Chemolithotropic Metabolism and Inorganic Energy Sources

Core Idea

Chemolithotrophic bacteria oxidize inorganic compounds (H₂S, NH₃, Fe²⁺, H₂) to gain energy, while fixing CO₂ for carbon. These 'autotrophs' include nitrifiers, sulfur oxidizers, and iron oxidizers, and are essential for nutrient cycling. Some are facultative (can also use organic substrates), while others are obligate chemolithotrophs.

How It's Best Learned

Study enrichment cultures of nitrifying bacteria and sulfur oxidizers. Examine sulfur globule inclusions in sulfur bacteria under microscopy.

Common Misconceptions

Chemolithotrophs are not common pathogens—most are environmental bacteria. 'Autotrophy' refers to carbon fixation from CO₂, not energy generation; some organotrophs are also autotrophs.

Explainer

From your study of bacterial metabolism, you know that all organisms need two things: an energy source to drive cellular work and a carbon source to build biomolecules. Most organisms you have encountered so far — including the heterotrophs covered in earlier topics — get both from organic compounds like glucose. Chemolithotrophs break this pattern entirely: they harvest energy by oxidizing inorganic molecules and fix CO₂ from the atmosphere for carbon. They are, in effect, living off rocks and air.

The logic is the same redox chemistry you already understand. In aerobic respiration, glucose is the electron donor and oxygen is the terminal electron acceptor, with the energy from electron transfer captured as a proton gradient that drives ATP synthase. Chemolithotrophs use inorganic electron donors instead of glucose. Nitrifying bacteria like *Nitrosomonas* oxidize ammonia (NH₃) to nitrite, and *Nitrobacter* oxidizes nitrite to nitrate — each step releasing electrons that feed into an electron transport chain. Sulfur-oxidizing bacteria like *Thiobacillus* oxidize hydrogen sulfide (H₂S) or elemental sulfur to sulfate. Iron-oxidizing bacteria convert ferrous iron (Fe²⁺) to ferric iron (Fe³⁺). In each case, oxygen typically serves as the terminal electron acceptor, and the resulting proton motive force drives ATP synthesis by the same chemiosmotic mechanism used in mitochondria.

The energy yields from these inorganic oxidations are typically much lower than from glucose oxidation, because the redox potential difference between donor and acceptor is smaller. This means chemolithotrophs grow slowly compared to heterotrophs — but they can thrive in environments where no organic carbon exists. They are the primary producers in deep-sea hydrothermal vent ecosystems, acid mine drainage, and deep subsurface rock formations. Some are obligate chemolithotrophs, unable to use organic compounds at all, while others are facultative, switching to organic substrates when available.

The ecological importance of these organisms is difficult to overstate. Nitrifying bacteria drive the nitrogen cycle by converting ammonia (which plants cannot easily use) into nitrate (which plants absorb readily). Sulfur oxidizers prevent toxic H₂S accumulation in soils and aquatic sediments. Iron oxidizers influence mineral weathering and soil formation. Without chemolithotrophs cycling these inorganic compounds, the biogeochemical cycles that sustain all life on Earth would grind to a halt. They also have practical applications: bioleaching uses iron- and sulfur-oxidizing bacteria to extract metals from low-grade ores, and nitrifying bacteria are essential in wastewater treatment for removing ammonia.