Introduction
Sulfur is an element that is found abundantly on earth. Sulfur can be found both as an elemental form and as a compound of sulfur. Sulfur is naturally present and additionally, it is also present in coal, crude oil, natural gas, oil shale and many other minerals. The most abundant of all sulfur is a compound of sulfur and iron called pyrite. Sulfur exists in various forms such as elemental sulfur, sulfides, sulfites, sulfates, oxides of sulfur, etc. However, the elemental form of sulfur, sulfides, and sulfates are the most common. Since the element is important and life support, they are involved in various bio-geo-chemical cycles.
Researchers who have carried out their studies on the sulfur cycle suggest that some of the earliest forms of life derived energy from the metabolism of sulfur compounds (Stetter, 1996). In fact, the study of the evolution of organisms helped to understand the basic metabolic pathway which in turn has brought about changes in the surface chemistry of this planet. Besides, it is also responsible for creating new metabolic pathways. One of the best examples to quote here is the discovery of anaerobic photosynthesis that leads to sulfate that is required for the evolution of sulfate-reducing bacteria (Schidlowski, 1989).
Studies have provided enough proof that during the pre-historic times there were well-defined sulfur cycles wherein the reduced sulfur gases were received from the mantle of the earth. The ocean crust and terrestrial volcanoes with the thermophilic and hyperthermophilic environment are good examples of such sources. In these environments, anaerobic photosynthesis needs to occur in the presence of light. On the other hand, when there is no light source the production of organic matter took place by chemolithoautotrophic metabolisms with the use of an electron donor that is H2. In this process, the elemental sulfur that is converted into H2S or carbon dioxide that is converted to form methane is the electron acceptor. Chemoheterotrophic metabolisms took place when the organic compounds were oxidized. In this process the reduction of elemental sulfur and sulfate together with the fermentation of acetate, producing methane took place.
When H2S and SO2 reacted, it resulted in the formation of sulfur in the element form and anaerobic photosynthesis was the major source of sulfate. It was through aerobic or oxygenic photosynthesis that the production of organic material increased and there was a proliferation of prokaryotic life. Basically, there was an expansion of the ecosystem that benefited the evolution of new bacterial life that included the bacteria in the sulfur cycle. It is only in the late Proterozoic era that there was an increase in the levels of oxygen that increased the formation of a number of non-photosynthetic, sulfide-oxidizing bacteria.
The Sulfur Cycle
Sulfur is linked with both sedimentary and gaseous cycles. While the gaseous cycle is a short-term cycle, the sedimentary cycle is long-term. Additionally, the sulfur cycle in the sedimentary phase is linked with the deposition of both organic and inorganic matter. As a result during the weathering and decomposition processes, the sulfur is released from the deposits which are then transferred or carried to the surrounding land and the aquatic system. When sulfur enters the gaseous phase of a cycle there is widespread circulation. This is the reason why during the combustion of fossil fuels, volcanic eruptions, the surface of the oceans, and gases released by decomposition enter the atmosphere. For instance, the terrestrial volcanic fields such as those found in the North Island of New Zealand, Yellowstone Park, and Iceland are major ecosystems sustaining elemental sulfur reduction (Canfield and Raiswell, 1999).
Several chemical reactions are involved in the sulfur cycle. For instance, sulfate reduction to form sulfide and followed by oxidation is an important step in this cycle. The amalgamation of sulfide into reduced inorganic and organic sulfur is another important step. There are also links between the inorganic and organic pools through sulfur compounds that form intermediate oxidation states. As mentioned earlier, sulfur results from both biotic and abiotic reactions. The following are some of the common laboratory reactions of organic sulfur that probably are of great environmental significance.
Reactions taking place during the formation of organic sulfur from inorganic sulfur
Oxidation reactions of reduced organic sulfur compounds
Note: For reactions l(b), 2(a) and 2(b), metal oxide phases present in sediments may be effective oxidants of reduced organic sulfur.
The sulfur cycle is a significant biogeochemical cycle that has a tremendous influence on the climate and life. Life depends on low levels of sulfur as a nutrient and is also an important part of proteins and amino acids. It is a life-supporting element that is essential for the basic survival of plants and animals, the soil is the main source of sulfur for plants and it is from the plants that these elements are passed on to the animals in the higher food chain. Plants including cabbage, onion, and soybean flour have high levels of sulfur. Methionine an essential amino acid contains sulfur and is present in protein-rich foods such as eggs, dairy products and meats.
The sulfur cycle begins with hydrogen sulfide (H2S) undergoing oxidation to form sulfur dioxide (SO2). Once this step is complete the SO2 that is present in the atmosphere is dissolved in the water and is brought back to the surface of the earth as rainwater. This rain is also called acid rain. When sulfur is in a solution with water it is mostly in the ionic form of sulfate (SO42-) ion, which is taken up by the plant roots easily. Plants then take up these ions and form their tissues. The major source of amino acids from the plants is taken up by the animals. When the animals excrete the excess sulfur as fasces it is again transferred to the land or the nearby water bodies. Additionally, organic sulfur from plants and animals is also transferred to the nearby environment through death and decay. The sulfur cycle has a major dependency on the bacteria as they play an important role in the conversion of organic sulfur into hydrogen sulfide in a gaseous form. H2S is further oxidized in an aerobic environment to the ionic form of SO42- by the action of other bacteria which is adapted for this purpose. Hence in this way, the cycle keeps continuing (Figure 1). One of the important reactions is as follows:
Bacteria take part in the process of anaerobic photosynthesis. This can be easily proven as both the Green non-sulfur bacteria and the Green sulfur bacteria are involved in anaerobic photosynthesis from prehistoric times (Woese, 1987). Additionally, studies have shown that in the sedimentary portion of the lakes there exists an anaerobic environment wherein these bacteria are not able to perform the task of oxidizing sulfides. However, this trend changes in the presence of UV radiation in these environments. Photosynthetic bacteria utilize sulfide H2S to produce carbohydrates and oxidize H2S to sulfur or to sulfate. One of the significant geological consequences of anaerobic photosynthesis is the production of sulfate. This sulfate undergoes further reactions and can be of great significance as an electron acceptor in the process of re-mineralization of organic carbon by sulfate-reducing bacteria (Schidlowski, 1989). The following are the chemical reactions that occur during this process:
There are also several other bacteria that are involved in the conversion of elemental sulfur to sulfate. It is important to note that an aerobic condition is essential for rapid conversion.
Studies have shown that there exist specific phytoplanktons in the oceans that can produce a chemical that transforms to SO2 in the atmosphere. Further, these gases continuously undergo these transformations in the air, water, and soil, and the sulfur cycle continues. It is also found that in the presence of nitrate the elemental sulfur is oxidized to sulfate. However, for this reaction to occur the precondition is the presence of an anaerobic condition together with the bacteria that helps in this reaction.
Significance of Sulfur Cycle
There are several significant roles that sulfur play. Sulfur forms an important part of the amino acids that form essential proteins. Sulfur in the atmosphere is responsible for the formation of acid rains. The oxidised state of sulfur is responsible for this. In fact, it can be said that this is one of the most important links that makes sulfur significant to the geochemical, atmospheric, and biological processes. There are several natural processes such as weathering of rocks, acid rain, and the process of denitrification, etc. depends on the presence of sulfur.
There are also chemical reactions that occur in the sedimentary regions. For instance, when sulfur is removed from the organic phase as elemental sulfur it becomes insoluble and accumulates in sediments. Further, in the presence of iron, sulfide may combine with it to form FeS, all of which are insoluble. The following reactions occur:
Researchers have found sulfur and also its role in various other unique environments. For instance, pyrite forms an important storage area for sulfur and also other trace metals in salt marsh sediments and also in marine sediments (Lord and Church, 1983). Another example is in the wetlands where sulfur is an important energy component (Howarth, 1984). The sulfur cycle also plays an important role in salt marshes. These environments are also systems to aid in the basic understanding of the cycle in the marine environment.
Figure 2 illustrates the sulfur cycle and its processes that can occur in an estuarine ecosystem. The high sulfate concentrations particularly from the sediments of the ocean and high sulfate reduction rates are the reasons for this (Howarth, 1984). The process of pyritization takes place with the reaction of hydrogen sulfide with detrital iron minerals (Canfield, 1989). FeS2 is insoluble under neutral and alkaline conditions and is tightly under arrest in mud or wet soil. It is also found that there is some FeSO4 in the sedimentary rocks overlying coal deposits. When FeSO4 is out in the open, it produces oxidizes and ferric sulfate in presence of water.
Therefore, it is said that sulfur in pyrite rocks when undergoes sudden weathering discharges high amounts of sulfur, sulfuric acid, ferric sulfate, and ferrous hydroxide into marine ecosystems. This results in the formation of an acidic environment in the sea and is harmful to most aquatic life.
Human Interferences on Sulfur Cycle
Anthropogenic activities are a major source of sulfur in the atmosphere. One of the main sources of compounds of sulfur in the atmosphere is the burning of fossils, industrial activates, and also natural disasters such as volcanic eruptions. However, it is anthropogenic activities that cause serious concerns to the environmentalists. Studies suggest that the emissions of sulfur in recent years are of such high magnitude that it is much more than the release from natural processes. Sulfuric acid particles are a major source of polluting smog that is a nuisance in urban areas and industrialized cities (Figure 3). The sulfate aerosols that are a result of this process cause various health problems such as respiratory diseases.
Atmospheric sulfur has a very short life span on the other hand the sedimentary sulfur cycle has a very long-life span. For instance, the processes such as erosion, sedimentation, and uplift of rocks containing sulfur take several millions of years. When sulfur compounds are released from volcanic eruptions into the atmosphere, there is a sudden increase in the concentration of sulfur in the atmosphere. However, industrial activities put a burden on sulfur in a continuous manner that causes serious health as well as environmental hazards. In fact, these compounds mix with water vapor and form sulfuric acid smog which in turn causes acid rain destroying plants, animals, and some of the most significant structures of the world like the Taj Mahal in India. Additionally, the sulfuric acid that is present in the atmosphere during smog cause irritation of the eyes. It is also a cause for the reflection of solar radiation. As a result of the reflection of solar radiation, the earth’s surface has a cooling effect.
Conclusion
Though in prehistoric times sulfur was considered a source of energy for many organisms, today it is a serious concern. Several problems still need further research to find a solution. For instance, the sulfur cycle and acid rain and smog are linked and is a serious problem in many parts of the world. However, that natural sulfur cycle involving the bacteria is of great significance when it is talked about the concentration of sulfur in the environment. Sulfur forms a main part of the structure of proteins and vitamins and play important role in their proper functioning.
Numerous researchers have worked on the element sulfur and the sulfur cycle in various environments. Still, there is a lot more in this field that needs to be explored. It is therefore essential that the research is encouraged to enhance the knowledge of sulfur chemistry in natural ecosystems. Another reason for the lack of knowledge on this subject is said to be the presence of a range of compounds of sulfur both in the oxidized as well as the reduced form. It is, therefore, important that intense research can only help us to overcome the problems caused by sulfur.
References
Canfield D E, and Raiswell R, (1999), The evolution of the sulfur cycle: American Journal of Science, 299, 1999, P. 697–723.
Canfield E. (1989). Reactive iron in marine sediments. Geochim. Cosmochim. Acta, 53, 619-32.
Howarth R W. (1984). The ecological significance of sulfur in the energy dynamics of salt marsh and coastal marine sediments. Biogeochemistry, 1, 5-27.
Lord C J. and Church T M. (1983). The geochemistry of salt marshes: sedimentary ion diffusion, sulphate reduction, and pyritization. Geochim. Cosmochim. Acta, 47, 1381-1391.
Schidlowski M, (1989), Evolution of The Sulphur Cycle in The Precambrian, in Brimblecombe, P. and A. Yu Lein, editors, Evolution of the global biogeochemical sulphur cycle: New York, John Wiley & Sons, p. 3–19.
Stetter K O, (1996), Hyperthermophiles in the History of Life, in Bock, G. R., and Goode, J. A., editors, Evolution of Hydrothermal Ecosystems on Earth (and Mars?): New York, Wiley and Sons, p. 1–10.
Woese C R, (1987), Bacterial evolution: Microbiological Review, 51, p. 221–271.