Role of microbes in coal, gas and mineral formation, prospecting for coal, oil and gas and recovery of minerals from low grade ores using microbes is included here.

Microorganisms as Geochemical Agents

The mid of the nineteenth century witnessed an ever growing interest in the pivotal role of microorganisms in carrying out not only the various processes related to fermentations but also tackling some of the human diseases. Nevertheless, Pasteur’s articulated contributions on fermentation evidently
proved and established that microorganisms in particular may cater as highly specific entities in performing a host of chemical transformations.

Winogradsky and Beijerinck legitimately shared the overall merit and credibility for establishing the precise role of microbes in the critical transformations of N and S.

Windogradsky (1856-1953) : He critically examined and observed that there exist a plethora of distinct and discrete categories of microorganisms each of which is invariably characterized by its inherent capability to make use of a specific inorganic energy source.

Examples :
(a) Sulphur Microbes : They oxidize inorganic sulphur containing entities exclusively.
(b) Nitrogen Microbes : They oxidize inorganic nitrogen containing compounds solely.

Interestingly, Winogradsky caused to be seen that there are certain microorganisms which either in association with free living or higher plants may exclusively make use of gaseous nitrogen for the synthesis of the specific cell components.

Hellriegel and Wilfarth (1888) : They showed explicitely that a predominantly mutual and immensely useful symbiosis does exist between bacteria and the leguminous plants particularly.

Beijerinck (1901) : He meticulously observed, described, and even enumerated the usefulness of the very presence of the ‘free-living nitrogen fixing’ organism Azotobacter** in maintaining the fertility of the soil.

In Earth science, a biogeochemical cycle or substance turnover or cycling of substances is a pathway by which achemical substance moves through both biotic (biosphere) and abiotic (lithosphereatmosphere, and hydrosphere) compartments of Earth. A cycle is a series of change which comes back to the starting point and which can be repeated. Water, for example, is always recycled through the water cycle, as shown in the diagram. The water undergoes evaporationcondensation, and precipitation, falling back to Earth. Elements, chemical compounds, and other forms of matter are passed from one organism to another and from one part of the biosphere to another through biogeochemical cycles.

The term "biogeochemical" tells us that biological, geological and chemical factors are all involved. The circulation of chemical nutrients like carbon, oxygen, nitrogen, phosphorus, calcium, and water etc. through the biological and physical world are known as biogeochemical cycles. In effect, the element is recycled, although in some cycles there may be places (called reservoirs) where the element is accumulated or held for a long period of time (such as an ocean or lake for water).

Important cycles

The most well-known and important biogeochemical cycles, for example, include

·         the carbon cycle,

·         the nitrogen cycle,

·         the oxygen cycle,

·         the phosphorus cycle,

·         the sulphur cycle,

·         the water cycle,

·         and the rock cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere,pedospheregeospherehydrosphere, and atmosphere of the Earth. Along with the nitrogen cycleand the water cycle, the carbon cycle comprises a sequence of events that are key to making the Earth capable of sustaining life; it describes the movement of carbon as it is recycled and reused throughout the biosphere.

·         The global carbon budget is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere ↔ biosphere) of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide information about whether the pool or reservoir is functioning as a source or sink for carbon dioxide.

·         The carbon cycle was initially discovered by Joseph Priestley and Antoine Lavoisier, and popularized by Humphry Davy.

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixationammonificationnitrification, and denitrification. The majority of Earth's atmosphere (78%) is nitrogen, making it the largest pool of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitroge fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle.

The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere (air), the total content of biological matter within the biosphere (the global sum of all ecosystems), and the lithosphere (Earth's crust). Failures in the oxygen cycle within the hydrosphere (the combined mass of water found on, under, and over the surface of planet Earth) can result in the development of hypoxic zones. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life on earth (see the Great Oxygenation Event)

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithospherehydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs only in specialized, local conditions.

On the land, phosphorus (chemical symbol, P) gradually becomes less available to plants over thousands of years, because it is slowly lost in runoff. Low concentration of P in soils reduces plant growth, and slows soil microbial growth - as shown in studies of soil microbialbiomass. Soil microorganisms act as both sinks and sources of available P in the biogeochemical cycle. Locally, transformations of P are chemical, biological and microbiological: the major long-term transfers in the global cycle, however, are driven by tectonic movements ingeologic time

Humans have caused major changes to the global P cycle through shipping of P minerals, and use of P fertilizer, and also the shipping of food from farms to cities, where it is lost as effluent.

The sulfur cycle is the collection of processes by which sulfur moves to and from minerals (including the waterways) and living systems. Such biogeochemical cycles are important in geology because they affect many minerals. Biogeochemical cycles are also important for life because sulfur is an essential element, being a constituent of manyproteins and cofactors.

The Sulfur cycle (in general)

Steps of the sulfur cycle are:

·         Mineralization of organic sulfur into inorganic forms, such as hydrogen sulfide (H2S), elemental sulfur, as well as sulfide minerals.

·         Oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to sulfate (SO42–).

·         Reduction of sulfate to sulfide.

·         Incorporation of sulfide into organic compounds (including metal-containing derivatives).

Structure of 3'-phosphoadenosine-5'-phosphosulfate, a key intermediate in the sulfur cycle.

These are often termed as follows:

Assimilative sulfate reduction (see also sulfur assimilation) in which sulfate (SO42–) is reduced byplantsfungi and various prokaryotes. The oxidation states of sulfur are 6 in sulfate and –2 in R–SH.

Desulfurization in which organic molecules containing sulfur can be desulfurized, producing hydrogen sulfide gas (H2S, oxidation state = –2). An analogous process for organic nitrogen compounds is deamination.

Oxidation of hydrogen sulfide produces elemental sulfur (S8), oxidation state = 0. This reaction occurs in the photosynthetic green and purple sulfur bacteria and some chemolithotrophs. Often the elemental sulfur is stored as polysulfides.

Oxidation of elemental sulfur by sulfur oxidizers produces sulfate.

Dissimilative sulfur reduction in which elemental sulfur can be reduced to hydrogen sulfide.

Dissimilative sulfate reduction in which sulfate reducers generate hydrogen sulfide from sulfate.

There are many biogeochemical cycles that are currently being studied for the first time as climate change and human impacts are drastically changing the speed, intensity, and balance of these relatively unknown cycles. These newly studied biogeochemical cycles include

·         the mercury cycle, and

·         the human-caused cycle of atrazine, which may affect certain species.

Biogeochemical cycles always involve hot equilibrium states: a balance in the cycling of the element between compartments. However, overall balance may involve compartments distributed on a global scale.

As biogeochemical cycles describe the movements of substances on the entire globe, the study of these is inherently multidiciplinary. The carbon cycle may be related to research in ecology and atmospheric sciences. Biochemical dynamics would also be related to the fields of geology and pedology (soil study)