Anaerobic respiration

Anaerobic respiration is a type of cellular respiration where the final electron acceptor in the electron transport chain is not molecular oxygen. While aerobic organisms use oxygen, which is an excellent electron acceptor, anaerobes rely on less oxidizing substances like nitrate, fumarate, sulfate, or elemental sulfur. Because these terminal electron acceptors have smaller reduction potentials than oxygen, less energy is released per oxidized molecule, making anaerobic respiration less efficient than aerobic respiration.

Anaerobic respiration should not be confused with fermentation, as they generate ATP through very different mechanisms. In cellular respiration—both aerobic and anaerobic—highly reduced compounds like NADH and FADH₂ are oxidized by a series of membrane proteins with increasing reduction potentials. This process establishes an electrochemical gradient, usually a proton gradient, across a membrane. The resulting proton motive force drives protons through ATP synthase, producing ATP from ADP and inorganic phosphate. Fermentation, on the other hand, relies solely on substrate-level phosphorylation and does not use an electrochemical gradient. Instead, it regenerates NAD⁺ by reducing oxidized compounds, often formed within the fermentation pathway itself. For example, in homofermentative lactic acid bacteria, NADH is oxidized back to NAD⁺ by reducing pyruvate to lactic acid, while in yeast, acetaldehyde is reduced to ethanol for the same purpose. Two important anaerobic microbial methane formation pathways exist: one through carbon dioxide or bicarbonate reduction (respiration) and another through acetate fermentation.

Ecologically, anaerobic respiration is vital to the global nitrogen, iron, sulfur, and carbon cycles by reducing oxyanions of these elements into more reduced compounds. This biogeochemical cycling significantly influences the carbon cycle and global warming. Anaerobic respiration occurs in many oxygen-poor environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface areas, and biofilms. Even oxygen-rich environments like soil contain micro-environments lacking oxygen due to its slow diffusion. For instance, dissimilatory denitrification—using nitrate as a terminal electron acceptor—is the main pathway returning fixed nitrogen to the atmosphere as nitrogen gas and is also important in host-microbe interactions. Some anaerobic ciliates even use denitrifying endosymbionts for energy, similar to how mitochondria function in oxygen-respiring organisms. Methanogenesis, a form of carbon dioxide respiration, produces methane through anaerobic digestion. While biogenic methane can serve as a sustainable alternative to fossil fuels, uncontrolled methanogenesis in landfills releases large amounts of methane, a potent greenhouse gas. Sulfate respiration generates hydrogen sulfide, which gives coastal wetlands their characteristic "rotten egg" smell and can precipitate heavy metal ions, leading to the deposition of sulfidic metal ores.

In applications, dissimilatory denitrification is widely used to remove nitrate and nitrite from municipal wastewater. Excess nitrate can cause eutrophication in waterways, and elevated nitrite levels in drinking water are toxic; denitrification converts both into harmless nitrogen gas. Specific types of anaerobic respiration are also critical in bioremediation, where microorganisms convert toxic chemicals into less harmful ones to clean up contaminated beaches, aquifers, lakes, and oceans. For example, anaerobic bacteria can reduce toxic arsenate or selenate into less harmful compounds, and chlorinated pollutants like vinyl chloride and carbon tetrachloride are also reduced through anaerobic respiration. Additionally, anaerobic respiration is used in microbial fuel cells, where bacteria respire solid electron acceptors like oxidized iron to transfer electrons from reduced compounds to an electrode, simultaneously degrading organic waste and generating electricity.