In the world at large, the heterotrophic processes of decomposition (catabolism) approximately balance the autotrophic metabolism (anabolism), as was indicated in this section’s statement, but the balance varies widely locally.
If decomposition is considered in the broad sense as “any energy-yielding biotic oxidation”, then several types of decomposition roughly parallel the types of photosynthesis when oxygen requirements are considered:
i. Type 1. Aerobic respiration—gaseous (molecular) oxygen is the electron acceptor (oxidant).
ii. Type 2. Anaerobic respiration—gaseous oxygen is not involved. An inorganic compound other than oxygen, or an organic compound, is the electron acceptor (oxidant).
iii. Type 3. Fermentation—also anaerobic, but the organic compound oxidized is also the electron acceptor (oxidant).
Aerobic respiration (Type 1) is the reverse of photosynthesis; it is the process by which organic matter (CH2O) is decomposed back to CO2 and H2O with a release of energy. All of the higher plants and animals and most of the Monerans and Protists obtain their energy for maintenance and for the formation of cellular material in this manner. Complete respiration yields CO2, H2O, and cellular material, but the process may be incomplete, leaving energy-containing organic compounds to be used later by other organisms.
The equation for aerobic respiration is typically written as follows:
C6H12O6 + 6O2 –> 6CO2 + 6H2O
As you will recall, during photosynthesis, solar energy is captured and stored in high-energy bonds in carbohydrates; oxygen is released in the process. The carbohydrate (such as a monosaccharide sugar, C6H12O6) is used by the autotroph or is ingested by the heterotroph. The energy contained in the carbohydrate is released during respiration via glycolysis and the Krebs cycle; carbon dioxide and water are also released. In virtually all ecosystems, photosynthetic autotrophs provide energy for the total system. Thus, the ultimate source of energy for the system is the Sun.
Respiration without O2 (anaerobic respiration) is largely restricted to the saprophages, such as bacteria, yeasts, molds, and protozoa, although it occurs as a dependent process in certain tissues of higher animals (muscle contraction, for example). The methane bacteria are examples of obligate anaerobes that decompose organic compounds with the production of methane (CH4) through reduction of either organic or mineral (carbonate) carbon (thus employing both types of anaerobic metabolism).
In aquatic environments, such as freshwater marshes and swamps, the methane gas, often known as “swamp gas,” rises to the surface, where it can be oxidized or, if it catches fire, may be reported as a UFO (unidentified flying object). The methane bacteria are also involved in the breakdown of forage in the rumen of cattle and other ruminants. As we deplete supplies of natural gas and other fossil fuels, these microbes may be domesticated to produce methane on a large scale from manure or other organic sources.
Desulfovibrio and other varieties of sulfate-reducing bacteria are ecologically important examples of anaerobic respiration (Type 2), because they reduce SO4 in deep sediments and in anoxic waters, such as the Black Sea, to H2S gas. The H2S can rise to shallow sediments or surface waters, where it can be oxidized by other organisms (the photosynthetic sulfur bacteria, for example). Alternatively, H2S can combine with Fe and Cu and many other minerals.
Millions of years ago, the microbial production of minerals may have been responsible for many of our most valuable metal ore deposits. On the negative side, sulfate-reducing bacteria cause billions of dollars damage annually through corrosion of metals by the H2S they produce. Yeasts, of course, are well-known examples of organisms using fermentation (Type 3). They are not only commercially important but also abundant in soil, where they help decompose plant residues.
Many kinds of bacteria are capable of both aerobic and anaerobic respiration, but the end products of the two reactions will be different, and the amount of energy released will be much less under anaerobic conditions. For example, the same species of bacterium, Aerobacter, can be grown under anaerobic and aerobic conditions with glucose as the carbon source. When oxygen is present, almost all of the glucose is converted into bacterial biomass and CO2, but in the absence of oxygen, decomposition is incomplete, a much smaller portion of the glucose ends up as cellular carbon, and a series of organic compounds (such as ethanol, formic acid, acetic acid, and butanediol) is released into the environment.
Additional bacterial specialists would be required to oxidize these compounds further and recover additional energy. When the rate of input of organic detritus into soils and sediments is high, bacteria, fungi, protozoa, and other organisms create anaerobic conditions by using up the oxygen faster than it can diffuse into water and soil. Decomposition does not stop then but continues, often at a slower rate, provided an adequate diversity of anaerobic microbial metabolic types is present.
Figure 2-12 illustrates the end products of aerobic and anaerobic metabolism when an input of nutrients (such as untreated municipal sludge) enters a stream or river. Before the point-source input of sludge, the stream is characterized by an abundance of dissolved oxygen and high species diversity. The input of sludge results in a biological oxygen demand (BOD) caused by bacterial respiration during the decomposition of waste products.
Thus, the stream system becomes more anaerobic as a result of the decomposition process and is characterized by decreased oxygen and reduced biotic diversity. Notice that the end products of anaerobic metabolism contain acids, alcohols, and products that may damage aquatic life in the stream.
Although the anaerobic decomposers (both obligate and facultative) are inconspicuous components of the community, they are nonetheless important in the ecosystem because they alone can respire or ferment organic matter in the dark, in oxygen less layers of soils, and in aquatic sediments. Thus, they “rescue” energy and materials temporarily lost in the detritus of soils and sediments.
The anaerobic world of today may be a clear model of the primordial world, because it is believed that the earliest life-forms were anaerobic prokaryotes.
Rich (1978) described the two-step evolution of life as follows – First, Precambrian life evolved as the free energy from lengthening electron transport increased (the quality of energy available to organisms increased). In the second step, the realm of conventional multicellular evolution, the energetic value of a unit of organic matter was fixed (ultimate electron acceptor = oxygen) and life evolved in response to the quantity of energy available to organisms.
In today’s world, the reduced inorganic and organic compounds produced by anaerobic microbial processes serve as carbon and energy reservoirs for photosynthetically fixed energy. When later exposed to aerobic conditions, the compounds serve as substrates for aerobic heterotrophs.
Accordingly, the two lifestyles are intimately coupled and function together for mutual benefit. For example, a sewage disposal system, which is a human-engineered decomposing subsystem, depends on the partnership between anaerobic and aerobic decomposers for maximum efficiency.