Download biogeochemical cycle-carbon,sulphur and phosphorus cycles

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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
BIOGEOCHEMICAL CYCLE
In ecology and Earth science, a biogeochemical cycle or substance turnover or cycling of substances is a
pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic
(lithosphere, atmosphere, and hydrosphere) compartments of Earth. 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). The water undergoes evaporation, condensation, and precipitation,
falling back to Earth clean and fresh. 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 the biogeochemical cycles.
Systems
All chemical elements occurring in organisms are part of biogeochemical cycles. In addition to being a part of
living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water
(hydrosphere), land (lithosphere), and the air (atmosphere). The living factors of the planet can be referred to
collectively as the biosphere. All the nutrients—such as carbon, nitrogen, oxygen, phosphorus, and sulfur—used
in ecosystems by living organisms operate on a closed system; therefore, these chemicals are recycled instead of
being lost and replenished constantly such as in an open system.
The flow of energy in an ecosystem is an open system; the sun constantly gives the planet energy in the form of
light while it is eventually used and lost in the form of heat throughout the trophic levels of a food web. Carbon is
used to make carbohydrates, fats, and proteins, the major sources of food energy. These compounds are
oxidized to release carbon dioxide, which can be captured by plants to make organic compounds. The chemical
reaction is powered by the light energy of the sun.
It is possible for an ecosystem to obtain energy without sunlight. Carbon must be combined with hydrogen and
oxygen in order to be utilized as an energy source, and this process depends on sunlight. Ecosystems in the deep
sea, where no sunlight can penetrate, use sulfur. Hydrogen sulfide near hydrothermal vents can be utilized by
organisms such as the giant tube worm. In the sulfur cycle, sulfur can be forever recycled as a source of energy.
Energy can be released through the oxidation and reduction of sulfur compounds (e.g., oxidizing elemental sulfur
to sulfite and then to sulfate).
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
Although the Earth constantly receives energy from the sun, it's chemical composition is essentially fixed, as
additional matter is only occasionally added by meteorites. Because this chemical composition is not replenished
like energy, all processes that depend on these chemicals must be recycled. These cycles include both the living
biosphere and the nonliving lithosphere, atmosphere, and hydrosphere.
Reservoirs
The chemicals are sometimes held for long periods of time in one place. This place is called a reservoir, which, for
example, includes such things as coal deposits that are storing carbon for a long period of time.When chemicals
are held for only short periods of time, they are being held in exchange pools. Examples of exchange pools
include plants and animals.Plants and animals utilize carbon to produce carbohydrates, fats, and proteins, which
can then be used to build their internal structures or to obtain energy. Plants and animals temporarily use carbon
in their systems and then release it back into the air or surrounding medium. Generally, reservoirs are abiotic
factors whereas exchange pools are biotic factors. Carbon is held for a relatively short time in plants and animals
in comparison to coal deposits. The amount of time that a chemical is held in one place is called its residence.
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
THE CARBON CYCLE
When examining the biogeochemical cycling of an individual element, it is useful to consider the global reservoirs
of this element, the size of these reservoirs, and whether or not these reservoirs are being actively cycled. The
natural rates of carbon cycling in oceans and on land are close to a steady state; that is, the rates of movement of
carbon between the atmosphere and trees or between algae and the dissolved inorganic carbon of the oceans do
not change measurably from year to year and tend to balance each other. However, human activities have
recently introduced changes in the carbon cycle that are large enough to be measured. Today, the global carbon
cycling is a mixture of the natural steady-state rates and reservoirs and the changing rates and reservoirs
affected by human activities.
Table: Major carbon reservoirs
Reservoir
Amount (billions of metric
Carbon is taken from the atmosphere in several
tons of carbon)
ways:
Atmosphere before 1850
560—610
Atmosphere in 1978
692
When
the
sun
is
shining,
plants
perform
photosynthesis to convert carbon dioxide into
carbohydrates, releasing oxygen in the process. This
Oceans and fresh water
process is most prolific in relatively new forests
Inorganic
35,000
Dissolved organic
1,000
strongest in deciduous forests during spring leafing
Land biota
600—900
out. This is visible as an annual signal in the Keeling
Soil organic matter
1,500
Sediments
10,000,000
more
Fossil fuels
10,000
hemisphere than in the southern.
where tree growth is still rapid. The effect is
curve of measured CO2 concentration. Northern
hemisphere spring predominates, as there is far
land
in
temperate
latitudes
in
that

Forests store 86% of the planet's above-ground carbon and 73% of the planet's soil carbon.

At the surface of the oceans towards the poles, seawater becomes cooler and more carbonic acid is
formed as CO2 becomes more soluble. This is coupled to the ocean's thermohaline circulation which
transports dense surface water into the ocean's interior (see the entry on the solubility pump).
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES

In upper ocean areas of high biological productivity, organisms convert reduced carbon to tissues, or
carbonates to hard body parts such as shells and tests. These are, respectively, oxidized (soft-tissue
pump) and redissolved (carbonate pump) at lower average levels of the ocean than those at which they
formed, resulting in a downward flow of carbon (see entry on the biological pump).

The weathering of silicate rock. Carbonic acid reacts with weathered rock to produce bicarbonate ions.
The bicarbonate ions produced are carried to the ocean, where they are used to make marine carbonates.
Unlike dissolved CO2 in equilibrium or tissues which decay, weathering does not move the carbon into a
reservoir from which it can readily return to the atmosphere.
Carbon can be released back into the atmosphere in many different ways:

Through the respiration performed by plants and animals. This is an exothermic reaction and it involves
the breaking down of glucose (or other organic molecules) into carbon dioxide and water.

Through the decay of animal and plant matter. Fungi and bacteria break down the carbon compounds in
dead animals and plants and convert the carbon to carbon dioxide if oxygen is present, or methane if not.

Through combustion of organic material which oxidizes the carbon it contains, producing carbon dioxide
(and other things, like water vapor). Burning fossil fuels such as coal, petroleum products, and natural gas
releases carbon that has been stored in the geosphere for millions of years. Burning agrofuels also
releases carbon dioxide.

Production of cement. Carbon dioxide is released when limestone (calcium carbonate) is heated to
produce lime (calcium oxide), a component of cement.

At the surface of the oceans where the water becomes warmer, dissolved carbon dioxide is released back
into the atmosphere.

Volcanic eruptions and metamorphism release gases into the atmosphere. Volcanic gases are primarily
water vapor, carbon dioxide and sulfur dioxide. The carbon dioxide released is roughly equal to the
amount removed by silicate weathering; so the two processes, which are the chemical reverse of each
other, sum to roughly zero, and do not affect the level of atmospheric carbon dioxide on time scales of
less than about 100,000 yr.

Forests and crops in the process of growing absorbs lots of carbon, while an old and stable forest
consumes as much CO2 during the day as they produce during the night.
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
In the biosphere
Around 1,900 gigatons of carbon are present in the biosphere. Carbon is an essential part of life on Earth. It plays
an important role in the structure, biochemistry, and nutrition of all living cells.

Autotrophs are organisms that produce their own organic compounds using carbon dioxide from the air
or water in which they live. To do this they require an external source of energy. Almost all autotrophs
use solar radiation to provide this, and their production process is called photosynthesis. A small number
of autotrophs exploit chemical energy sources in a process called chemosynthesis. The most important
autotrophs for the carbon cycle are trees in forests on land and phytoplankton in the Earth's oceans.
Photosynthesis follows the reaction 6CO2 + 6H2O → C6H12O6 + 6O2

Carbon is transferred within the biosphere as heterotrophs feed on other organisms or their parts (e.g.,
fruits). This includes the uptake of dead organic material (detritus) by fungi and bacteria for fermentation
or decay.

Most carbon leaves the biosphere through respiration. When oxygen is present, aerobic respiration
occurs, which releases carbon dioxide into the surrounding air or water, following the reaction C 6H12O6 +
6O2 → 6CO2 + 6H2O. Otherwise, anaerobic respiration occurs and releases methane into the surrounding
environment, which eventually makes its way into the atmosphere or hydrosphere (e.g., as marsh gas or
flatulence).

Burning of biomass (e.g. forest fires, wood used for heating, anything else organic) can also transfer
substantial amounts of carbon to the atmosphere

Carbon may also be circulated within the biosphere when dead organic matter (such as peat) becomes
incorporated in the geosphere. Animal shells of calcium carbonate, in particular, may eventually become
limestone through the process of sedimentation.

Much remains to be learned about the cycling of carbon in the deep ocean. For example, a recent
discovery is that larvacean mucus houses (commonly known as "sinkers") are created in such large
numbers that they can deliver as much carbon to the deep ocean as has been previously detected by
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
sediment traps. Because of their size and composition, these houses are rarely collected in such traps, so
most biogeochemical analyses have erroneously ignored them.
Carbon storage in the biosphere is influenced by a number of processes on different time-scales: while net
primary productivity follows a diurnal and seasonal cycle, carbon can be stored up to several hundreds of years in
trees and up to thousands of years in soils. Changes in those long term carbon pools (e.g. through de- or
afforestation or through temperature-related changes in soil respiration) may thus affect global climate change.
An idealized food web showing transfers
between
trophic
levels.
Organic
carbon
formed by primary producers is transferred to
grazers and predators. Decomposers and
respiration of grazers and predators return
CO2 to primary producers. The diagram shows
that the supportable biomass declines at
progressively higher trophic levels.
Carbon Transfer Through Food Webs
Every food web is based on primary producers. The net fixation of CO 2 to form organic compounds is carried out
by autotrophic organisms. Among the microorganisms, this includes photosynthetic and chemolithotrophic
organisms. The most important groups of microorganisms, in terms of their abilities to convert CO 2 to organic
matter, are the algae, the cyanobacteria, and the green and purple photosynthetic bacteria. Chemoautotrophic
microorganisms contribute to a lesser extent. The principal metabolic pathway for photosynthetic CO 2 fixation is
the Calvin cycle, in addition, microorganisms are capable of incorporating CO2 through the phosphoenol pyruvate
carboxylase system. In the case of heterotrophic microorganisms, exchange but no net CO2 fixation occurs, but
some chemolithotrophic microorganisms use this system either instead of or in addition to the pentose
phosphate cycle for net CO2 fixation. Methanogenic archaea play an important role in the anaerobic reduction of
CO2 .Only a limited number of microorganisms can utilize the resulting methane. These methylotrophs are
ecologically important in minimizing methane transfer to the atmosphere.
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
Carbon dioxide convened to organic carbon by primary producers represents the gross primary pro duction of
the community. This process is carried out predominantly by photosynthetic organisms that convert light energy
to chemical energy; the chemical energy is stored within the organic compounds that are formed. The conversion
of radiant energy to chemical energy in organic compounds is the essence of primary production.
A portion of the gross primary production is converted back to CO2 by the respiration of the primary
producers. The remaining organic carbon is the net primary production available to heterotrophic consumers;
the heterotrophs complete the carbon cycle, ultimately converting organic compounds formed by primary
producers back to CO2 during respiration.
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
Carbon Cycling Within Habitats
The degradation and recycling of organic matter in habitats is accomplished by heterotrophic macro- and
microorganisms. Microbial activities are crucial in terms of not only the quantity but also the quality of their
contributions. Under aerobic conditions, macro- and microorganisms share the ability to biodegrade simple
organic nutrients and some biopolymers, such as starch pectin, proteins, and so on, but microorganisms are
unique in their capacity to carry out anaerobic (fermentative) degradation of organic matter. They are also
responsible for the recycling of most abundant but difficult-to-digest biopolymers, such as cellulose and lignin.
In the ocean
The oceans contain around 36,000 gigatonnes of carbon, mostly in the form of bicarbonate ion (over 90%, with
most of the remainder being carbonate). Extreme storms such as hurricanes and typhoons bury a lot of carbon,
because they wash away so much sediment. Inorganic carbon, that is carbon compounds with no carbon-carbon
or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important
in controlling pH in the ocean and can also vary as a source or sink for carbon. Carbon is readily exchanged
between the atmosphere and ocean. In regions of oceanic upwelling, carbon is released to the atmosphere.
Conversely, regions of downwelling transfer carbon (CO2) from the atmosphere to the ocean. When CO2 enters
the ocean, it participates in a series of reactions which are locally in equilibrium:
Solution:CO2(atmospheric) ⇌ CO2(dissolved)
Conversion to carbonic acid:
CO2(dissolved) + H2O ⇌ H2CO3
First ionization:
H2CO3 ⇌ H+ + HCO3− (bicarbonate ion)
Second ionization:
HCO3− ⇌ H+ + CO3−− (carbonate ion)
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
This set of reactions, each of which has its own equilibrium coefficient determines the form that inorganic carbon
takes in the oceans[5]. The coefficients, which have been determined empirically for ocean water, are themselves
functions of temperature, pressure, and the presence of other ions (especially borate). In the ocean the equilibria
strongly favor bicarbonate. Since this ion is three steps removed from atmospheric CO 2, the level of inorganic
carbon storage in the ocean does not have a proportion of unity to the atmospheric partial pressure of CO 2. The
factor for the ocean is about ten: that is, for a 10% increase in atmospheric CO 2, oceanic storage (in equilibrium)
increases by about 1%, with the exact factor dependent on local conditions. This buffer factor is often called the
"Revelle Factor", after Roger Revelle.
In the oceans, bicarbonate can combine with calcium to form limestone (calcium carbonate, CaCO3, with silica),
which precipitates to the ocean floor. Limestone is the largest reservoir of carbon in the carbon cycle. The
calcium comes from the weathering of calcium-silicate rocks, which causes the silicon in the rocks to combine
with oxygen to form sand or quartz (silicon dioxide), leaving calcium ions available to form limestone.
The greenhouse effect is the process in which the emission of infrared radiation by the atmosphere warms a
planet's surface.. The greenhouse effect was discovered by Joseph Fourier in 182
There is concern today that the continued in atmospheric C0 2 currently at a rate of about 1 per year, might
intensify the “greenhouse effect. C02 is transparent to visible radiation, but absorbs s in the infrared range.
Visible sunlight striking earth irradiated back as longer-wavelength infrared radiation. An increase in CO2 in the
Earth’s atmospheric blanket would retain more of this radiation and thus would bring about a warming trend the
climate.
Contributing to the greenhouse effect is atmospheric methane released by human activities such as drilling for oil
and natural gas, land filling of solid waste and large scale cattle raising and wetland rice cultivation. Although
much lower in amount than CO2 the burning of fossil fuels, methane traps heat five times as effectively as CO 2 .
Thus even relatively small amounts, it can contribute to the green house effect significantly.
Global warming is the increase in the average measured temperature of the Earth's near-surface air and oceans
since the mid-20th century, and its projected continuation.
The average global air temperature near the Earth's surface increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the
100 years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the
observed increase in anthropogenic (man-made) greenhouse gas concentrations" via an enhanced greenhouse
effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect
from pre-industrial times to 1950 and a small cooling effect from 1950 onward.
On Earth, the major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse
effect (not including clouds); carbon dioxide (CO2), which causes 9–26 percent; methane (CH4), which causes 4–9
percent; and ozone, which causes 3–7 percent. The issue is how the strength of the greenhouse effect changes
when human activity increases the atmospheric concentrations of some greenhouse gases.
Human activity since the industrial revolution has increased the concentration of various greenhouse gases,
leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs and nitrous oxide.
Fossil fuel burning has produced approximately three-quarters of the increase in CO2 from human activity over
the past 20 years. Most of the rest is due to land-use change, in particular deforestation.
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
Phosphorus cycle
A phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the
lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not
play a significant role in the movements of phosphorus, because phosphorus and phosphorus-based compounds
are usually solids at the typical ranges of temperature and pressure found on Earth.
Phosphorus is an essential nutrient for plants and animals in the form of ions PO 43- and HPO42- . It is a part of
DNA-molecules, of molecules that store energy (ATP and ADP) and of fats of cell membranes. Phosphorus is also
a building block of certain parts of the human and animal body,
such as the bones and teeth.
Phosphorus normally occurs in nature as part of a phosphate ion,
consisting of a phosphorus atom and some number of oxygen
atoms, the most abundant form (called orthophosphate) having
four oxygens: PO43-. Most phosphates are found as salts in ocean
sediments or in rocks. Over time, geologic processes can bring
ocean sediments to land, and weathering will carry terrestrial .
Plants absorb phosphates from the soil. The plants may then be
consumed by herbivores who in turn may be consumed by
carnivores. After death, the animal or plant decays, and the
phosphates are returned to the soil. Runoff may carry them back
to the ocean or they may be reincorporated into rock.
The primary biological importance of phosphates is as a
component of nucleotides, which serve as energy storage within
cells (ATP) or when linked together, form the nucleic acids DNA
and RNA. Phosphorus is also found in bones, whose strength is derived from calcium phosphate, and in
phospholipids (found in all biological membranes).
Phosphates move quickly through plants and animals; however, the processes that move them through the soil
or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.
However, recent findings suggest that phosphorus is cycled through the ocean on the timescale of 10,000yr,
suggesting that the phosphorus cycle may play a role in global warming.
Unlike other cycles of matter compounds, phosphorus cannot be found in air as a gas. This is because at normal
temperature and circumstances, it is a liquid. It usually cycles through water, soil and sediments. In the
atmosphere phosphorus is mainly small dust particles.
Phosphorus is one of the longest cycles, and takes a long time to move from sediments to living organisms and
back to sediments.
Eutrophication is an increase in chemical nutrients -- typically compounds containing nitrogen or phosphorus -in an ecosystem. It may occur on land or in water. The term is however often used to mean the resultant increase
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
in the ecosystem's primary productivity (excessive plant growth and decay), and further effects including lack of
oxygen and severe reductions in water quality, fish, and other animal populations.
Eutrophication is frequently a result of nutrient pollution such as the release of sewage effluent and run-off from
lawn fertilizers into natural waters (rivers or coasts) although it may also occur naturally in situations where
nutrients accumulate (e.g. depositional environments) or where they flow into systems on an ephemeral basis
(e.g. intermittent upwelling in coastal systems). Eutrophication generally promotes excessive plant growth and
decay, favors certain weedy species over others, and is likely to cause severe reductions in water quality . In
aquatic environments, enhanced growth of choking aquatic vegetation or phytoplankton (that is, an algal bloom)
disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of oxygen in the water,
needed for fish and shellfish to survive. The water then becomes cloudy, colored a shade of green, yellow,
brown, or red. Human society is impacted as well: eutrophication decreases the resource value of rivers, lakes,
and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered. Health-related
problems can occur where eutrophic conditions interfere with drinking water treatment.
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
Sulphur Cycle
Sulfur cycle is one of the constituents of many proteins, vitamins and hormones. It recycles as in other
biogeochemical cycles.Sulphur like nitrogen, is an essential element for all living systems but because of its inert
nature is not utilized by plants. To be used, sulphur has to be first oxidized or reduced. In the soil it occurs both in
organic (sulphur amino acids, vitamins etc.) as well as in the inorganic form (sulphur, sulphates etc.) and is readily
metabolized.Sulphur .
Pools in Nature
Land
amount, 1018 g S yr-1
Pools
amounts, 1018 g sulphur
Sedimentary
rocks: ....
evaporites
2470
...... shales
4970
seawater
1280
land plants
0.0085
soil organic
matter
0.0155
atmosphere
0.0000025
TOTAL
8270
industry to air
90 (as SO2)
desert dust to air
8 (as sulphate salts)
volcanoes to air
5 (H2S > SO2)
biogenic activity to
air
4 (H2S)
air to land
90
human extraction
160
Ocean
air to ocean
180 (mainly sea
salt)
sea salt to air
144
biogenic activity to air
16 (DMS)
marine volcanoes to air
5 (H2S )
marine volcanoes to air
5
pyrite deposition
39
hydrothermal sulphides
96
Land - Ocean
air transport to sea
20 (pollution)
air transport to land
4 (cyclic salts)
weathering to rivers
72
rivers to ocean
130
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
The essential steps of the sulfur cycle are:




Mineralization of organic sulfur to the inorganic form, hydrogen sulfide: (H 2S).
Oxidation of sulfide and elemental sulfur (S) and related compounds to sulfate (SO 42–).
Reduction of sulfate to sulfide.
Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of
sulfur.
These are often termed as follows:
Assimilative sulfate reduction in which sulfate (SO42–) is reduced to organic sulfhydryl (otherwise known
as thiol) groups (R–SH) by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in
sulfate and –2 in R–SH.
Desulfuration in which organic molecules containing sulfur can be desulfurated, producing hydrogen
sulfide gas (H2S), oxidation state = –2. Note the similarity to deamination.
Oxidation of hydrogen sulfide produces elemental sulfur (So), oxidation state = 0. This reaction is done by
the photosynthetic green and purple sulfur bacteria and some chemolithotrophs.
Further 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.
Bacteria carry out various transformations of sulphur:
1. Sulphate reduction in anaerobic environments
2CH2O + 2H+ + SO42- --> H2S + 2CO2 + 2H2O
This reaction is analagous to aerobic respiration but with SO42-. rather than oxygen, acting as the terminal
electron acceptor in the oxidation reaction. The H2S produced may form reduced sulphur compounds
such as pyrite or undergo either of the reactions that follow;
2. Sulphur-based (anaerobic) photosynthesis
2H2S + CO2 --> CH2O + 2S + 2H2O
This reaction is probably the earliest form of photosynthesis using H2S, rather than water H2O, as the
hydrogen donor in the reduction of CO2. It (or something similar) is employed by green and purple
suphur-bacteria today;
Chemoautotrophy in aerobic conditions:
4H2S + CO2 + O2 --> CH2O + 4S + 3H2O
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
This reaction is performed by species of Thiobacilli in environments with free elemental sulphur or with H2S, for
example in the vicinity of deep sea hydrothermal vents.
Human impact on the sulfur cycle is primarily in the production of sulfur dioxide (SO2) from industry (e.g. burning
coal) and the internal combustion engine. Sulfur dioxide can precipitate onto surfaces where it can be oxidized to
sulfate in the soil (it is also toxic to some plants), reduced to sulfide in the atmosphere, or oxidized to sulfate in
the atmosphere as sulfuric acid, a principal component of acid rain.
In these systems sulphur is found mostly as a component of sulphur containing amino acids such as methionine
and cysteine and to a small extent in some vitamins. When plant and animal proteins are degraded, the sulphur
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BIOGEOCHEMICAL CYCLE-CARBON,SULPHUR AND PHOSPHORUS CYCLES
is released from the amino acids and accumulates in the soil which is then oxidized in the presence of oxygen
while under anaerobic conditions H2S may accumulate.
H2S can also accumulate during the reduction of sulphates under anaerobic conditions which can be oxidized to
sulphate under aerobic conditions. Plants utilize sulphur in the form of sulphates and then reduce it within the
cells to H2S before it is utilized mainly in the synthesis of sulphur amino acids and vitamins (biotin, thiamine,
lipoic acid). The assimilation of sulphur therefore, in many ways resembles the assimilation of nitrates.
The inorganic sulphur compounds which are transformed biologically represent various oxidation states from 2
of sulphide to +6 of sulphate. Not all the stages involved are biological. The biological oxidation of elemental
sulphur' and inorganic sulphur compounds such as H2S, sulphite and thiosulphate is brought about by
chemoautotrophic and photosynthetic bacteria. The oxidation of H2S is characteristic of certain pigmented
sulphur bacteria which use this compound as an electron donor in photosynthesis.
Members of the genus Thiobacillus are the main organisms involved in the oxidation of elemental sulphur.The
ability to oxidize sulphur is not restricted to only the genus Thiobacillus. Heterotrophic bacteria, actinomycetes
and fungi are also reported to oxidize sulphur compounds. For example species of Bacillus, Pseudomonas,
Arthrobacter and Flavobacterium are known to oxidize elemental sulphur or thiosulphate to sulphate.sulhur is
first
converted
enzymatically
to
sulphite
which
is
then
oxidized
to
sulphate.
It is believed that some of the sulphite from the first reaction reacts with sulphur to yield thiosulphate which is
then either cleaved to sulphur and sulphite or converted into tetrathionite. The latter is then metabolized to
sulphur or sulphite which are then oxidized to sulphate.under anaerobic conditions, sulphate is first reduced to
H2S by sulphate reducing microorganisms, mostly the bacteria. Many bacteria including species of Bacillus and
Pseudomonas are known to reduce sulphur or sulphate to H2S but among these, Desulfovibrio desulfuricans
seems to be the most important.
The mechanism by which sulphate is reduced involves the conversion of sulphate to sulphite, a reaction that
needs A TP. The sulphite is then reduced to H2S.The dissimilation of sulphur as sulphide and its release into the
atmosphere has been recognized in recent years as a pollution problem. In fact, it is believed that the microbial
volatilization of sulphur as H2S far exceeds the total amount of H2S produced from all other pollution sources.
The sulphur reducers have therefore become a recognized source of atmospheric sulphur.
Acid rain is rain or any other form of precipitation that is unusually acidic. It has harmful effects on the
environment and on structures. Acid rain is mostly caused by emissions due to human activity of sulfur and
nitrogen compounds which react in the atmosphere to produce acids. In recent years, many governments have
introduced laws to reduce these emissions.
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PASS WORD : biogeocycle