The following is an account of carbon, oxygen, hydrogen, nitrogen, phosphorus, calcium, sulphur and water cycles:
1. Carbon Cycle:
Carbon is contained in all organic compounds: carbohydrates, proteins, fats and nucleic acids which make up a living being.
(a) Carbon Dioxide goes into the Living Beings from Atmosphere:
The main source of carbon of living beings is the free atmospheric carbon dioxide. The producers are the first of the living organisms to entrap carbon from carbon dioxide.
During photosynthesis, plants combine carbon dioxide with water to make carbohydrates as follows –
About 4 to 9 × 1013 kg of carbon is fixed in photosynthesis annually. The simple carbohydrates synthesised by plants are converted into complex carbohydrates (polysaccharides) which are stored in plant tissues. Plants are eaten by herbivores who digest these and resynthesise these carbon compounds into their own types of carbon compounds. Carnivores feed upon herbivores, they digest the carbon compounds of herbivores and resynthesise them into their own.
(b) Carbon gets back into the Atmosphere:
Carbon is returned back to the atmosphere mainly by the following ways:
(i) By respiration.
(ii) The dead remains of plants and animals undergo decay process due to the activity of decomposers. This releases the locked up carbon dioxide into the atmosphere.
(iii) Part of the organic carbon gets burried in the earth’s crust and gives rise to fossil fuel (peat, coal, oil and gas) in course of time. By burning of this fuel, carbon dioxide is returned to the atmosphere.
(iv) Part of the organic carbon gets burried in the earth’s crust and gives rise to limestone rocks, by the weathering of these rocks, carbon dioxide is returned into the atmosphere.
(v) Hot springs and volcanic activity pours out about 100 million tonnes of carbon dioxide back into the atmosphere in a year.
(c) Carbon Dioxide Dissolved in Water:
Carbon dioxide dissolved in water of the oceans is the second important source of the gas. Carbon dioxide of atmosphere is in a dynamic equilibrium with that of oceans, so if its concentration in the atmosphere goes down, there is a movement of the gas from the oceans to the atmosphere and vice-versa.
2. Oxygen Cycle:
(a) Oxygen Enters into the Living Beings:
The main source of oxygen is the green plants which give out oxygen during photosynthesis according to the equation:
Oxygen which is thus released into the atmosphere constitutes about 21% of air. From the atmosphere, oxygen enters into the living beings as a respiratory gas. In course of respiration, oxygen combines with hydrogen to form water which becomes a part of the general water content of the living substance.
(b) Oxygen gets back into the Atmosphere:
Oxygen is returned back to the atmosphere, after death and decay of organisms, not as free oxygen, but as water or carbon dioxide.
3. Hydrogen Cycle:
(a) Hydrogen Enters into the Living Beings:
The only source of hydrogen is the water molecule in the atmosphere. Hydrogen enters into the living beings through photosynthesis during which water molecule is split up into oxygen and hydrogen. Hydrogen enters into the composition of glucose molecule and oxygen is released into the atmosphere. Through glucose, hydrogen moves into various organic components that are synthesised directly or indirectly from glucose and becomes part of the living matter.
(b) Hydrogen gets back into the Atmosphere:
(i) During respiration, hydrogen is oxidised to form water, some of which forms a part of the air which is breathed out.
(ii) The dead organisms and the organic compounds of the waste matter undergo decomposition by the activity of decay bacteria, during this process, hydrogen atoms are oxidised to form water again.
It will be seen that carbon, hydrogen and oxygen cycles are inseparably related to one another. All these cycles involve producers, consumers and decomposers, hence are combined into a single carbon-oxygen-hydrogen cycle which may be called as energy cycle as it involves flow of energy from beginning to the end in the biosphere.
4. Nitrogen Cycle:
(a) Nitrogen Enters into the Living Beings:
The main source of nitrogen is the atmosphere. Nitrogen is the most abundant of all components of the atmospheric air. There being about 78 parts of nitrogen in every 100 parts of air by volume, i.e., 4/5th of the atmospheric air is pure nitrogen. Before this free nitrogen is introduced into the producers, it should be fixed up into an inorganic ion- the nitrate (NO3).
As Nitrates, nitrogen is taken up by most of the plants in solution form. Some plants may absorb nitrites and use it to some degree though nitrogen is relatively toxic as nitrites. Some autotrophic and many heterotrophic marine bacteria are capable of utilising ammonia to synthesise their proteins.
Some plants absorb nitrogen as organic nitrogen, (i.e., nitrogen in the protoplasm of organisms), e.g., many amino acids have been reported to be used by clover and tomato plants, the aqueous spray of urea on apple in full bloom may enhance fruit set probably due to the absorption of urea directly by shoots and flowers.
Nitrogen fixation is a chemical process in which the atmospheric nitrogen chemically combines with other elements to from soluble nitrogen compound called nitrates. The nitrates are taken up by plants through roots, thus nitrogen from the atmosphere is introduced into living system and is assimilated.
Nitrogen assimilation involves many steps. In the living cells of the green plants, nitrates are at first reduced to ammonia, some intermediate compounds are formed during the reduction process, the first being nitrite (NO2) the second being hyponitrite (HNO). The enzymes, called nitrate reductase and nitrite reductase, catalyse the reactions.
Hydroxylamine (NH2OH) is the next compound in the sequence of intermediates and this is converted into ammonia, the reaction being catalysed by an enzyme called hydroxylamine reductase. Now ammonia enters as an amino group (-NH2) in the formation of various amino acids (R-NH2).
The other component of amino acid is derived from carbohydrate. Amino acids link together to form plant proteins and nucleic acids. The herbivores get their supply of nitrogen in the form of plant proteins; the carnivores get their protein supply in the form of animal proteins. The animals digest these proteins into amino acids which serve as building blocks for their own proteins and nucleic acids.
In the total cycle about 4-7 tonnes of nitrogen per hectare is added to the soil each year. There is some loss from the soil through the leaching of nitrates into fresh water courses and the seas. However, by all these processes the nitrogen cycle is balanced and thus the N2 concentration in the atmosphere is relatively constant. It should be noted that this natural cycle is upset by modern agricultural practices which dump about 40 M tonnes of nitrogenous fertilisers each year to the worlds cultivated land.
Fresh water resources viz. streams, rivers and lakes get polluted by agricultural run-off i.e. leaching of excessive nitrate from agricultural lands.
(b) Nitrogen gets back into the Atmosphere:
Nitrogen gets back into the atmosphere by the following ways:
(i) In the body of animals, excess of proteins are broken down to urea, uric acid or ammonia which pass out through urine.
(ii) Proteins of dead plants and animals undergo decay by the action of decomposers and change into ammonia (ammonification) then to nitrites, and then to nitrates.
Some of the nitrates go to the ground water, some remain in the soil and are thus available for recycling, while others undergo denitrification and are reduced to nitrogen which gets back into the atmosphere. Following bacteria help in this process.
Ammonifying bacteria convert proteins into ammonia, the process being known as ammonification. Nitric bacteria like Nitrosomonas found in the soil convert ammonium ions into nitrites. Nitrate bacteria like Nitrobacter convert nitrite ions into nitrate ions.
The process of conversion of ammonium ions into nitrate ions is known as nitrification. The energy released during nitrification is used by bacteria for their vital activities.
Denitrifying bacteria like Pseudomonas reduce nitrate ions of soil into gaseous nitrogen (NO2 and N2) which returns to the atmosphere. The process is known as denitrification.
Gain and Loss to Nitrogen Cycle:
(i) Nitrogen may enter into the atmosphere through volcanic gases.
(ii) The nitrifying bacteria (= nitrite and nitrate bacteria) which play a key role in the cycle, inhabit the top soil. Soil erosion results in the loss of these bacteria.
(iii) The nitrogen compounds in the soil may be leached or washed and are not available for absorption by plant roots.
(iv) Nitrates may be lost through rains to the sea but this nitrate may be re-obtained through marine fishes and birds.
Types of Nitrogen Fixation:
Nitrogen fixation is of the following types:
(a) Non-biological fixation of nitrogen
(i) Photochemical fixation or physico-chemical fixation
(ii) Industrial fixation
(b) Biological fixation of nitrogen
(i) Symbiotic fixation
(ii) Non-symbiotic fixation (fixation by free living organisms)
(a) Fixation by soil and water bacteria
(b) Fixation by blue-green algae
(c) Fixation by yeasts.
(a) Non-Biological Fixation of Nitrogen:
(i) Photochemical Fixation of Nitrogen:
During a thunderstorm, under the impact of electric sparks, nitrogen of the air reacts with oxygen to form nitric oxide which combines with oxygen of air to form nitrogen peroxide which in turn is dissolved by subsequent rains to form nitrous and nitric acids.
These acids are washed down to the soil and here they unite with some mineral substances like calcium or potassium and form their nitrates or nitrites. The nitrate is directly absorbed by the roots. The nitrites are at first oxidised to nitrates by organisms inhabiting the soil and then absorbed.
(ii) Industrial Fixation of Nitrogen:
Man manufactures fertilisers in the form of nitrate or ammonium compounds in the industries and their applications in agricultural fields to increase the yield are also a major nitrogen fixing process.
(b) Biological Fixation of Nitrogen:
(i) Symbiotic Fixation of Nitrogen:
Symbiotic fixation of nitrogen is done by symbiotic nitrogen fixers which are found only on land and not in the aquatic habitats. The best known among these are the species of root nodule bacteria (legume bacteria) Rhizobium which are not only exclusively terrestrial but are also – specific to particular leguminous plants (peas, beans, etc.).
The bacteria invade the broken or damaged root hairs or the soft tips of root hairs of leguminous plants by lysing the cell walls and plasma membrane. They multiply and in response to this, differential growth of the root occurs resulting in enlargements called root nodules. These nodules contain a red pigment called leg haemoglobin which is involved in the fixation of nitrogen to ammonium ions (NH4+).
The sole purpose of symbiotic nitrogen fixation is the production of NH+4 for the host. In this connection the bacteria in the nodule may resemble energy producing organelles of plant cells-the mitochondria. The bacteria in the nodule seem to lack a rigid cell wall and are osmotically labile. The pigment is a product of Rhizobium legume complex.
Some ammonia (NH4+) is absorbed by the host plants, some is used by the bacteria to synthesise its own proteins and nucleic acids while some remains locked up in the nodules. The soil therefore becomes richer in nitrogen, more particularly so, if the nodule bearing leguminous plants are ploughed into the soil.
The association of Rhizobium with the roots of leguminous plants is a symbiotic association as the bacteria get Carbohydrates from the plants while the plants get nitrogen food from the bacteria. So important is the process of symbiotic nitrogen fixation by legume bacteria that it has become a standard practice by fanners to grow leguminous crops in rotation to build up nitrogen supply in the soil.
Leguminous crops like pulses, beans, and peas are usually grown in the fields in rotation with non-leguminous crops like cereals (rice, wheat, maize, barley, oats, etc.). For the same reason, leguminous plants like Tephrosia and Derris are grown in tea gardens. In this way 50 to 400 lbs. of nitrogen per acre per year is added to the soil.
(ii) Non-Symbiotic Fixation of Nitrogen:
Non-symbiotic fixation of nitrogen is done by free living nitrogen fixers which are soil and water bacteria, blue-green algae, yeasts and perhaps by some other micro-organisms.
(a) Fixation by Soil and Water Bacteria:
Nitrogen fixing aerobic bacteria, Azotobacterium and anaerobic bacteria Clostridium are distributed widely in soil, fresh water and marine waters. These bacteria, fix nitrogen of the soil air in their own bodies in the form of amino acids from which they build their own proteins. When these bacteria die, their proteins are released into the soil.
(b) Fixation by Blue-Green Algae:
Blue-green algae are important as nitrogen fixers in deserts and countries of South-East Asia. In India, rice is grown year after year on the same land without adding fertilisers. During monsoons, luxuriant growth of blue-green algae occurs as gelatinous masses in the water-logged fields, they fix up atmospheric nitrogen in the soil. Aulosira fertilissima, the dominant algae of rice fields is the most powerful nitrogen fixer.
(c) Fixation by Yeasts:
Certain yeast cells and mycorrhiza fungi, fix up a very small quantity of atmospheric nitrogen.
5. Phosphorus Cycle:
Phosphorus is a constituent of nucleic acids, phospholipids, ATP ADP and numerous phosphorylated compounds which play a vital role in the biological system. Ecologically, phosphorus is more significant than other elements because the ratio of phosphorus to other elements in organisms is considerably greater than its ratio in the available primary sources.
The main source of phosphorus is not air; it is the rocks or other phosphate deposits which were formed in the past geological ages due to volcano apatite, fossil bone deposits, etc. These are gradually eroding and releasing phosphates to the ecosystems.
(a) Phosphorus Enters into the Living Beings:
Phosphates released from rocks and deposits, dissolve in water and the phosphate ions are absorbed by plants from soil through root hairs. In course of time phosphate gets incorporated into different phosphate-containing compounds. From plants phosphates are transferred to animal consumers as organic phosphates.
(b) Phosphorus gets back into the Soil:
The dead plants, animals and their waste are acted upon by decomposers (fungi and bacteria) which simplify the organic phosphates step by step thus releasing phosphates and phosphoric acid which break down into phosphate ions which are recycled.
Loss and Gain to the Phosphorus Cycle:
From the cycle, much phosphorus is lost through physical and biological processes and also due to the activities of man.
(a) Physical Process:
Due to soil erosion, much phosphorus escapes to the sea through the streams and rivers. Here some of it is deposited in shallow sediments of the photosynthetic zone (zone to which sunlight penetrates) and some which is deposited in the deep sediments is a definite loss to the cycle.
The phosphates deposited in the photosynthetic zone may be recycled as follows:
(i) Phosphorus may be taken up by phytoplankton and then deposited again in the sediments by their death and decay.
(ii) Phytoplankton may be ingested by zooplankton which excretes phosphates and these may be deposited in sediments.
(iii) The zooplankton may be eaten by fish which in turn may be eaten up by fish-eating birds. Thus some phosphorus from ocean is returned to land through guano deposits (excreta of bird of common occurrence in Peru) of these birds. Guano deposits may form phosphate rocks in course of time and erosion of these makes phosphate available for recycling. Part of phosphates may sink in the deep marine sediments and may be lost.
(b) Biological Process:
Some phosphorus is lost in the formation of teeth and bones both of which are more or less resistant to weathering. They escape to the sea and are deposited in sediments which are not available for recycling.
(c) Activities of Man:
Man mines more than two million tonnes of phosphate rock per year. Most of this is washed away and lost from the cycle.
6. Calcium Cycle:
(a) Calcium Enters into the Living Beings:
The main source of calcium is a number of calcium compounds found in rocks. Most of them being soluble are also found in water.
Plants absorb calcium from soil and build their organic calcium compounds. Through food chains, the organic calcium compounds move to different animals. Organisms also procure the dissolved calcium compounds with water they take in.
(b) Calcium gets back into the Soil:
The dead plants and animals and their excreta are acted upon by decomposers which release calcium into soil and water from where it is recycled.
Loss to the Cycle:
Through streams and rivers, calcium may move out of the biosphere into the sea.
7. Sulphur Cycle:
Sulphur is an essential constituent of certain amino acids like cystine, cysteine and methionine, of vitamins like biotin and thiamine and of active compounds like coenzyme—A. Plants do not require as much sulphur as they require phosphorus and nitrogen. Sulphur is not so often limiting to the growth of plants and animals.
There are two main sources of sulphur:
(i) A large pool deep in the soil and sediments.
(ii) A smaller pool in the atmosphere.
The sulphur cycle thus links air, water and soil. Sulphur gets into living beings and gets back into the soil, sediments and air.
In the large reservoir pool, the key role is played by specialised micro-organisms working in groups, each carrying out a particular chemical oxidation (O) or reduction (R). This is depicted by the ‘Centre Wheel’.
The large pool (centre wheel) can be further subdivided into two constituent parts as follows:
(i) An ‘available’ pool consisting of sulphate SO4– – ions which are directly available to the plants.
(ii) A ‘reservoir’ pool consisting of iron sulphide (FeS) which cannot be directly utilised by plants but have to be converted first to sulphate.
Thus exchanges occur between the available and reservoir pools. Through these exchanges sulphur gets into the living beings and then into the pool. The exchanges occur by means of oxidation and reduction reactions carried out by specialised micro-organisms.
The reactions occurring are depicted in the figure and may be elaborated as follows:
(a) Fate of Iron Sulphide:
(b) Fate of H2S:
Hydrogen sulphide thus produced is acted upon by sulphur bacteria. These may be divided into three groups—green, purple and colourless sulphur bacteria. Their actions on H2S are various as shown below but finally lead to the production of SO4– – either directly from H2S or indirectly through elemental sulphur (S).
(i) H2S + O2 → S + H2O
The formation of elemental sulphur under aerobic conditions is carried out by sulphur bacteria called Beggiatoa.
Conversion of elemental sulphur into sulphuric acid is done by bacteria Thiobacillus thioxidants which have also been grouped as the aerobic sulphide oxidisers since they may also produce SO4 directly from H2S. These bacteria and the Beggiatoa are grouped under chemosynthetic bacteria.
They obtain energy for CO2 assimilation, not by photosynthesis but by the oxidation of inorganic compounds like NH4+ to NO2– then to NO3–, Fe(ous) to Fe(ic) and sulphide to sulphur. Ecologically the chemosynthetic bacteria are intermediate between the autotrophic and heterotrophic bacteria.
The purple and green sulphur bacteria utilise H2S to produce energy for photosynthesis in the presence of light and CO2. In the process, they produce SO42- and H+ ions as shown by the above reaction. The process may proceed also by the formation of elemental sulphur.
(iv) Hydrogen sulphide may also be converted into iron pyrites. This occurs under anaerobic conditions unlike the formation of elemental sulphur occurring aerobically. Pyrites formation occurs by the reversal of the reactions, leading to H2S formation from pyrites. H2S reacts with iron hydroxides resulting into pyrite formation.
(v) Some H2S may be lost to the fresh and marine water bodies as it rises up in the soil. In water it converts into SO4 which comes out of water and may fall back again into it or be utilised by plants.
(c) Fate of Elemental ‘S’:
Elemental ‘S’ produced under aerobic conditions gets converted to SO4– –. This has already been dealt with.
(d) Fate of Organic ‘S’:
This must also be considered because organic sulphur largely contributes to SO4– – formation. Organic ‘S’ may be obtained from a variety of sources. It is obtained from soil by the death and decay of plants, from sediments by the death and decay of marine and fresh water animals.
Bacteria and other microorganisms which cannot manufacture their own food from inorganic substances but have to depend upon sources like other living organisms or dead organic matter, are called heterotrophs. These are responsible for the fate of sulphur present in organic compounds.
The aerobic heterotrophic micro-organisms convert organic ‘S’ in dead remains of organisms into SO4– – in the presence of water and oxygen. The anaerobic heterotrophic micro-organisms convert it into H2S which in turn may form SO4– – through many different routes or may be converted to iron pyrites and thus get recycled.
Some degree of overlapping occurs in the roles of different organisms. Certain photosynthetic bacteria which are facultative anaerobes can also function as heterotrophs in the absence of light.
(e) Fate of SO4:
Apart from H2S, elemental ‘S’ and organic ‘S’, SO4 may come into soil and sediments, also from minerals present naturally or from fertilisers added to soil as per human needs. Animal excretion is a source of recycled SO4.
Sulphate may have different fates. The most important is the incorporation of SO4– – into aquatic (marine and fresh water) and terrestrial plants. It is incorporated into proteins and also contributes to mineral formation.
Conversion of SO4 into H2S occurs by a chemical reduction process brought about anerobically by desulphovibrio bacteria. These bacteria are an aerobes as they do not use free O2 as the final hydrogen acceptor in respiration. Instead oxygen and a part of SO4– – is utilised in the process of producing H2S.
The H2S produced rises upto shallow sediments and can be acted upon by various micro-organisms. In dead plants and animals, SO4– – may be converted by reduction process into organic sulphur which again may form H2S when acted upon by anaerobic heterotrophs.
In the small atmospheric pool, sulphur is present as SO2 in small amounts and plays only a transitory role in the sulphur cycle.
This SO2 has the following three fates:
(i) Sulphur dioxide is lost into water bodies as it is readily soluble in water.
(ii) It gets into the soil when it comes down with rain water.
(iii) It can be taken up by plants directly from the atmosphere.
It may be recalled that though sulphur is not required in the ecosystem in as much quantity as nitrogen and phosphorus, the sulphur cycle is nevertheless an important one. It is a key cycle in the general pattern of production and decomposition. It has its own importance, e.g., when iron sulphides are formed, phosphorus is released from insoluble to soluble form and becomes available to organisms. The sulphur cycle is therefore an excellent illustration of how one cycle influences another.
8. Hydrologic or Water Cycle:
Water covers about 73% of the earth’s surface. It is the major constituent of the lithosphere and atmosphere and is an essential requirement of all living organisms. Plants and animals draw water from the reservoirs present in the earth such as rivers, lakes, streams, ponds, soil, etc. Water gets into the living beings from the hydrosphere and gets back into the hydrosphere.
Plants absorb large quantities of water from the soil, an extremely low percentage of the total water absorbed, is consumed in the process of photosynthesis, some becomes a part of the general water content of the plant body while most of the water is lost into the atmosphere through the process of transpiration.
In animals, some water becomes a part of the general water content of the body, some is excreted as liquid water back into the hydrosphere, and some goes back into the atmosphere as water vapour and raises its moisture content. It is interesting to note that a given amount of water goes from the hydrosphere to the atmosphere more faster through the metabolic processes of living organisms than through the process of evaporation from the hydrosphere.
(iii) Death and Decay:
After the death of terrestrial organisms, the decay process releases all the water present in their bodies, back to the hydrosphere.
(iv) Evaporation and Condensation:
Due to solar radiations, millions of tons of water vapour reaches the atmosphere from the hydrosphere daily. As the evaporated water vapour rises into the upper atmosphere, it cools and condenses to form a colloidal solution of water in air (cloud). The clouds, on further cooling, drop down as liquid rain water or snow or hail (wind storm with snow), depending upon the degree of cooling. Thus water comes back to the hydrosphere from the atmosphere and is again absorbed by plants and used by living organisms.