Post on 18-Mar-2022
ZOOLOGY Principles of Ecology
Ecosystem: Ecosystem Processes-I (Part-4)
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Paper : 12 Principles of Ecology
Module : 32 Ecosystem: Ecosystem Processes-I (Part-4)
Development Team
Paper Coordinator: Prof. D.K. Singh Department of Zoology, University of Delhi
Principal Investigator: Prof. Neeta Sehgal Department of Zoology, University of Delhi
Content Writer: Dr. Kapinder Kirori Mal College, University of Delhi
Content Reviewer: Prof. K.S. Rao Department of Botany, University of Delhi
Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi
ZOOLOGY Principles of Ecology
Ecosystem: Ecosystem Processes-I (Part-4)
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Description of Module
Subject Name ZOOLOGY
Paper Name Principles of Ecology: Zool 012
Module Name/Title Ecosystem
Module Id M32: Ecosystem: Ecosystem Processes-I (Part-IV)
Keywords Biogeochemical cycles, Nutrient cycles, Gaseous cycle,
sedimentary cycle, Sulfur cycle, Phosphorous cycle.
Contents
1. Learning Outcomes
2. Introduction
3. Types of biogeochemical cycle
4. Sedimentary cycle
4.1. Phosphorous cycle
4.1.1. Impact of human activities on phosphorous cycle
4.2. Sulfur cycle
4.2.1. Impact of human activities on sulfur cycle
5. Summary
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Ecosystem: Ecosystem Processes-I (Part-4)
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1. Learning Outcomes
After studying this module, you shall be able to
Explain nutrient cycle and biogeochemical cycles.
List the major types of biogeochemical cycles.
Explain sedimentary cycles such as Phosphorus and sulfur cycle.
Understand impact of human activities on these sedimentary cycles.
2. Introduction
All living organisms are made up of various chemical elements present in the nature. Out of
all elements occurring in the nature, between 30 and 40 are known to be required by all living
organisms (essential elements). There are certain elements like carbon, hydrogen, oxygen,
and nitrogen are required by organisms in large amount while other elements are required by
organisms in small or in minute quantities. Irrespective of the quantitative need, all essential
elements show definite cycles. The non essential elements which are less closely coupled
with the organism are also flow along with essential elements either in water cycle or because
of their chemical affinity with them.
The biological community operates as a complex processor through which organisms move
nutrients from one place to another within the ecosystem. These biological exchanges of
nutrients interact with physical exchanges and for this reason nutrient cycles can also be
considered as biogeochemical cycle. Bio refers to living organisms and geo refers to earth.
Geochemistry is related with chemical composition of earth and the exchange of various
elements between different parts of earth crust, its atmosphere, and water bodies like sea,
lake, river etc. The concept of the geochemistry is given by Russian Polynov (1937) which
can be explained as the role of chemical elements in the production and decomposition of all
materials with special emphasis on weathering. Biogeochemistry was founded by Russian
V.I. Vernadskij (1926) which involves the study of exchange of materials between biotic and
abiotic components of the ecosphere.
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Ecosystem: Ecosystem Processes-I (Part-4)
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Figure.1: General pattern of nutrient cycles.
The figures 1 demonstrate the general pattern of nutrients cycles on a global scale. All
nutrient cycles are closed on a global scale but they are open at the local scale. The individual
elements that form the cycle are imperishable and can be recycled by plants and animals. It is
very important to understand and measure the global nutrients cycles because these cycles are
continuously shifting due to anthropogenic activities with possible effects on global climate.
Thus, analysis of nutrient cycling ends with an assessment of human impact on nutrient
cycles and their consequences for animals and plants. Global nutrient cycles correspond to
the summation of local events occurring in different biotic communities. Thus, to understand
the global nutrient cycle, the study must begin at the level of local community.
All nutrients are located in the compartments which represent a distinct space in nature.
These compartments can be defined broadly or very specifically. A compartment is made up
of certain quantity of nutrients present in the standing crop. For example, in lake ecosystem,
Volatiles
bioelements only Evaporation
Marine
food web
Dead organic
matter
H2O and volatile biochemicals
Sinking
OCEAN
Precipitation
Dead organic
matter
Terrestrial
food web
Bioelements
in solution
Loss by water runoff
Decomposition
Weathering Terrestrial biosphere
Volatiles
Bioelements only
Death Uptake
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Ecosystem: Ecosystem Processes-I (Part-4)
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the phosphorous dissolved in water forms one pool and the phosphorous which is present in
the bodies of consumers is another pool.
The compartments exchange nutrients and thus it is important to measure the uptake and
outflow of nutrients for each compartment. The rate of movement of nutrients between two
compartments is known as flux rate and it is measured as the quantity of nutrients moving
from one pool to another per unit time. The flux rate and pool size together constitutes
nutrient cycle for any particular ecosystem. The ecosystems are not isolated from each other
and nutrients move from one ecosystem to another through meteorological, geological or
biological transport mechanism. Meteorological input includes dissolved matter in the rain
and snow, atmospheric gases and dust particles carried by the wind, geological input includes
weathering and subsurface drainage and biological inputs consists of movements of animals
between different ecosystems.
3. Types of Biogeochemical Cycles
From the view point of the ecosphere as a whole, biogeochemical cycles fall into two basic
groups:
1) The gaseous cycle in which the reservoir is in the atmosphere or hydrosphere (ocean) and
2) Sedimentary or mineral cycle in which the reservoir is in the lithosphere i.e. earth crust.
The gaseous cycle has already been discussed in the previous module. In this module we will
learn about sedimentary biogeochemical cycles.
4. Sedimentary or mineral cycle
4.1. Phosphorous cycle
Phosphorus is an important constituent of cell membrane, phospholipids, nucleic acid, bone,
and teeth etc. which make it important to both plants and animals. The reservoir of
phosphorus is present in rock and other natural phosphate deposits (figure 2). These
reservoirs slowly erode and release phosphate from the rocks and minerals by the process of
weathering, erosion, leaching and mining. Only a small fraction of the total phosphorus in the
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soil is available to plants. It occurs naturally as phosphate (PO4¯), either as soluble inorganic
phosphate ions, soluble organic phosphate, as particulate phosphate (insoluble organic or
inorganic molecules) or as mineral phosphate (mineral grain present in rock or sediment).
Phosphate, present in the soil, is absorbed by plants through their roots and incorporated into
tissues. From autotrophs, phosphorus is moved along the grazing food chain with excess
phosphate is excreted trough feces. For example, guano deposits of birds on the desert west
coast of South America. The detritivores in the detritus food chain degrade large organic
molecules containing phosphate and released inorganic ionic phosphate. This form is
immediately absorbed by autotrophs or it incorporated into sediment. The sedimentary phase
remains comparatively slow than the organic phase.
Figure.2: The global phosphorus cycle.
Soil erosion and leaching of dissolved forms release phosphate into the rivers or lakes. Some
phosphate precipitated in the lake sediments whereas, majority of phosphate escapes into the
oceans. In marine and freshwater ecosystems, phytoplanktons absorb organic phosphates
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Ecosystem: Ecosystem Processes-I (Part-4)
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which are being eaten by zooplankton and detritivores. Zooplanktons may excrete as much
phosphorus daily as it stores in its biomass, returning it to the cycle. It is estimated that more
than half of the phosphorus excreted by zooplanktons are taken up by phytoplanktons. The
remaining phosphorus in organic compounds is used by bacteria, which fail to regenerate
large amount of dissolved inorganic phosphate. Some part of the phosphate is deposited in
the shallow sediments and some is lost to the deep sediments. During the process of ocean
upwelling, the movement of deep waters to the surface brings small amount of phosphates
from the depths to shallow water, where light is available to phytoplanktons for doing
photosynthesis. When the organisms die and sink to the bottom, the level of phosphate at
surface of water became depleted. During upwelling of deep water brings some amount of
phosphorus to the surface of water. Through the uplifting of sediments and harvesting of
fishes from sea, the phosphorus is returned back to land from sea. According to one estimate,
through the fish we consume 60,000 tons of elementary phosphorous returns annually. Sea
birds, by depositing their fecal material (guano deposits), on land also significantly
contributes to returning of phosphorus into the cycle. But this is insufficient to compensate
for the loss from the land to the sea. On a geological timescale, uplifting and subsequent
weathering return this phosphorus to the active cycle (figure 3).
Major reservoir of Phosphorus is found in rocks which releases phosphorus by the
process of weathering. This is carried to the soil by water or air as inorganic
phosphate.
Inorganic phosphate is absorbed and assimilated by plants. In most soils, only 0.01%
of phosphorus is available of the total phosphorus in soil. From autotrophs, organic
form of phosphorus moves through the food chain.
The dead organic matter is acted upon by the phosphatising bacteria to release
inorganic phosphorus from bound organic form. Phosphorus is also lost by runoff
water in deep-ocean sediments.
Phosphorus is returned from shallow marine deposits in fish harvest and guano
deposits of fish eating birds and geological uplift.
The precipitation of phosphorus in marine habitats restricts the primary productivity.
The lake which consists of limited amount of phosphorus forms the oligotrophic lake.
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Figure. 3: Diagrammatic representation of phosphorous cycle.
4.1.1. Impact of human activities on phosphorous cycle
1) Large quantities of phosphate are mobilized by extracting it from rocks for the
manufacturing of fertilizers and detergents. The yield of crops can be increased only
by the use of the fertilizers, so the consumption of these fertilizers will further
increase in future.
2) Deforestation also causes loss of available phosphate from the soil.
3) Release of sewage waste, runoff of fertilizers from soil and animal waste disrupt the
functioning of aquatic ecosystems (causes eutrophication).
4) The use of phosphorous at the present rate will diminish the reserves within 60-160
years.
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Ecosystem: Ecosystem Processes-I (Part-4)
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4.2. Sulfur cycle
Like nitrogen, sulfur also forms an important constituent of proteins and amino acids and is
characteristic of organic compounds. It exists in various forms like sulfur, sulfide, sulfur
monoxide and sulfates. Sulfate (SO4) is the most common and important biological form
which is utilized by the plants to incorporate into proteins, sulfur being an essential
constituent of some amino acids like cystine. Sulfur is required by ecosystem in small amount
for the growth of plants and animals. Nevertheless, the sulfur cycle is the key one in the
general pattern of production and decomposition. The sulfur cycle has both sedimentary
phase and gaseous phase. The sedimentary phase of sulfur cycle is tied up in the inorganic
and organic deposits. The sulfur is released from these deposits by the process of weathering
and decomposition which is carried to the terrestrial and aquatic ecosystems. The sulfur cycle
is less pronounced in the gaseous phase and it allows circulation of sulfur on a global scale.
The sulfur enters into atmosphere through combustion of fossil fuels, volcanic activities,
decomposition process and exchange from surface of the seas. It is released into the
atmosphere as hydrogen sulfide (H2S) and reacts promptly with oxygen to form sulfur
dioxide (SO2). It is soluble in water and is carried back to the earth surface through rain as
weak sulfuric acid (H2SO4).
Figure.4: The global sulfur cycle.
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Whatever its source, sulfur in a soluble form, frequently as sulfate (SO4) is absorbed by plant
roots where it is incorporated into various organic molecules like proteins and certain amino
acids (cystine). From these autotrophs, the sulfur is transferred to the consumer level with
excess being excreted in the fecal matter. These excretory materials and dead plants and
animals carrying sulfur releases back into the soil and to the bottoms of ponds, lakes and seas,
acted upon by detritivores (bacteria and fungus).
One group of bacteria called sulfur bacteria reduces hydrogen sulfide (H2S) to elementary
sulfur which is then oxidizes into sulfuric acid. In the presence of light, some green and
purple bacteria use hydrogen sulfide (H2S) during photosynthesis. The purple bacteria which
are found in salty marshes and in the mudflats of estuaries, can transform hydrogen sulfide
into sulfate. Green bacteria can transform hydrogen sulfide into elemental sulfur.
In an aerobic condition, the hydrogen sulfide is oxidized to sulfate by certain bacteria which
are used by the autotrophs. In an anaerobic environment such as bottom of lakes, oxidation
process cannot be occurred due to absence of oxygen. However, in the presence of infrared
radiation, some photosynthetic bacteria can oxidize sulfide into elementary sulfur or sulfate.
The elementary sulfur can also be utilized by other bacteria to form sulphate. Under
anaerobic conditions, elementary sulfur can also be converted into sulfate by certain bacteria
in the presence of nitrate. Under certain conditions, sulfate can also be reduced into sulfide or
sulfur by bacteria.
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Figure. 5: The fate of elementary sulfur in sulfur cycle.
These numbers of reaction in the organic phase of the sulfur cycle provides a mechanism for
regulating the availability of elementary sulfur to the autotrophs (figure 5). If iron is present
in the sediments, sulfur will precipitate as ferrous sulfide (FeS2) in an anaerobic condition.
FeS2 is insoluble in neutral and acidic pH and it is firmly held in mud and wet soil.
Some ferrous sulfide is present in the sedimentary rocks called pyritic rocks may overlying
coal deposits. The FeS2, when exposed to the air in deep and surface mining, oxidizes and in
the presence of water produces ferrous sulfate (FeSO4) and sulfuric acid (H2SO4). In this way,
sulfur present in pyrite rocks, abruptly exposed to weathering by humans, discharges heavy
slugs of sulphur, sulphuric acid, ferric sulphate and ferrous hydroxide into aquatic
ecosystems. These compounds devastate aquatic life and decreases pH of water (acidic).
Consequently, the routes of sulfur cycle can be divided into following main steps:
Sulfur is present in the nature as elementary sulfur or sulfides and sulfates. The fossil
fuels and volcanic activities release H2S and SO2 gas in atmosphere which eventually
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returns to the soil as sulphuric acid along with rain, forming sulfate compounds by the
activities of various bacteria.
The elementary sulfur present in the rocks is also converted into sulfate by weathering
process and is also released into the soil. Sulfur in the form of sulfate is absorbed by
plants and form various organic molecules.
The sulfate utilized by the plants is transferred to consumers of the food chain. After
the death of plants and animals as well as their waste materials are decomposed by
detritivores into sulfur. The elementary sulfur is converted into sulfate by certain
bacteria which is again available for the plants and cycle continues.
4.2.1. Impact of human activities on sulfur cycle
As in the case of other biogeochemical cycles, sulfur cycle is also being affected by human
activities, such as air pollution, mining etc.
1) The oxides of sulfur are toxic and constituted about one third of the industrial air
pollutants discharged into the air. Burning of sulfur containing coal, automobile
exhausts and oil to produce electric power increases the concentration of these
oxides (SO2 and SO4) into the air which are also damaging to photosynthesis.
2) The oxides of sulfur and nitrogen interact with water vapour to produce sulfuric
and nitric acid (H2SO4 and H2NO3) that falls to earth surface in the form of acid
rain. It has greatest impact on fresh water bodies like lakes or streams and acidic
soils that lacks pH buffers (such as carbonates, calcium salts etc.).
3) The conversion of sulfur containing metallic mineral ores into free metals like
copper, lead, and zinc releases large amount of SO2 into the environment.
4) Refining of sulfur containing petroleum to make gasoline, heating oil etc. also
releases large amount of SO2 into the atmosphere.
5. Summary
All living organisms are made up of various chemical elements such as C, H, N, O, present in
the nature. Out of all elements occurring in the nature, 30 to 40 elements are known to be
ZOOLOGY Principles of Ecology
Ecosystem: Ecosystem Processes-I (Part-4)
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required by living organisms. The biological community operates in a complex manner in
which organisms move nutrients from one place to another within the ecosystem. These
biological exchanges of nutrients interact with physical exchanges and for this reason nutrient
cycles are also considered as biogeochemical cycle. The ecosystems are not isolated from
each other and nutrients move from one ecosystem to another through meteorological,
geological or biological transport mechanism.
From the view point of the ecosphere as a whole, biogeochemical cycles can be divided into
two basic categories, gaseous cycle in which the reservoir is in the atmosphere or
hydrosphere and sedimentary cycle in which the reservoir is in the lithosphere.
The sedimentary cycle includes phosphorus and sulfur cycle. Phosphorus is an important
constituent of cell membrane, phospholipid, nucleic acid, bone and teeth etc. The reservoir of
phosphorus is present in rock and other natural phosphate deposits which releases phosphorus
by the process of weathering. This is carried to the soil by water or air as inorganic phosphate
which is absorbed and assimilated by plants. From autotrophs, organic form of phosphorus
moves through the food chain. Dead organic matter is acted upon by the phosphatising
bacteria to release inorganic phosphorus from bound organic form. Phosphorus is returned
from shallow marine deposits in fish harvest and guano deposits of fish eating birds and
geological uplift.
Like nitrogen, sulfur also forms an important constituent of proteins. Sulfate (SO4) is the
most common and important biological form which is utilized by the plants to incorporate
into proteins. Sulfur is present in the nature as elementary sulfur or sulfides and sulfates. The
fossil fuels and volcanic activities release H2S and SO2 gas in atmosphere which eventually
returns to the soil as sulfuric acid along with rain, forming sulfate compounds by the
activities of various bacteria. The elementary sulfur present in the rocks is also converted into
sulfate by weathering process and is also released into the soil. The sulfate utilized by the
plants is transferred to consumers of the food chain. When plants and animals die, their body
is decomposed by detritivores and free sulfur is released. The elementary sulfur is converted
into sulfate by certain bacteria which is again available for the plants and cycle continues.