We have alluded to their role in the decay of organic matter, and we shall expand the idea by briefly examining the place of microorganisms in three vital cycles of nature:
1. The carbon,
2. Sulfur, and
3. Nitrogen cycles.
1. Carbon Cycle:
The earth is composed of numerous elements, among which is a defined amount of carbon that must constantly be recycled to allow the formation of organic compounds of which all living things are made. Photosynthetic organisms take carbon in the form of carbon dioxide and convert it into carbohydrates using the sun’s energy and chlorophyll pigments.
The vast jungles of the world, the grassy plains of the temperate zones, and the plants of the oceans show the results of this process. Photosynthetic organisms, in turn, are consumed by grazing animals, fish, and humans who use some of the carbohydrates for energy and convert the remainder to cell parts. To be sure, some carbon dioxide is released back in respiration, but a major portion of the carbon is returned to the earth when the animal or plant dies.
It is here that the microorganisms exert their influence, for they are the primary decomposers of dead organic matter. Working in their countless billions in the water and soil, bacteria, fungi, and other microorganisms consume the organic substances and release carbon dioxide for reuse by the plants. This activity results from the concerted action of a huge variety of microorganisms, each with its own nutritional pattern of protein, carbohydrate, or lipid digestion.
Without the microorganisms, the earth would be a veritable garbage dump of animal waste, dead plants, and organic debris accumulating in implausible amounts.
But there is more. Microorganisms also break down the carbon-based chemicals produced by industrial processes including herbicides, pesticides, and plastics. In addition, they produce methane, or natural gas, from organic matter and are probably responsible for the conversion of plants to petroleum and coal deep within the recesses of the earth.
Moreover, many microorganisms trap CO2 from the atmosphere and form carbohydrates to supplement the results of photosynthesis. In these activities, the microorganisms represent a fundamental underpinning of organic creation.
2. Sulfur Cycle:
The sulfur cycle may be defined in more specific terms than the carbon cycle. Sulfur is a key constituent of such amino acids as cystine, cysteine, and methionine, all of which are important components of proteins. Proteins are deposited in water and soil as living things die, and bacteria decompose the proteins and break down the sulfur-containing amino acids to yield various compounds including hydrogen sulfide.
Sulfur may also be released in the form of sulfate molecules commonly found in organic matter. Anaerobic bacteria such as those of the genus Desulfovibrio subsequently convert sulfate molecules to hydrogen sulfide.
The next set of conversions involves several genera of bacteria, including members of the genera Thiobacillus, Beggiatoa, and Thiothrix. These bacteria release sulfur from hydrogen sulfide during their metabolism and convert it into sulfate. The sulfate is now available to plants where it is incorporated into the sulfur-containing amino acids. Consumption by animals and humans completes the cycle.
3. Nitrogen Cycle:
The cyclic transformation of nitrogen is of paramount importance to life on Earth. Nitrogen is an essential element in nucleic acids and amino acids. Although it is the most common gas in the atmosphere (about 80 percent of air), animals cannot use nitrogen in its gaseous form, nor can any but a few species of plants. The animals and plants thus require the assistance of microorganisms to trap the nitrogen.
The nitrogen cycle begins with the deposit of dead plants and animals in the soil. In addition, nitrogen reaches the soil in urea contained in urine. A process of digestion and putrefaction by soil bacteria and other microorganisms follows, thus yielding a mixture of amino acids. Amino acids are further broken down in microbial metabolism, and the ammonia that accumulates may be used directly by plants.
Next, mineralization takes place. In this process, complex organic compounds are finally converted to inorganic compounds and additional ammonia. Much of the ammonia is converted to nitrite ions by Nitrosomonas species, a group of aerobic Gram-negative rods. In the process, the bacteria obtain energy for their metabolic needs.
The nitrite ions are then converted to nitrate ions by species of Nitrobacter, another group of aerobic Gram-negative rods, which obtain energy from the process. Nitrate is a crossroads compound: it can be used by plants for their nutritional needs, or it can be liberated as atmospheric nitrogen by certain microorganisms.
For the nitrogen released to the atmosphere, a reverse trip back to living things is an absolute necessity for life to continue as we know it. The process is called nitrogen fixation. Once again microorganisms in water and soil play a key role because they possess the enzyme systems that trap atmospheric nitrogen and convert it to compounds useful to plants. In nitrogen fixation, gaseous nitrogen is incorporated to ammonia that fertilizes plants.
Two general types of microorganisms are involved in nitrogen fixation: free-living species and symbiotic species. Free-living species include bacteria of the genera Bacillus, Clostridium, Pseudomonas, Spirillum, and Azotobacter, as well as types of cyanobacteria and certain yeasts. Generally, the free-living species fix nitrogen during their growth cycles. The nitrogen-fixing ability of these species cannot be overemphasized.
Symbiotic species of nitrogen-fixing microorganisms live in association with plants that bear their seeds in pods. These plants, known as legumes, include peas, beans, soybeans, alfalfa, peanuts, and clover. Species of Gram-negative rods known as Rhizobium infect the roots of the plants and live within swellings, or nodules, in the roots.
Although complex factors are involved, the central theme of the relationship is that Rhizobium fixes nitrogen and makes nitrogen compounds available to the plant while taking energy-rich carbon compounds in return. The bulk of the nitrogen compounds accumulates when Rhizobium cells die.
Legumes then use the compounds to construct amino acids and, ultimately, protein. Animals consume the soybeans, alfalfa, and other legumes and convert plant protein to animal protein, thereby completing the cycle.
Humans have long recognized that soil fertility can be maintained by rotating crops and including a legume. The explanation lies in the ability of rhizobia to fix nitrogen within the nodules of legumes. So much nitrogen is captured, in fact, that the net amount of nitrogen in the soil actually increases after a crop of legumes has been grown. When cultivating legumes, there is no need to add nitrogen fertilizer to the soil.
In addition, when crops such as clover or alfalfa are plowed under, they markedly enrich the soil’s nitrogen content. Thus, humans are indebted to microorganisms for such edible plants as peas and beans, as well as for the indirect products of nitrogen fixation, namely, steaks, hamburgers, and milk.