While more complex organisms tend to engage in aerobic respiration, some microorganisms can effectively break down organic matter without any oxygen at all. Good thing, since they can produce methane and break down organic materials in situations where there is no oxygen available, such as in stagnated water.
"Anaerobic" refers to a process that does not involve oxygen. Organic materials that break down without oxygen lead to fermentation. Fermentation is the process of organic material breaking down through living anaerobic organisms. Anaerobic organisms use phosphorus, nitrogen and other nutrients to develop protoplasm. Unlike aerobic bacteria, anaerobic bacteria turn organic nitrogen into acids and ammonia. They release carbon in the form of methane gas. A small amount of carbon disperses as CO2.
Organic Matter Decomposition
Marshes contain large amounts of methane because of anaerobic fermentation. When the organic matter decomposes through putrefaction, undesirable odors appear, resulting from the hydrogen sulfide and organic compounds that contain sulfur, such as mercaptans. Organic material that breaks down through anaerobic processes eventually becomes humus.
Humus is an important material for gardening and farming. This humus experiences a small amount of oxidation, which does not negatively affect the material as a gardening or farming aid. Organic material contains so much energy that anaerobic decomposition can actually produce heat through the process. The heat produced by the aerobic process can sometimes be high enough to kill pathogens, but the anaerobic process is usually not hot enough. However, pathogens often leave the environment because it is unsuitable for their growth and has biological antagonists.
Anaerobic composting happens most often in environments where there is a large amount of moisture, because the moisture acts as a barrier, preventing oxygen from entering. People engaged in anaerobic composting might notice unpleasant odors, but submerging the material in water causes the gases to dissolve and enter the atmosphere more slowly, reducing the smells.
Anaerobic digestion goes through four stages. The first stage is hydrolysis, when organic material breaks into simple organic molecules through water. The water breaks the chemical bonds. Next, fermentation, or acidogenesis, occurs, accelerating the decomposition of carbohydrates through bacteria, enzymes, molds and yeasts. Then, acetogenic bacteria convert the products of the fermentation to acetate in a process called acetogenesis. Meanwhile, methanogenesis occurs as bacteria break down acetate, hydrogen and carbon dioxide. Methanogenic and acetogenic bacteria work together, with the methanogenic bacteria keeping the hydrogen levels down well enough for the acetogenic bacteria to break down the fermentation products.
Anaerobic digestion plays an important role in biological sewage treatment as well as humus production. This digestion process converts wastewater pollutants into carbon dioxide, methane and small amounts of biosolids. Anaerobic digestion can also create methane fuel. Since some bacteria found in the decomposing materials produce acids that lower the pH, people using anaerobic decomposition for methane gas production need to add sodium bicarbonate to keep the mixture basic.