. FEMS Microbiology Reviews, 33(6), 999–1043. https://doi.org/10.1111/j.1574-6976.2009.00187.x

8. Jones, S., & Santini, J. M. (2023). Mechanisms of bioleaching: Iron and sulfur oxidation by acidophilic microorganisms. Essays in Biochemistry, 67(4), 685–699. https://doi.org/10.1042/EBC20220257

9. Kato, S., Chan, C., Itoh, T., & Ohkuma, M. (2013). Functional gene analysis of freshwater iron-rich flocs at circumneutral pH and isolation of a stalk-forming microaerophilic iron-oxidizing bacterium. Applied and Environmental Microbiology, 79(17), 5283–5290. https://doi.org/10.1128/AEM.03840-12