There are more than 60,000 different kinds of bacteria, and billions can inhabit a single gram of soil. Two categories of soil-dwelling bacteria include aerobic bacteria, which require oxygen, and anaerobic bacteria, which do not need oxygen to survive. Anaerobic bacteria tend to dwell in areas with poorly drained soil. There are two main types of anaerobic bacteria found in soil and other environments: facultative anaerobes and obligate anaerobes.
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Facultative Anaerobic Bacteria
Facultative bacteria can exist with or without oxygen and are a common type of anaerobe found in soil. They play a key role in carrying out fermentation in soil that is temporarily anaerobic and wet. Fermentation can be a positive or negative effect of soil bacteria. It may be a useful process in decomposition and breaking down materials in the soil, but may also produce compounds that are toxic to plant and animal life.
Most denitrifying bacteria are facultative anaerobes. Denitrifying bacteria convert nitrate to nitrogen or nitrous oxide, releasing it back into the atmosphere as part of the nitrogen cycle.
Obligate Anaerobic Bacteria
Obligate anaerobes, which require an oxygen-free environment, will die in the presence of oxygen. The most abundant type of obligate anaerobe found in soil belongs to the genus Clostridium. In soil, clostridium bacteria ferment compounds such as carbohydrates or amino acids. Some strains can also convert dinitrogen as part of the nitrogen cycle.
Other Types of Anaerobic Bacteria
Lesser types of anaerobic bacteria include obligate anaerobes such as methanogens and sulfate-reducing bacteria.
Methanogens are a kind of archaebacteria that produce methane through a particular metabolic pathway. They constitute an important final step in the carbon cycle, but are also contributors to the greenhouse effect. Methanogens are also significant to biotechnology because methane is a source of clean, high-energy fuel.
Sulfate-reducing bacteria convert sulfates found in groundwater to hydrogen sulfide. Hydrogen sulfide produces an unpleasant smell akin to rotten eggs; if consumed, it may cause nausea or illness in humans.