An anaerobe is an organism that does not require oxygen (\(text{O}_2\)) to grow and survive. This ability challenges the common assumption that all life depends on oxygen, a gas that is actually toxic to many microbial species. This trait is held by certain bacteria, archaea, fungi, and protozoa. These organisms utilize alternative chemical processes for metabolism, often inhabiting environments where oxygen is scarce or absent. They demonstrate life’s biochemical flexibility, thriving across a vast range of environments, from deep ocean sediments to the human body.
Types of Organisms That Shun Oxygen
The classification of anaerobes depends on how they react to oxygen exposure.
Obligate Anaerobes
Obligate anaerobes are the most sensitive group; exposure to even small amounts of oxygen can be lethal. These organisms lack the necessary enzymes, such as superoxide dismutase and catalase, used by other life forms to neutralize toxic oxygen byproducts like superoxide and hydrogen peroxide. Consequently, they are restricted to environments like deep soils, puncture wounds, or the lower gastrointestinal tract where oxygen levels are virtually zero.
Facultative Anaerobes
Facultative anaerobes are metabolically flexible and can switch their energy production methods based on their surroundings. They perform efficient aerobic respiration when oxygen is available, but switch to anaerobic pathways when oxygen disappears. This dual capability allows them to grow throughout a medium, though they often concentrate where oxygen is most abundant because aerobic respiration yields the greatest energy.
Aerotolerant Anaerobes
Aerotolerant anaerobes cannot use oxygen for metabolism but are not harmed by its presence. Unlike obligate anaerobes, they possess superoxide dismutase, which allows them to detoxify harmful oxygen derivatives. They rely exclusively on anaerobic metabolism, even when oxygen is present, because they cannot use it as a terminal electron acceptor. This resilience permits them to inhabit oxygenated areas, such as the surface of the skin.
How Anaerobes Generate Energy
Anaerobes generate adenosine triphosphate (ATP) through biochemical pathways that do not rely on oxygen as the final electron acceptor. These methods, fermentation and anaerobic respiration, are distinct from the highly efficient aerobic respiration used by oxygen-breathing organisms.
Fermentation
The most straightforward pathway is fermentation, a less efficient process that uses only glycolysis to generate ATP. Glucose is partially broken down into pyruvate, yielding a small net gain of two ATP molecules. The final step converts pyruvate into end products like lactic acid or ethanol, which regenerates the electron carrier molecules needed to keep glycolysis running. For example, fermentation by certain bacteria produces lactic acid, causing milk to sour and curdle into yogurt.
Anaerobic Respiration
Anaerobic respiration is a more complex and energy-yielding process. It utilizes a membrane-bound electron transport chain, similar to aerobic respiration. However, instead of oxygen, these organisms use an inorganic molecule other than \(text{O}_2\) as the final electron acceptor. Common non-oxygen acceptors include nitrate (\(text{NO}_3^-\)), sulfate (\(text{SO}_4^{2-}\)), or carbon dioxide (\(text{CO}_2\)). The electron transport chain allows anaerobic respiration to generate significantly more ATP than fermentation, though generally less than aerobic respiration. For instance, some archaea use carbon dioxide as the terminal electron acceptor, reducing it to methane (\(text{CH}_4\)) in a process called methanogenesis. This ability allows them to colonize environments devoid of oxygen, such as deep-sea vents and waterlogged soils.
The Critical Roles of Anaerobes in Nature and Our Bodies
Anaerobes play fundamental roles in shaping planetary chemistry and maintaining the health of complex life forms.
Ecological and Biogeochemical Roles
In natural ecosystems, anaerobes are the primary drivers of biogeochemical cycles for elements like carbon, nitrogen, and sulfur in marine sediments and deep aquifers. Sulfate-reducing bacteria, for instance, cycle sulfur by using sulfate as an electron acceptor, essential for decomposition in anaerobic aquatic environments. Methanogens, the \(text{CH}_4\)-producing archaea, generate a significant portion of the planet’s atmospheric methane. These organisms thrive in the digestive tracts of ruminant animals and in landfills, recycling carbon by breaking down organic matter in the absence of oxygen.
Roles in the Human Body and Industry
Anaerobes dominate the microbial community in the human large intestine, often outnumbering aerobic species by 100 to 1. This extensive gut microbiota breaks down complex carbohydrates that human enzymes cannot digest and produces short-chain fatty acids that serve as energy sources for the host. They also contribute to the synthesis of vitamins, such as vitamin K and B-vitamins.
Anaerobes are also relevant in medicine and industry. Pathogens like Clostridium tetani, the causative agent of tetanus, cause serious infections when introduced into oxygen-poor tissue environments. Conversely, they are harnessed industrially for wastewater treatment, where they break down sludge, and in the production of fermented foods like sauerkraut and certain cheeses, where their metabolic byproducts create unique flavors and textures.