Introduction:

Life on earth would not be probable devoid of microbes. Soil micro-organisms serve up as biogeochemical agents for the transformation of complex organic compounds into simple inorganic compounds or into their constituent's elements. The total procedure is termed as Mineralization. Biogeochemical cycling signifies to the biological and chemical methods that elements like nitrogen, carbon, sulphur, iron and magnesium experience throughout microbial metabolism. This transformation of complex organic compounds to inorganic compounds or elements gives for the continuity of elements (or their compounds) as nutrients for animals and plants comprising man.

Biogeochemical Cycling:

Biogeochemical cycling is the movement of materials through biochemical reactions via biospheres. The biosphere is that part of the earth and its atmosphere in which the living organisms take place. The activities of microorganisms in the biosphere encompass a direct impact on the quality of human life. Micro-organisms are in particular significant in recycling the materials.

The metabolism taken out by micro-organisms frequently transfers materials from one place to the other. Changes in the chemical forms of different elements can lead to the physical movement of the materials. At times causing transfer among the atmosphere (air) hydrosphere (water) and lithosphere (land).

Biogeochemical cycling as well signifies to the biological and chemical methods which elements like nitrogen, carbon, sulphur, iron and magnesium experience all through microbial metabolism.

It can as well be stated as cyclical path which elements take as they flow via living (biotic) and non-living (abiotic) components of the ecosystem. Such cycles are significant as a fixed and limited amount of the elements which make up living cells exists on the earth and in the atmosphere. Therefore in order for an ecosystem to maintain and sustain its features and life forms elements should continuously be recycled.  For illustration: when the organic carbon which animals use as an energy source and blow out as carbon dioxide (CO₂) were not eventually transformed back to an organic form we would run out of organic carbon to form cells.

Elements comprised in the biogeochemical cycles are employed for three general aims.

1) Biomass Production:

In the production of biomass, the element transferred (example: N, C and so on) is incorporated into cell materials for instance, all organisms need nitrogen to generate amino acids, therefore plants and most of the prokaryotes assimilates nitrogen through incorporating ammonia (NH₃) to synthesize the amino acid glutamate.

2) Energy Source:

A reduced form of the element is employed to produce energy in form of ATP. For illustration reduced carbon compounds like lipids and amino acids are employed as energy source through heterotrophs.

3) Terminal Electron Acceptor that is, Carbohydrate Oxidized to CO₂:

The electrons from energy source are transferred to an oxidized form of the element throughout respiration, this decreases the terminal electron acceptor in aerobic conditions, O₂ is employed as a terminal electron acceptor.

Features of Biogeochemical Cycles:

1) Elements needed are in five forms and mostly from the non-living reservoir in the atmosphere. They can as well be gotten from the sedimentary rock or water.

2) The elements go in cycle and are for all time free in inorganic state in abiotic environment and when required in biotic environment, they are turned to organic state. They can be joined with other elements.

3) The recycling of such elements maintains an essential balance of nutrient and they are maintained all the way through.

4) The cycles (that is, biogeochemical) are complex and they comprise the action of producers, consumers and decomposers.

5) All the organisms participate directly in recycling by eliminating, adding or changing nutrients. Microorganisms are noted for the metabolic conversion particularly in changing some elements from one nutritional form to the other.

Carbon Cycle:

Carbon is a much significant element as it makes up organic matter that is a part of all life. Carbon follows a certain route on earth termed as the Carbon Cycle.

The carbon cycle mainly comprises the transfer of carbon-dioxide and organic carbon between the atmospheres where carbon takes place principally as inorganic CO₂ and the hydrosphere and lithosphere that have varying concentrations of inorganic and organic compounds.

The carbon cycle starts having carbon fixation that is the conversion of CO₂ to organic matter. Plants are thought of as the principal CO₂ fixing organisms however at least half of the carbon on earth is fixed through microbes; specifically marine photosynthetic prokaryotes and protists.

Cyanobacteria like Prochlorococcus and Synechococcus are comprised in carbon fixation by employing energy from sunlight. Chemolithoautotrophic microorganisms like Thiobacillus and Beggiatoa as well fix CO₂ to organic matter whereas metabolizing compounds like H₂S for energy.

Once carbon is fixed to organic compounds, the next phase in the cycle comprises its transfer from population to population in the biological community, supporting the growth of a diversity of heterotrophic organisms, that is, heterotrophs like animals and protozoa which eat autotrophs and might in turn be eaten by other animals. Therefore, they get organic carbon to build biomass and to oxidize to achieve the energy.

Decomposers make use of the remains of primary producers and consumers for the similar purposes.

The respiratory and fermentative metabolism of the heterotrophic organisms returns inorganic carbon-dioxide to the atmosphere completing the carbon-cycle. If animals and plants die, such organic compounds are decomposed through fungi and bacteria and through decomposition the organic compounds are oxidized and CO₂ is returned to the cycle.

Carbon is stored in the rocks like limestone (CaCO₃) and is dissolved as carbonate ions (CO₃) in the oceans. Vast deposits of fossil organic matter exist in the form of fossil fuel like petroleum and coal. Burning such fuel discharges CO₂ resultant in an increased amount of CO₂ in the atmosphere.

Nitrogen Cycle:

Nitrogen is a necessary constituent of DNA, RNA and proteins that are the building blocks of life; therefore all the organisms need nitrogen to live and grow.

Nitrogen, the richest substance in the atmosphere or air (around 80%) is not directly useable by most organisms. This is due to the reason of strong triple bond between the nitrogen atoms in the molecule makes it relatively inert. Merely a few bacteria are capable to make use of nitrogen directly. For animals and plants to be able to use N₂, N₂ gas should first be transformed to more chemically available forms like ammonium (NH₄⁺), Nitrate (NO₃^-1), (NO₂^-1) and organic nitrogen having compounds like proteins and amino acids. Microorganisms as well are capable to utilize these other forms of nitrogen. 

The conversion of nitrogen compounds mainly through microorganisms changes the oxidation states of nitrogenous compounds and set up a nitrogen cycle.

Three methods taken out by microorganisms are important in the nitrogen cycle. They are:

Nitrogen fixation:

Technique by which free nitrogen (N₂) is extracted from the atmosphere and transformed (fixed) into nitrogen compounds which are plant nutrients (that is, fertilizer). In nature, this method is carried out by some bacteria (present in the root nodules of legumes like peas and beans), blue-green algae and the lightning flash.

Nitrification:

It is the oxidation of ammonium (NH₄⁺) ions into nitrite (NO₂^-) ion and then nitrate (NO₃^-) ions through microorganisms in water and soil. Nitrate ions are absorbed through the plants as necessary nutrients and by the help of oxygen, transformed (or synthesized) into plant protein (that is, amino acids).

Denitrification:

It is the conversion of nitrates to nitrogen gas that is then discharged into the atmosphere. This is caused by means of bacteria and how they achieve their energy. A small quantity is transformed to usable forms by lightning in a procedure termed as atmospheric nitrogen fixation.

Sulphur Cycle:

Sulphur can be present in several oxidation states in organic and inorganic compounds. Oxidation-Reduction reactions mediated through micro-organisms modify the oxidation state of sulphur in different compounds establishing the sulphur cycle.

Micro-organisms are able of eliminating sulphur from organic compounds beneath aerobic conditions, the elimination of sulphur (that is, desulphurization) of organic compounds outcomes in the formation of sulphate, while beneath anaerobic conditions hydrogen sulphide is generally produced from the mineralization of organic sulphur compounds. Hydrogen sulphide might as well be formed by sulphate-reducing bacteria which employ sulphate as the terminal electron acceptor through anaerobic respiration. Hydrogen sulphide reacts with metals.

Being negatively charged, it complexes or reacts without difficulty by cations in the environment like aluminum, iron and calcium. Such compounds are relatively insoluble and most available to plants and microbes between pH 6 and 7 beneath these conditions. Such organisms readily and rapidly change phosphate to its organic form so that it becomes accessible to animals. The microbial transformation of phosphorus features the transformation of simple orthophosphate (PO₄^-) that bears phosphorus in the +5 valence state to more complex forms. Such comprise the polyphosphates seen in the metachromatic granules and also more well-known macromolecules.

The Phosphorus Cycle:

Biogeochemical cycling of phosphorus is significant as all the living cells need phosphorus for nucleic acids, lipids and a few polysaccharides. 

Though, most of the environmental phosphorus is present in low concentration, locked in the earth's crust; therefore it is the nutrient which limits growth. Dissimilar the Nitrogen and Carbon cycles, the phosphorus cycle has no gaseous constituent. Phosphorus is derived only from the weathering of phosphate - having rocks, therefore in soil. Phosphorus exists in both organic and inorganic forms. Organic phosphorus is found in the biomass, humus and other organic form. The phosphorus in such organic materials is recycled through microbial activity.

Inorganic phosphorus, on the other hand is negatively charged, therefore it complexes or reacts simply with cations in the environment like iron, aluminum and calcium. Such compounds are relatively insoluble and most available to plants and microbes among pH 6 and 7. Beneath these conditions such organisms readily and rapidly transform phosphate to its organic form in such a way that it becomes accessible to animals. The microbial transformation of phosphorus features the transformation of the simple orthophosphate (PO₄^-) that bears phosphorus in the +5 valence state to more complex forms. These comprise the polyphosphates seen in the metachromatic granules and also more familiar macromolecules.

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