Nitrogen cycle
In nature, nitrogen is present in organic form as proteins, and in many inorganic states, the most reduced being NH3 and the most oxidized being NO3–. In the atmosphere, there is abundant molecular nitrogen. There are some nitrogen-fixing microbes in nature. Fixation of atmospheric nitrogen requires high energy input. Only prokaryotes are capable of biological nitrogen fixation.
The most important group is rhizobia which forms root nodules in leguminous plants. There is some specificity between the bacterial strain and the legume host. Through cross-fertilization, the nodulation of specific crops could be increased by inoculation of seeds with appropriate Rhizobium strains. Other associations are also of agricultural value, particularly root nodules in non-legumes by actinomycetes (Frankia) and biological nitrogen fixation by cyanobacteria in paddy fields.
Nitrogen fixation is a rate-limiting step of the nitrogen cycle. There should be efficient cycling between organic and inorganic forms of nitrogen. Inorganic nitrogen in the form of NH3 or NO3– is converted to organic form mainly by plants and microorganisms. NO3– nitrate is reduced first to NH3. Microbes differ in their ability to use an inorganic source of nitrogen.
The organic nitrogenous compounds are converted to ammonia mainly by microorganisms. For this mineralization of organic nitrogen, the microbes degrade dead organic matter and animal excretory products.
In traditional farming systems, mineralization has provided enough NH3 for plants. However, this could hardly satisfy the nitrogen requirements of modern, intensive agriculture. This led to a dramatic increase in the chemical fixation of nitrogen by the Haber process. Approximately 25% of all fixation is now carried out industrially for the production of ammonium-based fertilizers. Their application could, however, lead to environmental problems. Ammonia is converted first to nitrite and then to nitrate by nitrifying bacteria.
Ammonium ions are bound to negatively charged clay particles in the soil. However, nitrate ions are readily leached and can reach high concentrations in run-off waters and rivers. This can cause methemoglobinemia or blue baby disease in animals and infants and can lead to eutrophication. However, it must be clear that none of the nutrient cycles occurs in isolation.
Sulphur cycle
Sulphur, like nitrogen, is also present in organic and inorganic forms and can also be deficient in soils. Sulphate is the major form assimilated by microbes and plants. This is reduced in the cell before incorporation into proteins and other organic compounds.
Mineralization of these organic compounds results in the production of sulphate, or under anaerobic conditions H2S. H2S can also be formed anaerobically by dissimilatory reduction of sulphate. This process is carried out by sulphate-reducing bacteria-Desulphovibrio, Desulphomonas, and Desulphomaculum which use sulphate as an alternative electron acceptor to O2.
This process again exhibits the links between different nutrient cycles as sulphate reduction accounts for up to 50% of the carbon mineralised in marine sediments. This process gives rise to the black odor of mud and sediments of estuaries, the color developing from the precipitation of ferrous sulphide. Sulphide is believed to be involved in the corrosion of mild steel.
H2S is oxidised chemically to sulphate in the presence of O2 but can be oxidised under aerobic and anaerobic conditions by microbes. Anaerobic conversion is carried out by the photosynthetic purple sulphur and green sulphur bacteria, Chromatium and Chlorobium. Aerobic oxidation of sulphide and other inorganic forms such as thiosulphate, tetrathionate and elemental sulphur is carried out by sulphur oxidising bacteria-Beggiatoa and Thiobacillus. H2SO4 produced by Thiobacillus caused cracks in concrete cooling towers.
Interactions Among Microbial Populations
It is common in nature for microorganisms to be associated in some way with other organisms, which may also be microbes or higher life forms, plants, or animals. Such relationships may provide them with protection, nutrients, or other benefits.
From an evolutionary standpoint, such associations are directed towards meeting the challenge of the environment by adaption to existence in continuous close association with other forms of life. This biological phenomenon is known as symbiosis. We have used the term here in a wider sense of its original meaning of living together. (Growth & Distribution of Microorganisms Notes Study Material)
Types of Symbioses
The associations vary widely. On the basis of their degree of intimacy, these may be:
1. Ectosymbioses. The microbe remains external to the cells of its host. The term host is sometimes used for a larger of two symbionts.
2. Endosymbioses. The microbe grows within the cells of its host.
On the basis of the relative advantage accruing to each partner, the associations are classified as follows:
1. Neutralism. The host remains unaffected. Some persons do not recognize it under symbiosis.
2. Mutualism. Both partners benefit from the association.
3. Parasitism. One partner benefits, but the second gains nothing and often suffers mild or severe damage.
Functions of Symbiosis
A symbiont substitutes for part or all of the non-living environment which free-living organisms occupy. The symbioses may provide the partner with any of the following benefits:
1. Protection. Endo-as well as ectosymbionts living in the body cavities of animals is protected from adverse environmental conditions. These habitats protect the symbionts from desiccation and in the case of warm-blooded hosts, from extremes of temperature.
2. Provision of a favorable position. The association may provide one partner with a position that is favorable with respect to the supply of nutrients. Marine ciliates are attached to the body surfaces of crustaceans, where the host’s respiratory and feeding currents assure the microbe of a constant supply of food. Other examples are the favorable position provided by marine invertebrates to their photosynthetic algal symbionts. Other cases are the thick conical protuberances on the mantle of the tridacnid clams (family Tridacnidae).
Each protuberance contains one or more lenslike structures the hyaline organs made up of transparent cells. Each hyaline organ is surrounded by a dense mass of algae. The function of the lenslike haline organ is to permit light to penetrate deeply into the mass of algae.
3. Provision of recognition devices. Bioluminescence is widespread in animals such as jellyfish, earthworms, fireflies, squid, and fish. The emission of light appears to be a recognition device, promoting schooling, mating, or attraction of prey.
4. Nutrition. This is the most common function. The provision of nutrients may be indirect, as in the case of fungi infecting plant roots, where they increase the water-absorbing capacity of the root system. Generally, the association is direct, where the symbiont furnishes one or more essential nutrients to the partner. Some examples are, symbiotic nitrogen-fixing associations; microflora of animal guts, as those eating cellulosic foods; and lichens.