The following points highlight the top five types of biochemical cycle existing in ecosystem. The types are: 1. Hydrologic Cycle 2. Gascons Nutrient Cycle 3. Sedimentary Nutrient Cycle 4. Phosphorus Cycle 5. Sulphur Cycle.
Type # 1. Hydrologic Cycle:
In the hydrologic cycle there occurs an interchange of compounds between the earth’s surface and the atmosphere via precipitation and evaporation. The biota of the ecosystem plays an accessory role in the cycle and the presence or absence of the biota does not affect the movement of the cycle.
However, it is an established fact that a significant amount of water is incorporated by the-biota of the ecosystem in protoplasmic synthesis and also there is a substantial return to the atmosphere by way of transpiration.
According to him the world precipitation per year amounts to 4.46 geograms (1 geogram= 1020gm). Of this amount 0.99 geogram falls on land and 3.47 geogram falls on ocean surface. The water content of the earth’s surface is 266,069-88 geogram. The water content of the various parts of the earth is given below.
Characteristics of hydrological cycle:
a. The general world precipitation pattern is dependent upon the interaction of several forces. Primary of these forces is the interaction between atmospheric circulation and the topography. The distribution of the major ecosystems is dependent upon the interactions.
The rate and amount of precipitation being as critical as those of evaporation, the ratio of these two factors forms the crucial factor in determining the distribution of particular types of ecosystems.
b. More water evaporates from the ocean than that returns to it via rainfall and conversely less water evaporates from land and more returns to it via rainfall. This indicates that a part of the rainfall which supports land ecosystem comes from the water evaporated from the ocean. It has been estimated that in Mississippi valley as much as 90% of the rainfall comes from the ocean.
c. There is about 0.25 geogram of water in fresh-water ponds, lakes and rivers. The run-off is 0.2 geogram per year and the annual rainfall is 1.0 geogram. Thus the annual recharge rate of ground water is 1.0—0.2 geogram or 0.8 geogram.
d. The circulation pattern of atmosphere determines the pattern of precipitation distribution. The trade wind may be cited to explain this. The trade winds move from cooler latitudes towards the equator carrying moisture and depositing the moisture in the equatorial region.
This makes the equatorial region very wet whereas the coast line to the north and south of the equator remains relatively dry. The other example that may be cited is that of Rain shadow.
When moist air moves over a mountain range it rises and cools to supper saturation. Precipitation occurs in such cases on the windward side of the range. As the moisture depleted air continues to move it comes down, warms and picks up moisture by evaporation from ground and water surfaces.
This region thus receives less moisture than the windward region forming the rain shadow. Examples of rain shadow region are Rocky mountains and south side of the Himalayas.
e. In the hydrological cycle the H2O atmospheric compartment is small and it has a more rapid turnover rate and shorter time of residence in the atmosphere than CO2 cycle.
Type # 2. Gascons Nutrient Cycle:
A. Carbon cycle:
The carbon cycle is the simplest of all nutrient cycles. The major reservoir of carbon is the ocean where it remains stored as bicarbonate. The oceans contain more than 50 times as much carbon as that of air and the oceanic reservoir regulates the amount of carbon in the atmosphere.
Atmosphere if the minor reservoir though the basic movement of carbon is from atmosphere to producers, from producers to consumers, from consumers to decomposers and then back from the decomposers to the atmosphere. In the atmosphere carbon dioxide concentration is from 0.03 to 0.04 per cent. Annual fixation of carbon dioxide by green plants is about 4 to 9 X 1013 kg.
A considerable amount of biologically fixed carbon as gaseous CO2 is returned to the atmosphere by the respiratory activity on the part of the producers and consumers. But the most substantial return is accomplished by the respiratory activity of the decomposers and the way of their processing the waste materials and other trophic levels.
It has been estimated that 8 billion tons of CO2 is injected annually into the atmosphere. Of this 6 billion tons come from burning of fuel or accidental fire and 2 billion tons from cultivation of land for agriculture. Most of this is quickly passed into the sea and is kept there stored in the form of carbonate. The interplay between atmospheric and aquatic carbon dioxide is worth noting.
The interchange between the two phases occurs through the process of diffusion, the direction of flow is dependent upon the concentration. The aquatic phase of CO2 occurs through precipitation. A litre of rain water contains about 0.3 c.c. of CO2.
The dissolved CO2 combines with the water of the ecosystem and forms carbonic acid (H2CO3), the reaction being always reversible. The carbonic acid in its turn disassociates in a reversible reaction into H+ and HCO3– ions. The bicarbonate ions again disassociate reversibly into H+ and CO3– ions.
As all these reactions are reversible the direction of movement is dependent upon the concentration of the components.
The carbon cycle (Fig. 3.14) in reality is a perfect one and carbon is returned to the atmosphere as fast as it is removed. The cycle also shows that there are a number of avenues by which carbon is utilized and a much larger number by which it is restored to the atmosphere.
B. Nitrogen cycle:
The nitrogen cycle is a complex one though it is a complete and perfect cycle. The cycle exhibits marked difference from that of carbon cycle. The nitrogen cycle is extensive, complicated and at the same time ordered (Fig. 3.15).
The atmosphere contains as much as 79 per cent nitrogen. That means the organisms live in a nitrogen-rich atmosphere. But if one thinks that nitrogen reservoir is atmosphere, he is mistaken. Because most organisms are unable to use atmospheric nitrogen.
The real store-house of nitrogen for the use of organisms lies in inorganic forms like ammonia, nitrite and nitrate and also in organic forms like urea, protein and nucleic acids.
Living organisms can tap nitrogen largely in the form of nitrate. So fixation of atmospheric nitrogen in nitrate form is at the helm of the affair. The fixation of nitrogen occurs through physico-chemical and biological means.
Biological means of nitrogen fixation:
A few but abundant kinds of bacteria collectively called Nitrogen-fixing bacteria and some algae have the capacity to fix atmospheric nitrogen. At the same time there are many bacteria called Denitrifying bacteria which can free the nitrogen. It is by the action of this micro-organisms the nitrogen is continually entering the air and continually returning to the cycle.
The nitrogen-fixing organisms are:
a. Free-living bacteria—Ex. Azotobactor (aerobic) and Clostridium (anaerobic).
b. Symbiotic nodule bacteria on legume plant—Ex. Rhizobium.
c. Purple bacteria—Ex. Rhodospirillum.
d. Blue-green algae—Ex. Ambaena, Nostoc.
e. Some Lichen.
The denitrifying organisms ate Pseudomonas and some Fungi.
Course of events in nitrogen cycle:
a. The nitrogen in the air is transformed into nitrates by (1) the nitrogen-fixing bacteria and by (2) electrification and photo-chemical means. It has been estimated that electrification and photochemical nitrogen fixation is in the order of 35 mg/m2/year and biological fixation or fixation by nitrogen fixing bacteria is 140-700 mg/m2/year.
b. Nitrates and other simpler nitrogen compounds are used by plants for the synthesis of amino acid and protein which in turn go to the animals.
c. The animal metabolic wastes in the form of urea, uric acid, etc., are transformed into ammonia. The protoplasm of dead animals and plants is acted upon by bacteria, actinomycetes and fungi occurring both on land and water.
These organisms utilize the dead organisms or which are but organic nitrogen-rich substrates in their metabolism of it. They convert it to and release it in the organic form ammonia. The process is known as Ammonification or mineralization. A considerable amount of ammonia is also gained from volcanic action.
d. Many autotrophic and some heterotrophic bacteria only can use nitrogen occurring in ammonia form to synthesize their own protoplasm. But to most ammonia is not the accessible form. Ammonia or mostly ammonium salt is converted to nitrate by a process called nitrification. The nitrification process is slow, pH dependent and occurs in acid condition.
The transformation of ammonia to nitrates is not direct. The bacteria, Nitromonas convert ammonia to nitrite (NO2) and other bacteria such as Nitrobactor acton nitrite to complete the conversion to Nitrate.
e. The characteristic of the cycle is that the steps in the conversion of protein to nitrates provide energy to the organisms which accomplish the breakdown. But the return steps from nitrite to protoplasm require energy from other sources—sunlight or organic matter.
Amount of nitrogen fixation:
In 1944 it was estimated that the amount of nitrogen fixed from the air lies between 140 and 700 mg per square metre per year. Most of this fixation is biological in nature and only a small quantity (about 35 mg per square metre per year) is fixed by electrification and photo-chemical means.
Recent (1970) estimates show that biological fixation of nitrogen on the earth’s terrestrial surface is at least 1 gm per square metre per year and in fertile lands it may be as much as 20 gm per square metre per year.
Type # 3. Sedimentary Nutrient Cycle:
In addition to oxygen, hydrogen, carbon dioxide and nitrogen living organisms require at least 13 other elements like calcium, phosphorus, potassium, sodium, chlorine, sulphur, magnesium, iron, copper, manganese, iodine, cobalt and zinc.
Traces of these elements are only required by living organisms. But the requirement of calcium and phosphorus is needed a bit more than other elements. Phosphorus is a constituent of nucleoproteins, phospholipids and skeletons. Calcium is needed for skeletons, shells, antlers and other organs. These elements are obtained from food, water, salt licks and grit taken into the stomach.
Type # 4. Phosphorus Cycle:
Phosphorus is a necessary and important constituent of protoplasm. It has been evaluated that the ratio of phosphorus to other elements in organisms is considerably greater than the ratio of phosphorus in the available and primary sources. Ecologically phosphorus is very significant as it is the limiting or regulating element in productivity.
The phosphorus cycle is comparatively simple (Fig. 3.16). The reservoir of phosphorus is the rocks or other deposits that have been formed in past geological ages. Erosions of these reservoirs release phosphate to the ecosystems. But in the process much of the phosphate escapes into the sea, where part of it is deposited in the shallow sediments and part of it is lost to the deep sediments.
The means of return of phosphorus to the cycle is inadequate. The uplifting of sediments in most part of the world has become rare. However, sea-birds play an important role in bringing back phosphorus to the cycle through their ‘guano’ deposits. Man also harvests a lot of marine fish and this aids in the return of some phosphorus to the cycle.
Plants take inorganic phosphate as orthophosphate ions. This phosphorus is transferred to consumers. After death the protoplasm of plants and animals is acted upon by decomposers (phosphatizing bacteria) to make it available again as dissolved phosphate.
The excreta of animals also return some phosphorus to the cycle. The bones and teeth of animals being very resistant to weathering account for some loss of phosphorus.
A study of phosphorus cycle reveals that the return of phosphate to the cycle is inadequate to compensate the loss. It is man who has hastened the rate of loss of phosphorus and has made the phosphorus cycle inadequate. It has been calculated that one to two million tons of phosphate rock are mined per year.
But most of it is washed away and lost. The annual return against this is sixty thousand tons per year. The agronomists tell us that the situation is not alarming as phosphate deposits are still too large.
Type # 5. Sulphur Cycle:
Only a few organisms meet their sulphur requirements in such forms as amino acid and cystein. The source of biologically significant sulphur is inorganic sulphate.
The reservoir of sulphur lies in the soil and sedimentary rocks. The atmosphere is a minor reservoir formed by fuel combustion.
The centre wheel of the sulphur cycle (Fig. 3.17) rotates round the activity of a group of specialized micro-organisms which function as a relay team, each carrying out a particular chemical oxidation or reduction.
The sedimentary aspect of the cycle involves the precipitation of sulphur in the presence of iron in anaerobic condition. Ferrous sulphide is unsoluble in neutral or alkaline water and as a result the sulphur has the potential for being bound up under these conditions to the limits of the amount of iron present.
The biologically incorporated sulphur is mineralized by bacteria and fungi in ordinary decomposition. Some such sulphur is also reduced directly to sulphides including hydrogen sulphide by bacteria specially the Escherichia and Proteus.
Inorganic sulphate (SO4) is the source of elemental sulphur in the ecosystems. Under anaerobic condition the sulphate is reduced to elemental sulphur or to hydrogen sulphide by bacteria under the genus Desulphovibrio, Escherichia and Aerobactor. The presence of a large amount of hydrogen sulphide occurring in the anaerobic or deeper portion of aquatic ecosystem is inimical to animal life. The H2S rises to shallow sediments and is acted upon by other organisms.
Colourless sulphur bacteria such as species of Beggiatoa oxidize hydrogen sulphide to elemental sulphur. Species of Thiobacillus oxidize elemental sulphur to sulphate and other species of Thiobacillus oxidize sulphide to sulphur.
At the global level the regulation of sulphur cycle is dependent upon the interaction of geochemical and meteorological processes (erosion, sedimentation, leaching, rain absorption), and biological processes (production and decomposition). The interdependence of air, soil and water also aids in the regulation.