BIOCHEMICAL TRANSFORMATION OF SULFUR
Sulfur cycle is similar to the nitrogen cycle. Both of these elements exist in a number of oxidation states and undergo similar type of chemical reactions and biological transformations. The majority of S is found in the lithosphere. Most nitrogen on earth is also in the lithosphere; however, dinitrogen in the atmosphere is the major pool of biologically available nitrogen.
Only a small portion of S pool is found in the atmosphere, and most S that cycle through the atmosphere is because of human activities. In fact, since the industrial revolution, increased burning of fossil fuels has almost doubled the rate of S entering the atmosphere to approximately 1.5 x 1011 kg S yr-1.
Major reserves of the S are in organic form, which are released through biological decomposition. S enters the soil in the form of: plant residues, animal residues / wastes, chemical fertilizers, dissolved in rainwater, sulfide in primary minerals and elemental S applied to soil to control pathogens or for reclamation of alkali soil etc.
The organic forms of S include: S containing amino acids i.e. cystine, methionine, etheral sulfate, thiourea, glucosides and alkaloids.C:S of organic compounds is 100:1.
Amongst the inorganic forms of S, SO4 dominate in aerobic conditions but sulfide, elemental S, thiosulphate or tetrathionate have also been observed in small amounts.
Sulphur in its various organic and inorganic forms is readily metabolized in soil. The dominance of one or other transformation is governed to a large extent by environmental circumstances that affect the composition and activity of microflora.
Microorganisms bring about number of transformations of sulfur, which are;
Mineralization i.e. Decomposition of organic S compounds with the release of inorganic compounds.
Immobilization i.e. Microbial assimilation or immobilization of simple compounds of S and their incorporation into bacterial, fungal or actinomycetese cells.
Oxidation of inorganic compounds such as sulphide, Thiosulphate, polythionates and elemental S.
Reduction of SO4 and other anions to sulfide.
Volatilization of inorganic and organic sulfur compounds
The transformation of organic S compounds originating from plants or animals can proceed through both aerobic and anaerobic pathways. Microbial production of different enzymes e.g. sulphatase are responsible for mineralization reactions. Organic compounds added to soil are mineralized. Some S is used by microflora for their own cell synthesis and the remaining is released. If conditions are aerobic, the end product is SO4. In the absence of atmospheric O2 particularly during protein decomposition, H2S and the odoriferous mercaptan accumulate.
Mineralization and immobilization depend on C:S ratio of the decomposing substrate. Sulphate (SO4) accumulates only when S level in
organic matter exceeds the microbial needs.
There are various inorganic compounds in soil, which can serve as sources of S for microorganisms. These compounds include sulphate, hyposulphite, sulphoxylate, thiosulphate, tetrathionate and thiocyanate etc. Many heterotrophic microorganisms are unable to utilize sulfate. Instead, they use amino acids.
If C:S ratio is greater than 400:1, net immobilization takes place. The lower C:S ratio (less than 200:1) causes net mineralization. If soil is treated with carbohydrates, S deficiency in soil occurs as the microbial population increases due to carbohydrates which uses the S present in the soil.
Inorganic sulfur oxidation
Under aerobic conditions, reduced S is oxidized through a variety of intermediates by both chemical and biological pathways. Oxidation states range from +6 in SO4 to –2 in H2S and its derivatives. Oxidation of S in soil generates acidity, depending on the soil involved, this may be either helpful or harmful.
The oxidation of sulphide, S and thiosulphate is slower by chemical means compared to the rapid oxidation by microorganisms if the temperature and moisture are optimum.
Oxidation of inorganic S compounds in soils, waters, and sediments can be carried out by a diverse group of auto and heterotrophic bacteria including Thiobacillus, Sulfolobus, Thiomicrospora, Arthrobacter, Pseudomonas, some actinomycetes and filamentous fungi etc.
Reduction of inorganic sulfur compounds
Flooding of soil results in oxygen deficiency and SO 4 level in that soil decreases compared to an increase in sulfide level. Much of the sulphide comes from the reduction of sulphates but mineralization of organic S may also give sulphides. SO4 reduction increases by: increasing water level, adding organic material and rising temperature,
while it is depressed by: aeration, NO3 amendment and ferric or manganic salts.
The reduction of inorganic S compounds is usually mediated by anaerobic, organotrophic organisms including Desulfovibrio, Desulfotomaculum and desulfomonas etc. These organisms are responsible for sulphide formation in waterlogged soils and sediments. Sulphate reducing bacteria are found over an extensive range of pH and salt concentrations, in saline lakes, evaporation beds, deep-sea sediments and oil wells. The environmental consequences of sulphate reduction include: The reduction of sulphates in sewage systems leads to the formation of sulphides, which has been implicated in the corrosion of stone and concrete. H2S with an odor of rotten eggs, clogs and corrodes pumps and sewage.
Under anaerobic conditions, Fe3+ and SO42- are reduced to Fe2+ and HS-. These reduction processes can increase the pH of the system. The H2S can cause injury to the roots. Availability of SO4 is reduced so drain on fertility. Reduction of salinity by precipitation of SO4. Formation of S deposits.
A number of Sulfur gases are released from soils, marhes, peats, and sediments or from anthropogenic sources. These gases may be inorganic or organic and play an important role in the cycling of S through the atmosphere. Many different fungi and heterotrophic bacteria are responsible for the formation of these volatile compounds during the metabolism of organic sulfur compounds. Following are few examples of volatile sulfur comounds: Hydrogen Sulfide (H2S), Methyl mercaptan(CH3SH), Dimethyl sulfide (CH3SCH3), Dimethyl disulfide (CH3SSCH3), Carbon disulfide (CS2), Carbonyl sulfide (COS)
The most potent of the greenhouse gases is methane, 10 trillion tons of which is produced by microorganisms buried in the ocean floor. However, very little of the methane escapes to the atmosphere. Untill recently, it was not understood what happens to the methane. It was hypothesized that bacteria assimilated methane, but most known methane-eating bacteria are aerobic, which obviously cannot survive deep in the ocean floor. However, it is known that sulfur-reducing bacteria can function under these conditions.
Recently scientists recognized that sulfate disappeared at the same place in the sediment as methane. Investigations revealed that methanotrophic archaea lived in association with sulfate-reducing bacteria in the sediment layer. It is hypothesized that the consortia carry out the following reaction:
CH4 + SO42- HCO3 + HS- + H2O
This biochemical symbiosis results in a highly efficient method of transferring intermediates, allowing microorganisms to consume methane that would otherwise cause heating of the earth that would make unlivable for life as we know it.
Syed Shabbar Hussain Shah
B.Sc.(hons) Agri. Soil Science 7th Semester
Institute of Soil and Environmental Sciences
University of Agriculture Faisalabad
Fahim Ali Jawad
B.Sc(hons)Agri. Biotechnology 7th Smester
CABB,University of Agriculture Faisalabad