Sulfur cycle
Sulfur cycle is the circulation of sulfur in various forms through nature. Sulfur occurs in all living matter as a component of certain amino acids. The sulfur reservoir is in the soil and sediments where it is locked in organic (coal, oil and peat) and inorganic deposits (pyrite rock and sulfur rock) in the form of Sulphate, sulphides and organic sulfur. It is released by weathering of rocks, erosional runoff and decomposition of organic matter and is carried to terrestrial and aquatic ecosystems. The sulfur cycle is mostly sedimentary except two of its compounds, hydrogen sulphide (H2S) and sulfur dioxide (SO2), which are gaseous components.
Sulfur is present in three forms in the biosphere, Elemental Sulphur, Inorganic Sulphur (sulphate in aerobic soil and as sulphide in anaerobic soil) and Organic Sulphur (amino acids and plants/animal residues). Sulfur is an essential part of all living matter because sulfur containing amino acids are always present in almost all kinds of proteins.
Sulfur cycle in brief, Sulfur-containing proteins are degraded into their constituent amino acids by the action of a variety of soil organisms. The sulfur of the amino acids is converted to hydrogen sulfide (H2S) by another series of soil microbes. In the presence of oxygen, H2S is converted to sulfur and then to sulfate by sulfur bacteria. Eventually the sulfate becomes H2S. Hydrogen sulfide rapidly oxidizes to gases that dissolve in water to form sulfurous and sulfuric acids. These compounds contribute in large part to the “acid rain” that can kill sensitive aquatic organisms and damage marble monuments and stone buildings.
Different processes involved in Sulfur Cycle
Sulfur disproportionation is an ecologically and technologically important part of the sulfur cycle. It is also termed dismutation or “inorganic fermentation” as one sulfur compound serves as electron donor and acceptor.
Assimilatory sulfate reduction – algae and many heterotrophic microorganisms assimilate sulfur in the form of sulfate. Since direct uptake as sulfide is not feasible due to high toxicity of H2S, the reduced sulfur is immediately reacted with an acceptor, serine to form cysteine.
Organosulfur decomposition or desulfuration of organic compounds in soils and sediments yield mercaptans and H2S.
1. Degradation of proteins through proteolysis liberates amino acids containing sulfur
2. Heterotrophic bacteria use sulfur containing amino acids and release H2S. For example, cysteine desulphurase enzyme will convert cysteine to pyruvic acid and H2S and NH3
3. Some bacteria such as Desulfotomaculum can reduce sulphates to H2S.
In marine environments, the major decomposition product of organosulfur is DMS or dimethyl sulfide which is formed from dimethylsulfoniopropionate (DMSP), which is used by bacterioplankton (floating bacteria) as a sulfur source for protein synthesis and H2S. DMS, Mercaptans and H2S escape to atmosphere and upon photo oxidations become sulfate.
Oxidative sulfur transformations
The H2S may get subjected to microbial oxidation under aerobic conditions or phototrophically oxidized under anaerobic conditions.
Chemolithotrophic microbes such as Beggiatoa, Thioploca, Thiothrix, etc oxidises H2S to Sulfur. Sulfur globules are deposited in the cells. In the absence of H2S, these globules are slowly oxidized further to sulfate.
Some species of Thiobacillus oxidise H2S and other reduced sulfur compounds and deposit elemental sulfur since they are not highly acid tolerant. Some other members of Thiobacillus produce sulfuric acid from the oxidation of elemental sulfur and other sulfur compounds under aerobic conditions. Thiobacillus denitrificans carry out sulfur oxidation under anaerobic conditions utilizing nitrate as terminal electron acceptor.
H2S is also phototrophically oxidized under anaerobic conditions by photosynthetic sulfur bacteria (Chromatiaceae, Chlorobiaceae, Ectothiorhodospiraceae, etc). Chromatiaceae store sulfur globules intracellularly, Chlorobiacea and Ectothiorhodospiraceae excrete sulfur globules. Some cyanobacteria also participate in the phototrophic oxidation of H2S.
Reductive sulfur transformations
Sulfite can be reduced to sulfide by a wide variety of microorganisms, including Alteromonas, Clostridium, Desulfovibrio and Desulfotomaculum.
Desulfuromonas acetoxidans grow using acetate under anaerobic conditions by reducing sulfur to H2S.
Extremely thermophilic anaerobic archaea such as Thermoproteus, Pyrobaculum, Pyrodictium, etc are capable of sulfur respiration with hydrogen gas.
H2 + S ---> H2S
Some species of Bacillus, Pseudomonas and Sacharomyces is also capable of releasing H2S from sulfate.
Some obligately anaerobic bacteria carry out dissimilatory sulfate reduction and they are known as sulfate reducers or sufidogens. Examples are Sulfate reducing bacteria such as Desulfovibrio, Desulfotomaculum, etc reduce sulfate to H2S.
The production of H2S have a marked effect on the population in a habitat. H2S is highly toxic to aerobic organisms since it reacts with heavy metal groups on cytochromes. It kills nematodes and other animal populations in water logged soils and destroys plant roots and can kill plants. H2S also have antimicrobial activity.
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Phosphorous Cycle
The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Phosphorous Cycle is a sedimentary biogeochemical cycle. Phosphorous reservoir is mostly in the form of rock deposits and is not present in gaseous form in the biosphere. Phosphorous is one of the key element of the biosphere. It is an essential component of the cell in DNA, RNA, ATP and phospholipids. Phosphorous is present in the environment as phosphates of Calcium and Iron which are usually insoluble.
The dead remains of the flora and fauna including microbes act as the source of phosphorous in the soil environment. Phosphate can serve as terminal electron acceptor in the absence of sulfate, nitrate and oxygen. The final product of phosphate reduction is Phosphine (PH3). Phosphine is a volatile and toxic gas and ignite in presence of oxygen and produce green glow. This is liberated from swamps, soils, and marine regions where there is extensive decomposition and ignites when exposed to air and can ignite methane and give rise to ghostly light phenomena.
Phosphate which enters the aquatic ecosystem becomes part of the marine sediment.
The microbial transformation of phosphorus involves primarily the transformation of phosphorus from simple orthophosphate to various more complex forms, including polyphosphates found in metachromatic granules.
The phosphorus cycle has two main steps
Mineralization: Conversion of Organic Phosphorus into Insoluble Inorganic Phosphates
Solubilization: Conversion of Insoluble Inorganic Phosphates into Soluble Inorganic Phosphates
Both these processes are operated by the soil microbial population.
Mineralisation occurs with the help of enzymes phosphatases. Phosphorous mineralization is mainly carried out by the microbial population. Many soil microorganisms produce enzymes that attack many of the organic phosphorus compounds in the soil and release inorganic phosphate. Organic phosphorous reach soil from dead remains of plants and animals and also from animals excretions such as urine and faecal matter. Some bacteria and fungi produce phytase which releases soluble inorganic phosphates from inositol hexaphosphate or phytic acid.
Solubilization - Bacterial population such as Bacillus, Micrococcus, Pseudomonas and fungi such as Aspergillus, Penicillium and Mycorrhiza are involved in the solubilisation of phosphorous. Presence of Phosphate solubilizing microbial population increases the phosphate assimilation in plants. The mechanism of phosphate Solubilization is mainly by production of organic acids. Nitrosomonas and Thiobacillus produce nitrous and sulfuric acids respectively and solubilize inorganic phosphates. Phosphorous Solubilization is affected by a number of factors such as temperature, pH, aeration, carbon and nitrogen source etc. In marine environment the availability of Phosphorous depends on the temperature of the surface water. Here the dissolved Phosphorous gets incorporated in the phytoplanktons.
Microbial activities may also immobilize phosphorous. Thus it become unavailable to biological community. The assimilation of phosphorous into cell constituents such as membranes remove phosphate from available pool. The Phosphorous present in sediments deep in ocean also is not biologically available. When phosphorus-containing compounds from marine organisms sink to the floor of the ocean, they form new sedimentary layers. Over long periods of time, these sedimentary rock may be moved from the ocean to the land by a geological process called uplift, this is a very slow process.
Phosphate concentration highly influences primary productivity. Addition of phosphates from detergents into lakes makes the lakes eutrophic, there will be algal and cyanobacterial blooms, increased organic matter in the water body and subsequent decomposition of the organic matter deplete the water body of oxygen which result in fish kills.
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