anaerobic

Anaerobic describes processes, organisms, or environments that occur in the absence of oxygen, or in which exposure to oxygen is harmful. In microbiology, organisms are commonly classified as obligate anaerobes, facultative anaerobes, or aerotolerant anaerobes. Obligate anaerobes cannot withstand atmospheric oxygen; facultative anaerobes can use oxygen when available but do not require it for growth; aerotolerant anaerobes do not utilize oxygen but tolerate its presence.

Anaerobic metabolism includes fermentation, where ATP is generated by substrate-level phosphorylation with organic compounds as electron acceptors, and anaerobic respiration, in which alternative electron acceptors such as nitrate, sulfate, or carbon dioxide replace oxygen. Compared with aerobic respiration, ATP yield is typically lower. Common fermentation end products include lactic acid, ethanol, and various organic acids.

Obligate anaerobes inhabit oxygen-free environments such as deep soil, sediments, the gastrointestinal tracts of animals, and anaerobic digesters. Notable groups include Clostridium and Bacteroides bacteria, methanogenic archaea, and sulfate-reducing bacteria.

Industrially, anaerobic processes are exploited in fermentation, anaerobic digestion for biogas production, and wastewater treatment. Medically, anaerobic infections occur in poorly perfused tissues and deep wounds; pathogens such as Clostridium species can cause gas gangrene, botulism, or tetanus. Treatment emphasizes wound debridement, targeted antibiotics, and, in some cases, oxygen therapy or hyperbaric oxygen to inhibit anaerobes where appropriate.

Etymology: the term derives from Greek an- "without" and aer "air".