3.13: Metabolism of Chemolithotrophs

Chemolithotrophs are microorganisms that obtain energy by oxidizing inorganic molecules such as hydrogen gas (H₂), ammonia (NH₃), reduced sulfur compounds (H₂S, S²⁻), and ferrous iron (Fe²⁺). Unlike heterotrophic organisms that rely on organic carbon, chemolithotrophs transfer electrons from these inorganic donors to the electron transport chain (ETC), generating a proton motive force (PMF) that drives ATP synthesis through oxidative phosphorylation. However, because inorganic electron donors generally provide less energy than organic molecules, chemolithotrophs must oxidize large amounts of inorganic material to sustain growth.

The efficiency of energy production in chemolithotrophs depends on the redox potential of their electron donors and acceptors. Oxygen (O₂) is the most commonly used terminal electron acceptor, as it has a highly positive redox potential, maximizing energy yield. However, some chemolithotrophs can utilize alternative acceptors such as nitrate (NO₃⁻), sulfate (SO₄²⁻), or carbon dioxide (CO₂) under anaerobic conditions. Since inorganic molecules provide less energy than organic substrates, the electron transport chains in chemolithotrophs often pump fewer protons across the membrane, leading to lower ATP yields than heterotrophic organisms.

Most chemolithotrophs are autotrophs, meaning they use ATP and reducing power to fix CO₂ and synthesize organic molecules for growth. However, some inorganic electron donors, such as Fe²⁺ and NO₂⁻, have higher redox potentials than the NAD⁺/NADH pair, making it thermodynamically unfavorable to reduce NAD⁺ to NADH directly. To overcome this, many chemolithotrophs use reverse electron flow, an energy-consuming process that forces electrons against the thermodynamic gradient to produce NADH for biosynthetic reactions.

Chemolithotrophs are classified based on their preferred electron donors. Hydrogen-oxidizing bacteria use H₂ as an energy source and transfer electrons to NAD⁺ or the ETC via hydrogenase enzymes. Nitrifying bacteria play a crucial role in the nitrogen cycle by oxidizing ammonia (NH₃) to nitrite (NO₂⁻) and then to nitrate (NO₃⁻), a process known as nitrification. Sulfur-oxidizing bacteria metabolize sulfur compounds such as H₂S and S²⁻, generating sulfate (SO₄²⁻) and ATP through both oxidative phosphorylation and substrate-level phosphorylation. Additionally, anammox bacteria perform anaerobic ammonia oxidation, a unique process that converts NH₄⁺ and NO₂⁻ into nitrogen gas (N₂), contributing to nitrogen loss in aquatic environments.

Chemolithotrophs play a fundamental role in global biogeochemical cycles, particularly in the nitrogen, sulfur, and iron. They contribute to transforming and recycling essential nutrients in ecosystems, influencing soil and water chemistry. Additionally, these microbes are utilized in various industrial and environmental applications, such as bioremediation, bioleaching, and wastewater treatment. Their ability to thrive in extreme environments, including deep-sea hydrothermal vents and acidic mine drainage sites, highlights their metabolic versatility and ecological significance.