Anaerobic respiration

Anaerobic respiration is a biological process used by some organisms to generate energy without molecular oxygen. In aerobic respiration, oxygen acts as the final electron acceptor in an electron transport chain. Oxygen is an excellent electron acceptor, which allows aerobic organisms to extract a large amount of energy. Anaerobes, however, use different substances to complete their energy-making process. These substances include nitrate, fumarate, sulfate, or elemental sulfur. Because these chemical substances are less oxidizing than oxygen, they have smaller reduction potentials. This means that anaerobic respiration releases less energy per oxidized molecule than aerobic respiration does.

To understand how this works, we must look at the electrochemical gradient. During cellular respiration, cells use reduced chemical compounds like NADH and FADH2. These compounds are produced during stages like glycolysis and the citric acid cycle. These electrons are passed through a series of respiratory integral membrane proteins. Each protein in the chain has a sequentially increasing reduction potential. This movement of electrons helps establish a proton gradient across a membrane. This gradient creates an electrical potential or a difference in ion concentration. This force is known as the proton motive force.

This proton motive force drives protons down their concentration gradient. The protons move through a specific protein channel called ATP synthase. This movement acts like a current to drive the synthesis of ATP from ADP and inorganic phosphate. It is important to distinguish this from fermentation. While both processes can occur without oxygen, they generate ATP in very different ways. Fermentation does not use an electrochemical gradient at all. Instead, it relies on substrate-level phosphorylation to produce energy. In fermentation, the electron acceptor NAD+ is regenerated from NADH through the reduction of other compounds.

Microbes use many different types of electron acceptors to survive. For example, some bacteria perform denitrification. In this process, they use nitrate as a terminal electron acceptor to produce nitrogen gas (N2). Other microbes perform sulfate respiration, which produces hydrogen sulfide. This gas is responsible for the "rotten egg" smell often found in coastal wetlands. Some organisms engage in methanogenesis, a form of carbon dioxide respiration. This process produces methane gas through either carbon dioxide reduction or acetate fermentation. Different organisms specialize in different chemicals. For instance, *Geobacter* species can reduce iron, while *Desulfovibrionales* use sulfate.

These biological processes have a massive impact on the Earth's environment. Anaerobic respiration is a critical component of the global nitrogen, iron, sulfur, and carbon cycles. This movement of elements is known as biogeochemical cycling. These cycles significantly impact the carbon cycle and global warming. For example, uncontrolled methanogenesis in landfill sites releases large amounts of methane. Methane is a potent greenhouse gas that contributes to climate change. However, biogenic methane can also serve as a sustainable alternative to fossil fuels. The way these chemicals cycle through the Earth helps regulate our atmosphere and ecosystems.

Anaerobic respiration occurs in many diverse environments. You can find it in freshwater and marine sediments, soil, and subsurface aquifers. It also exists in deep subsurface environments and within biofilms. Even in environments that contain oxygen, such as soil, tiny micro-environments exist without it. This happens because oxygen gas has slow diffusion characteristics. This allows anaerobic life to thrive even in seemingly aerobic places. These small pockets are essential for the continuous cycling of nutrients across the planet.

Humans can also use these microbial processes for helpful applications. Dissimilatory denitrification is widely used to remove nitrate and nitrite from municipal wastewater. This is important because excess nitrate can cause eutrophication in waterways. Additionally, these microbes are used in bioremediation to clean up contaminated environments. Some bacteria can reduce toxic arsenate or selenate into less harmful molecules. They can also reduce chlorinated pollutants like vinyl chloride. Finally, scientists use microbial fuel cells to generate electricity. These cells use bacteria that respire solid electron acceptors, like oxidized iron, to transfer electrons to an electrode. This can clean organic waste while producing power.