Essay on bio-geochemical cycles

There exists a direct link between abiotic and biotic components. The abiotic components are in continuous cycle; from the environment they enter the living organisms and from these organisms return to the environment after their death and decay.

This cycle is known as ‘bio- geochemical cycle’. ‘Bio’ means living organisms and ‘geo’ means earth (rock, soil, air, water, etc.) and ‘chemical’ refers to all chemical elements.

There are around forty chemical elements and their salts called nutrients, which are essential for life. Some elements are required in large quantities and are known as ‘macro-nutrients’, namely, C, N, O, H, K, Ca, Mg, S, P, etc.

Some elements are required in smaller quantities such as Fe, Mn, Cu, Zn, B, Mo, V, Co, CI and Na. Some elements such as sodium, can act both as macro as well as micro-nutrient depending upon the species and their requirements.

The movement of the elements and the inorganic components is essential for life and is called ‘nutrient cycle’.

The biogeochemical cycle of iron in the ocean : Nature Geoscience ...

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In the consumption and return process, a number of water organisms and some physic-chemical phenomena are involved, making an orderly operating cycle. The three distinct bio-geochemical cycles are: (i) Hydrologic or water cycle (ii) Gaseous cycle (iii) Sedimentary cycle.

The hydrologic cycle deals with the interchange of water between living organisms and their environment (atmosphere, land, sea, ocean, etc.).

The gaseous cycle deals with the interchange of gases like oxygen, nitrogen, carbon dioxide, etc.


The sedimentary cycles such as sulphur and phosphorous are concerned with the interchange of minerals in between the lithosphere and living organisms through water cycle.

Through weathering of earth crust, the minerals are liberated, get dissolved in water, and either move to rivers, lakes, seas or again get deposited in the earth’s crust through sedimentation or get deposited separately to form bigger rocks.

After weathering, these again enter into the cycle. The minerals are consumed by the living organisms from their habitats and after their death, are returned to the soil through decomposers and transformers maintaining a mineral cycle.

Oxygen Cycle:


Oxygen (gO) is the group 16 or VIA element of the periodic table, the electronic configuration being ls22s22p4 or [He] 2s22p4. Oxygen forms stable p7t – pin multiple bonds and thus occurs in free form as gas (23 per cent by weight or 21 per cent by volume).

In combined state, it occurs as water, oxides and oxy-salts. Oxides and oxy-salts constitute 46.6 per cent by weight of the earth’s crust.

In plant and animal tissues, it is about 50-70 per cent. In fact, combined oxygen is much more plentiful than free oxygen.

Oxygen is present in large quantities in our atmosphere. Plants release oxygen during photosynthesis and this amount of oxygen is utilized by aquatic plants and heterotrophic organisms in respiration. Thus, there exists a balance between the oxygen production and its utilization. For aquatic plants and animals, dissolved oxygen is the source of oxygen.

Carbon Cycle :

Carbon (6C) is the element of group 14 or IVA element in the periodic table, the electronic configuration being 1 s22s22p2 or [He] 2s22p2. It has electro negativity of 2.5. In carbon sp3, sp2 and sp, hybridization can take place.

Carbon forms mainly covalent bonds, in which it exhibits maximum oxidation state of +4. It occurs in both free and as well as in combined state. In its free state the three allotropic forms of carbon are diamond (sp3 hybridization), graphite (sp2 hybridization) and amorphous carbon in different varieties as charcoal, coal, coke, gas coke, petroleum coke, etc. In its combined state it occurs as C02, CO, CH4 carbonates of sodium, potassium, calcium and magnesium.

In air, C02 is found to the extent of 0.03 per cent by volume. It is found in caves, mines, etc., and also evolves from volcanoes. Carbon dioxide has a unique property of absorbing infrared radiation, thus keeping the earth warm.

Carbon atom is the principal building block of many kinds of molecules that make up living organisms. The cyclic process through which carbon is converted to organic molecules, used up and released again from the organisms to the source is called the ‘carbon cycle’.

The C02 cycle or carbon cycle is maintained by the process of photosynthesis, respiration, decomposition and fossil fuel burning.

Plants utilize C02 from the atmosphere in the process of photosynthesis and eventually carbon compounds are formed.

Nitrogen Cycle :

Nitrogen (N) is the group 15 or YA element of the periodic table. The electronic configuration is ls22s2 2p3 or [He] 2s2 2p3. Nitrogen is non- metal and in the free-state exists as gaseous N2 and has the ability to form pn – p7t multiple bonds. In the oxide state they exist as N20, NO and N204. The oxidation state of nitrogen varies from -3 to +5.

In air, nitrogen content is 75 per cent by weight or 80 per cent by volume. In the combined state it is found as nitrites and ammonium salts.

Nitrogen is a very important constituent of amino acids, nucleic acid, lipids and proteins of living organisms. But living organisms, except some nitrogen-fixing bacteria cannot use elemental nitrogen directly from atmosphere.

The plants use nitrogen only in the form of nitrate (N03). The conversion of nitrogen into nitrates, known as nitrogen fixation, takes place in two ways: (a) Non-biological methods, where thunderstorm and lightening convert a small percentage of elemental nitrogen into nitrates. (b) Biological methods, where elemental nitrogen is converted to nitrates by symbiotic bacteria such as rhizobium, anabaena and non-symbiotic bacteria like azotobacter, derxia, etc. as well as some other microorganisms like blue green algae.

The biological nitrogen fixation is approximately twenty times greater than that of non-biological methods.

The nitrate (N03) so formed is taken up by plants and ultimately monoacids, nucleic acids and proteins are synthesized. Herbivores eat

After death, the plant and animal bodies, as well as their wastes are decomposed by bacteria like bacillus subtitles, bacillus mesenteric us and actinomycetes releasing ammonia. This process is known as ‘ammonification’. The ammonia may escape into the atmosphere or may be retained in the soil as NH4 + ion.

The ammonia or NH4 + ions get converted (oxidized) into nitrites (N02~) by bacteria such as nitrosamines, micrococcus, and subsequently to nitrates (N03~) by bacteria such as nitrobacteria and nitrocystes. The process is known as ‘nitrification’.

The nitrate (N03) thus formed gets converted (reduced) mainly to nitrogen and ammonia under anaerobic condition in the soil by some iron and sulphur bacteria like bacillus cereus, micrococcus, achromobacter and thiobacillus. The process is known as ‘gentrification’.

Sulphur Cycle:

Sulphur (S) is the group 16 or VIA element of the periodic table. The electronic configuration of sulphur is Is2 2s2 2p6, 3s23p4or [Ne] 3s23p4.

Sulphur is incapable of forming pn- pn multiple bonds and hence, occurs as solid and not gas. Sulphur occurs both in Free State (solid sulphur) as well as sulphides and sulphate as galena (PBS), zinc blended (ZnS), barites (BaS04), Epsom salt (MgS04.7H20), etc.

Organic compounds such as proteins, garlic, onion, mustard, hair all contains sulphur. Sulphur is very essential element for the living organisms for synthesizing amino acids like cystein, enzyme co-factors like thiamine, biotin and for certain vitamins. It is the secondary nutrient for plants and forms the part of the sedimentary cycle.

In the gaseous form, sulphur exists as H2S and S02, in solid form as elemental sulphur, sulphate, sulphides, and as organic sulphur in the body of living organisms.

The burning of fossil fuels and volcanoes release S02, H2S gas in the atmosphere, which ultimately returns to the soil as sulphuric acid along with rain, forming sulphate compounds of various captions.

Sulphur in the form of S04 -2 is absorbed by plants. The elemental sulphur converted to sulphate (S04 2) is also released into the soil through weathering of some sulphate containing rocks.

The sulphates (S04~2) are utilized by plants and passed on to heterotrophic organism through the food chain. After death, the bodies of plants and animals as well as their wastes are decomposed by microorganisms releasing sulphur as organic sulphur as well as in the form of sulphates (S04~2) to the soil. The organic sulphur gets converted to sulphate (S04~2) by sulphur bacteria. The sulphate (S04~2) is again utilized by plants and the cycle goes on.

Phosphorus Cycle:

Phosphorus (15P) is the group 15 or VA element of the periodic table. The electronic configuration is Is2, 2s2 2p3, 3s2 3p3 or [Ne] 3s2 3p3.

Phosphorus is non-metal, an extremely active element and hence, occurs only in the combined state as phosphates in the rocks and in the soil as phosphoresce (Ca3 (P04)2), chlorapatite (3Ca3 (P04)2. CaCl2) and Fluor apatite (3Ca3 (P04)2. CaF2).

In living organisms, it occurs as phosphor proteins. It is an essential constituent of bones, teeth, blood and nervous tissues and is also an extremely essential element for the growth of plants.

Phosphorus forms part of sedimentary cycle. Phosphorus is the main constituents of ATP and ADP and is very essential for the growth of plants, as well as animals. Many rocks contain phosphorus usually in the form of (P04~3).

Due to weathering, (P04~3) is released into the soil and becomes available to the plants. The phosphates (P04 -3) are utilized by plants in metabolism and passed on to heterotrophic organisms through the food chain.

After death, the bodies of, plants and animals as well as their wastes are decomposed by microorganisms, releasing phosphorus in the form of phosphates (P04~3) into the soil, making them available to plants once again.

Thus, phosphate cycle starts from (P04 -3) in the rocks, moves to plants due to weathering of rocks, then moves to other plants and animals, decomposes and returns back in the form of phosphate salts (P04~3) again.

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