Sulfur Cycle

P A Loka Bharathi, National Institute of Oceanography, Panaji, India © 2008 Elsevier B.V. All rights reserved.

Sulfur Cycle Fluxes of the Global Biogeochemical Sulfur Cycle

Sulfur Oxidation Summary

Sulfate Reduction Further Reading

Sulfur Cycle

Most elemental cycles are operative in both oxidative and reductive mode, each fueling the other, either in a dynamic instantaneous manner in space or sequentially over time. Sulfur and its species are important geochem-ical agents. While the element sulfur is the fourteenth most abundant element on Earth, sulfate ion is the second most abundant ion next only to chloride in seawater and carbonate in freshwater. Elemental sulfur is produced hydrothermally and also by oxidation of sulfide by weathering. The element is also formed as an intermediate of sulfide oxidation or sulfate reduction. Sulfides exist in a variety of forms, most of which are solids. However, dissolved sulfide can occur as bisulfide (HS—) at neutral pH, sulfide ions (S2—) at alkaline pH, and H2S at acidic pH, which is volatile and has a rotten egg smell.

Sulfur transformations govern the compositions of the oceans, and the redox balance on the Earth's surface. It is complex due to a variety of oxidation states. Besides, some transformations occur at significant rates both bac-teriologically as well as chemically.

The sulfur cycle involves eight electron oxidation/ reduction reactions between the most reduced H2S (—2) to the most oxidized SO4- (+6). It acts as either electron donor or acceptor in many bacterially mediated reactions.

The oxidation states of key sulfur compounds are given in the following table:

Organic

S (R-SH)

-2

Sulfide

(H2S)

-2

Elemental S

(S0)

0

Thiosulfate

(S2O32 - )

+2 (av./S)

Tetrathionate

(S4O62 - )

+2 (av./S)

Sulfur dioxide

(SO2)

+4

Sulfite

(SO32 - )

+4

Sulfur trioxide

(SO3)

+6

Sulfate

(SO42 - )

+8

Bacteria can mediate these oxidations. While bacterial oxidation of sulfur at the expense of oxygen or nitrate generally leads to chemosynthetic carbon fixation, the reductive S cycle is respiratory.

Sulfur Oxidation

Although a variety of oxidation states exist, only three forms are important, namely the sulfhydryl and elemental form besides the sulfate radical.

Reduced sulfur compounds can be used either by colorless sulfur bacteria or the colored photosynthetic bacteria. These bacteria notably belong to @ purple bacteria group, namely the colored Chromatium sp. or the colorless Thiobacillus sp. The others include Thiosphaera, Thiomicrospira, Thermothrix, Beggiatoa, and the archaean Sulfolobulus. The final product sulfate and different amounts of energy are available depending on the oxidation state of sulfur used as the electron donor:

H2S + 2O2 ! SO4- + 2H+ - 798.2 kJ HS- + I/2O2 + H+ ! S0 + H2O -209.4 kJ S0 + H2O + 1 I/2O2 ! SO4- + 2H+ -587.1 kJ S2O2- + H2O + 2O2 ! SO2- + 2H+ -822.6kJ

Those forms that can oxidize sulfur under acid conditions are also able to oxidize iron. Yet others grow at neutral pH.

The oxidation of sulfur involves the reaction of sulf-hydryl groups of the cell, like glutathione, with the formation of sulfide-sulfhydryl complex. The enzyme sulfide oxidase oxidizes the sulfide to sulfite.

Chemosynthetic Sulfur Oxidation

The chemotrophic pathways are involved in the oxidation of reduced sulfur compounds, H2S, S, and S2O^-. The oxidation of S2O3- to tetrathionates S4O2-, trithionate

S3O6-, pentathionate S5O6-, and elemental S0 depends on environmental factors like oxygen and pH. The trithionate and pentathionate are formed from tetrathionate with sulfite and thiosulfite, respectively:

The chemotrophic bacteria have to compete with the spontaneous oxidation of sulfide. However, this chemical oxidation rates are speeded up by bacterial intervention -for example, bacterial oxidation of sulfide ores by thioba-cilli. Thiobacilli-like bacteria play an important role in thiosulfate oxidation and sulfide oxidation, sometimes at the expense of nitrate as in Thiobacillus denitrificans:

Sulfide oxidation coupled to nitrate reduction could be an important process in some coastal ecosystems where sediment-produced sulfide encounters nitrate-rich overlying waters that have been depleted in oxygen.

In other ecosystems like deep-sea hydrothermal vents, sulfur/sulfide oxidation is one of the main processes that bacteria utilize for chemosynthetic production of organic matter. Gigantic tubeworms in and around the vent fields harbor symbiotic sulfide-oxidizing bacteria. Special hemoglobins that bind H2S as well as O2 transport both substrates to the trophosome where they are released to the bacterial symbiont, thus preventing sulfide poisoning of the host.

Sulfide concentration in some vents measured as the sum of H2S, HS", and S2 can be related to prevailing temperature, but where biological uptake is rapid, the relationship can become nonlinear. The proportion of these species is strictly determined by pH of the surrounding. At the near-neutral pH from 7 to 7.9 of low-temperature fluids, HS" is the prevailing species. Sulfide exposed to oxygen could be inorganically oxidized with a half-life of c. 380 h at 2 0C, and pH 7.8 and 110 mm O2 and 10 mm H2S. Biological oxidation of sulfide by macroorgan-isms and microorganisms at Galapagos vents could be 4-5 orders of magnitude greater than spontaneous sulfide oxidation in the laboratory. Generally in nature, the transition zones where the anaerobic zone meet the aerobic, the sulfide-oxidizing bacteria can form a sumptuous source of food to protozoans, microzooplanktons, and other higher forms of life. The oxidation of reduced sulfur compounds generates organic compounds (CH2O) from inorganic substrates and is akin to primary production. In the marine microbial sulfur cycle, there is no net gain of organic material (Figure 1). This is because organic material must be oxidized to generate the sulfide that is required for chemosynthetic production of organic carbon. However, at geothermal vents sulfide is released from the geochemical interaction of seawater and hot rock deep within the Earth crust. Under these conditions there is a net gain of organic material through the oxidation of

Valence +6

Sulfur Cycle

Ht o

Figure 1 The microbial sulfur cycle. Modified from Fenchel T and Blackburn TH (1979) Bacteria and Mineral Cycling. London: Academic Press.

Ht o

org S

V J

FeS

v y

Figure 1 The microbial sulfur cycle. Modified from Fenchel T and Blackburn TH (1979) Bacteria and Mineral Cycling. London: Academic Press.

sulfide and production of new biomass. The actual biochemical transformations are complex with light or chemical energy used to generate reducing power like NADH that is coupled to CO2 fixation generally through Calvin-Benson cycle. Though chemosynthesis was described more than a century ago by Winogradsky, it was only with the discovery of hydrothermal vents that its significant quantitative role got established.

Geochemical Implications of Sulfur Oxidation

The sulfur-oxidizing microbes act as competitors and sinks for inorganic sulfur along with other reduced compounds as producers of organic biomass as food for zooplankton and a variety of benthic organisms.

Just as dissolved sulfides support chemosynthetic production, particulate sulfides too can support autotrophic growth. Metal sulfides, including pyrite, FeS2, pyrrhotite, Fe5S6-Fe16S17, chalcopyrite, CuFeS2, and sphalerite, ZnS, form widespread active and relict hydrothermal sites. Though sulfide oxidation of metal sulfides generally takes place at low pH, nonacidophiles from vent's sulfides are capable of autotrophic growth. Thus massive sulfide deposits on the seafloor may serve as potential source of electrons for autotrophic growth even when the vents are extinct. Such systems are also known to support high biomass of invertebrates.

Chemosynthetic systems in extreme environments like hydrothermal vents are likened to extreme environment systems on other planets or other extraterrestrial bodies. The search is on for chemoautotrophic forms on Mars. Such systems are also suspected to occur on Europa, a moon of Jupiter. They could represent the type of none-quilibrium systems which are thought to have been important in the origin of life on our planet.

Applications

These microbes evoke deep scientific interest because of their metabolic diversity and their adaptability to extreme environments. These characteristics can be gainfully harnessed by biotechnological firms for their enzymes or other metabolic products.

Sulfur-oxidizing bacteria could be judiciously used to contain 'crude oil souring' due to excess sulfide production in oil wells.

Sulfur Bacteria in Symbiosis

The association of thioautotrophic (autotrophic at the expense of reduced sulfur compounds) symbiotic bacteria could be very specific. Most of them are related to subdivision of Gamma proteobacteria. Mariculture of symbiont-bearing invertebrate bivalves has also been suggested as a means of treating industrial sulfur waste.

Photosynthetic Sulfur Oxidation

Anoxygenic photosynthesis is also responsible for converting reduced sulfur to sulfate, thus forming habitats called 'sulfureta'. The purple sulfur bacteria like Chromatium can oxidize sulfide internally through elemental sulfur to sulfate. The green sulfur bacteria do so externally. Consequently, the former are less tolerant to sulfide (—0.8 to 4 M) as compared to the latter (4-8 mM). The purple nonsulfur bacteria are least tolerant (0.4-2 mM).

When an anoxygenic bacteria grows on CO2 as sole source of carbon, besides the formation of ATP, reducing-power NADPH must also be made available so that CO2 can be reduced to form cell material. The source of reducing power is not water as in photosynthetic plants but reduced sulfur compounds like H2S, S°, or thiosulfate. Hydrogen or organic compounds, such as lactate, succi-nate, butyrate, and malate, can also donate electrons for the activity:

Habitats in which oxygenic photosynthesis is important is relatively limited in distribution and is therefore restricted to shallow coastal sediments or coral lagoon or sediments on beaches. These bacteria are at the end of anaerobic food chain that operates within microbial level. Heterotrophs feed sulfate-reducing bacteria (SRBs) and SRBs in turn feed photosynthetic bacteria, which can act as food sources for protozoans or microzooplankton.

Mixotrophy

Sulfur oxidation is carried out not only by chemolitho-trophs but also by other groups like (1) mixotrophs (capable of autotrophic and heterotrophic growth); (2) che-molithotrophic heterotrophs; (3) heterotrophs which do not gain energy but derive benefits; (4) heterotrophs which gain nothing from the oxidation. Most pseudomo-nads are capable of growing mixotrophically on organic compound and reduced inorganic sulfur. Both marine and freshwater pseudomonads are capable of growing on thio-sulfate and oxidizing it to tetrathionate.

The metabolic capabilities of a microbe sometimes cannot be too specific. It is argued that many microbes could be facultative, autotrophic at times, and hetero-trophic at other, assimilating simple organic substrates that are available. Thus microbes like Pseudomonas sp. and Alcaligenes sp. have also been implicated in the hetero-trophic sulfide oxidation. They could also behave like Thiobacillus denitrificans-like organisms (TDLOs) oxidizing reduced sulfide at the expense of nitrate and fixing carbon dioxide in the process. Such metabolic flexibility increases their competitive edge.

Sulfate Reduction

Assimilatory Sulfate Reduction

Sulfate-reducing activity (SRA) that takes place for the incorporation of sulfide radical for biosynthetic cycle is referred to as assimilatory sulfate reduction (ASR). Sulfate is reduced to sulfide in the assimilatory cycle which combines with serine to form cysteine. This in turn can be converted to methionine. These two amino acids are the main constituents of sulfur-containing molecules in the cells. Sulfur content can vary from 0.3% in eel grass to 3.3% in marine algae. As the N:S ratio in land plants is only 30:1, the reductive assimilation of sulfate is less important than nitrate. Assimilatory reduction is common among organisms and does not lead to the production of sulfide.

The eight-electron reduction of sulfate to sulfide proceeds in different stages. As the ion is stable it needs to be activated with ATP. The enzyme ATP sulfurylase catalyzes the attachment of sulfate ion to phosphate of ATP to form adenosine phosphosulfate (APS). Another P is added to APS to form phosphoadenosine phosphosulfate (PAPS) before it gets reduced to form sulfite.

Dissimilatory Reduction

The SRA that takes place in anaerobic respiration is termed as dissimilatory sulfate reduction (DSR). Here sulfate is used as terminal electron acceptor leading to the production of sulfide. Here too the sulfate ion is activated by ATP to form APS. However, in this case, the sulfate moiety of APS is reduced directly to form sulfite with the release of AMP. Thus the first product of ASR and DSR is sulfite.

Dissimilatory SRBs act as agents of synergy in sulfur cycle and bring about syntrophic associations. The end products from organic substrate oxidation and sulfate reduction lead to the formation of sulfide and carbon dioxide. SRA can account for nearly 80% of organic carbon mineralization in marine environment, especially in coastal regimes where nearly 5 x 10 kgyr~ of sulfate gets reduced.

SRA follows zero-order kinetics in marine sediments with respect to sulfate up to a concentration of c. 2 mM . The rate of SRA depends on both quantity and type of organic matter and the sulfate ion available which is generally not limiting in the marine environment. There is in general 2:1 molar relationship between the labile carbon utilized and the sulfate reduced. Sulfate-reducing rates (SRRs) can span several orders of magnitude: 50-500nmcm~ day~ in coastal zones; 2785nmg~ day~ in salt pans; 4000 nm ml_ day~ in salt marshes; and up to 14 000nmml~ day~ in microbial mats.

The other environmental parameter that affects SRA is temperature. Though the rates of activity can vary by factors ranging from <5 in winter to >0.30 in summer in the temperate region, in the tropics it is not very marked.

SRA predominates in marine sediments but the accumulation of the end products of sulfate respiration is pH dependent. Both metal sulfide formation and rapid biological oxidation are responsible for controlling the amount of sulfide that eventually escapes any system. Sediments harboring vegetation tend to emit less sulfide due to rapid oxidation by oxygen emitted from roots.

Though SRA is largely anaerobic there have been observations of its occurrence in the surficial oxic layers of microbial mats. Sometimes the rate measured in these oxic layers can equal or exceed the SRA of the deeper anoxic layers.

Abundance, Physiological Groups, and Taxonomic Diversity

Though SRBs are high in activity, they are low in abundance contributing to a maximum of 5-6% of total counts of bacteria. While culturable forms retrieved as colony-forming units (CFUs) range from 102 to 10 in agar shake tubes, most probable numbers (MPNs) methods yield 106-108ml_1 and fluorescent in situ hybridization method (FISH) up to 107ml_1.

SRBs form two main groups based on their ability to utilize carbon sources completely or incompletely. In incomplete organic carbon oxidation, SRBs utilize a variety of organic substrates and oxidize it to acetate:

In complete organic carbon oxidation, the organic substrate is totally oxidized to carbon dioxide, water, and sulfide:

Though the above two groups of SRBs are physiologically distinct they can coexist with the latter using the metabolic end products of the former.

SRBs also fall in the following major groups, namely Gram-negative mesophilic, Gram-positive spore-forming thermophilic bacteria belonging to 6 subgroup of Proteobacteria and Archaea. The former includes two main families Desulfovibrionaceae and Desulfobacteriaceae. Desulfovibrionaceae includes the genera Desulfovibrio and Desulfomicrobium. Desulfobacteriaceae includes at least 20 genera, most of which are complete oxidizers of organic acids. The Gram-positive group comprises of Desulfotomaculum.

Desulfobacteriaceae are metabolically more versatile and are highly adapted to environments that undergo drastic redox changes as in salt marsh sediments or intertidal regions. Rhizosphere habitats are replete with Desulfobulbus species which are capable of sulfur disproportionation.

Sulfur-reducing activity

Bacteria like Desulfuromonas acetoxidans are capable of reducing sulfur at the expense of acetate. Some SRBs and iron-reducing bacteria are also capable of reducing sulfur. Many of these bacteria are able to generate ATP during sulfur reduction. These groups can also use organic disulfide molecules like cysteine or glutathione. Though sulfur and sulfate reducers can coexist, the latter can produce more sulfide. Most of these bacteria belong to Archaea. Methanogenic thermophilic Archaea reduce sulfur to sulfide while methane generation gets retarded. The process of sulfur reduction is an ancient process, as is suggestive from their presence in the deep branches of the phylogenetic tree. Though some sulfur reducers phylogenetically belong to 6 subclass of Proteobacteria, they show affinity to other unrelated classes as well. Metabolic flexibility assures ecological competitiveness.

Sulfur disproportionate

Inorganic fermentation or disproportionation of sulfur and sulfur compounds, thiosulfate, and sulfite has been frequently encountered in SRB:

S2O2- + H2O ! SO2- + HS- + H+ 4S° + 4H2O ! SO4- + 3HS- + 5H+

Disproportionation seems to be very important and therefore widespread. Thiosulfate is an important intermediate as it can act as an electron acceptor or donor and thus mediate both oxidative and reductive cycle. Thus the thiosulfate shunt provides for complete anaerobic sulfur cycling. Though this is not energetically very viable, the bacteria are able to grow in the presence of metal oxides which can scavenge sulfide.

Use of Heavy Isotopes in Ecology

Sulfur exists primarily as two stable forms of isotopes: 32S and to a certain extent as 34S. Heavier isotopes are discriminated against; that is, most biochemical reactions prefer the lighter isotope and this preference is useful in elucidating microbial interactions. Thus sulfide production by bacterial reduction is much lighter than sulfide of strictly chemical or geothermal origin. Also the biological oxidation of sulfide to sulfur or sulfate either aerobically or anaerobically shows a preference for the lighter isotope. However, this fraction is not as great as that occurring through sulfate reduction as respiratory rates are faster and higher than synthetic ones.

Geochemical Implications of SRBs and SRA

Though the abundance of SRBs is generally low, their high respiratory activity mediates many other activities. Sulfate reduction sets other geochemical reactions in pace. The sulfide formed is responsible for the precipitation of metal sulfides which are available for autotrophic sulfide-oxidizing bacteria. It has been argued that about 90% of the sulfide produced by SRA is recycled back to sulfate to complete the cycle. The rest gets buried to form FeS2. However, the energy gain from dissimilatory SRA is relatively low: at an energy yield AG' of —128 kJ with lactate and —48 kJ with acetate. Nevertheless, the sulfide produced by these bacteria can act as an energy source for other autotrophic bacteria.

The SRBs are also capable of using a variety of inorganic sulfur compounds as electron acceptors. These include dithionite, tetrathionite, thiosulfate, sulfite, bisulfite, metabisulfite, sulfur, sulfur dioxide, and dimethyl sulfoxide.

Sulfate reducers can use other electron acceptors like nitrate; group 4 oxyanions like molybdate and selenate; and even metals like uranium, chromium, technetium, gold, iron, and manganese(iv).

SRA Implications on Climate

SRBs can not only mediate synergistic reactions locally but also impact the climate on a wider scale. They are known to participate not only in the degradation of dimethylsulfoniopropionate (DMSP) but also in the flux of degradation product, dimethyl sulfide (DMS).

The SRBs are involved in the demethylation of DMSP to yield methylmercaptopropionate (MMPA), carbonate, and sulfide or oxidation of DMS to yield bicarbonate and sulfide.

DMSP demethylation

DMS oxidation

Intertidal sediments harbor algal osmolyte DMSP. DMSP could release DMS by the intervention of SRB. This could have countereffect on global warming. This reaction also decreases the effect of the potent greenhouse gas methane.

DMS emissions form an important bulk of sulfur that enters the atmosphere and affects the climate. It accounts for 90% of the biogenic sulfur emissions from the marine ecosystem. DMS produced from DMSP breakdown which reaches the atmosphere serves to decrease warming by radiative backscatter from aerosols. It also reflects radiation from increased cloud cover. Both methanogens and SRBs compete for DMS but methanogens outcompete SRBs when DMS concentrations are high.

Similarly, the breakdown of the osmoregulant glycine betaine in marine sediments releases acetate and tri-methylamine. The former is a preferred substrate for SRBs and the latter for methanogens. Other sulfur compounds like carbonyl sulfide (OCS) and carbon disulfide (CS2) species could be formed photochemically or biologically for bacterial consumption.

Thus, the gaseous products ofsulfur cycle interlink land, water, and atmosphere. These include hydrogen sulfide, DMS, methane thiol, carbonyl sulfide, and carbon disulfide. These volatile sulfur compounds get photochemically oxidized to produce acid rain or aerosol sulfate particles that decrease the incoming solar radiation and lead to cloud condensation nuclei. These processes influence the global radiative balance and consequently the climate.

Applications

The activity of SRB could be deleterious to all underground constructions because of their involvement in corrosion. The sulfide they produce is responsible for anodic corrosion, and their propensity to scavenge hydrogen generated in underwater metal structures could cause cathodic corrosion. However, some of these activities could be used in metal recovery from wastewater treatment as metal sulfides. The synergy existing between SRB and other microbes could be effectively used in bioremediation and ecosystem management. This trait could be exploited to contain mercury and other heavy metal contamination in water bodies.

Fluxes of the Global Biogeochemical Sulfur Cycle

The sulfur fluxes, both natural and anthropogenic, have been derived from various studies. A summary diagram of the global sulfur cycle with quantitative estimates of the sulfur fluxes is given in Figure 2. The numbers near the arrows designate the total sulfur flux in Tg S yr_1 for all compounds. The contributions from anthropogenic

The Sulfur Cycle
Sulfur flux with anthropogenic contributions in parenthesis

P1

Mining from lithosphere

P2

Fertilizers from soil

P3

Industrial sewage

P4

Anthropogenic sulfur to atmosphere

P5

Erosion

P6

Biogenic sulfur

P7

Volcanic

P8

Dust emission

P9

To land from atmospheric precipitation

P10

From river runoff

P11

Anthropogenic and natural flux from continent to oceans

P12

Biogenic H2S from shallow coastal sediments

P13

Marine sulfur from sea spray

P14

Marine sulfur to continents

P15

Ocean atmosphere to ocean

P16

Reduced sulfure mission from ocean

P17

Biomass from marine plants

P18

Mineralized sulfur from dead marine organisms

P19

Organic sulfur to sea bottom

P20

Sulfate oxidized from organic sulfur returns to sea

P21

Organic sulfur buried in marine sediment

P22

Sulfate buried in marine sediments

P23

Reduced sulfur buried in marine sediments

Figure 2 Fluxes of the global biogeochemical sulfur cycle. Modified from Ivanov MV (1981) Global biogeochemical sulfur cycle. In: G E Likens (ed.) Some Perspectives of the Major Biogeochemical Cycles, ch. 4. Chichester, UK: Wiley.

activities are indicated by numbers in parentheses. About 120 TgS are extracted annually by man from the lithosphere in fossil fuels and sulfur-containing raw materials for the chemical industry of which about 58% (70 TgS) gets emitted to the atmosphere. About half of the remaining 50TgS directly enters rivers through sewage and residual waters, and another part from fertilizers to agricultural land. Simultaneously, volcanic gases contribute markedly to the atmospheric sulfur cycle over continents amounting to 29Tgyr-1. The major transfer of sulfur from continents to the ocean by river runoff amounts to 224 Tg of which anthropogenic contribution is about 109 Tg. The total flux of various sulfur forms, that is, organic, sulfate, and pyrite from oceanic water to sediments and further to the lithosphere, amounts to 130Tgyr~\ Thus the estimates suggest that the anthropogenic sulfur fluxes to the atmosphere and hydrosphere have reached a level comparable with that of natural fluxes. The natural sulfur flux from the lithosphere, its main reservoir, is compensated by the reverse flux of sulfur compounds to the lithospheric sediments of the ocean. Further, there is also indication that by the end of this century the anthropogenic sulfur fluxes could notably increase all over the world.

All of the main reactions of the sulfur cycle involving living organisms are closely related to the carbon cycle. The amount of carbon involved in the fluxes of the sulfur cycle through biogenic processes varies depending on the type of organisms undertaking the metabolism of the sulfur compounds. In the processes of bacterial chemo-synthesis, which are characterized by low amounts of energy utilized for the CO2 assimilation, only relatively small amounts of carbon are transformed into organic matter. In anaerobic bacterial photoassimilation of CO2 where sulfur compounds are used as electron donors, the amounts of oxidized sulfur and assimilated carbon are comparable. In anaerobic sulfate reduction, 24 g of organic carbon is mineralized for each 32 g of reduced sulfate sulfur. Thus, in ecosystems with an advanced development of photoauotrophic bacteria and SRBs, both groups of microorganisms transform significant amounts of carbon compounds and, consequently, these organisms should be considered not only as participants in the sulfur cycle but also as active biogeochemical agents of the carbon cycle.

Summary

Microbes, especially bacteria, play an important role in oxidative and reductive cycle of sulfur. The oxidative part of the cycle is mediated by photosynthetic bacteria in the presence of light energy and chemosynthetic forms in the absence of light energy. At the end of the anaerobic food chain in bacteria they serve to purify the system of sulfide and other metabolic end products. In the process sulfur is returned to the system as sulfate. In transition zones from anaerobic to aerobic, photosynthetic bacteria can form a food source to protozoans and microzooplankton. Chemosynthetic sulfur-oxidizing bacteria are the dominant bacterial forms that support thriving ecosystems in hydrothermal vents. Scientists are seeking evidences from such extreme environment for similar life on other planetary bodies.

The reductive cycle on the other hand is mostly driven by the sulfate/sulfur-reducing bacteria which use sulfate as the electron acceptor in anaerobic respiration to produce sulfide. Their close association with other microbes can have profound geochemical influence. Their metabolic activity dictates the availability of trace metals to other forms of life. While sulfide gets precipitated, phosphate gets released into the systems. Nitrogen fixation by these anaerobes also adds to the nitrogen economy of the environment they inhabit. In sediments of continental shelves that hold the reserve of gas hydrates, these microbes can modulate the concentration of methane in such ecosystems. Most importantly, the interaction with DMSP, an osmolyte from phytoplankton, can have wideranging climatic implications.

The main reactions of the sulfur cycle involving living organisms are closely related to the carbon cycle. The amount of carbon involved in the fluxes of the sulfur cycle through biogenic processes varies depending on the type of organisms undertaking the metabolism of the sulfur compounds. The estimates suggest that the anthropogenic sulfur fluxes to the atmosphere and hydrosphere have reached a level comparable with that of natural fluxes. The natural sulfur flux from the lithosphere, its main reservoir, is compensated by the reverse flux of sulfur compounds to the lithospheric sediments of the ocean. Further, there is also indication that by the end of this century the anthropogenic sulfur fluxes could notably increase all over the world.

This is NIO Contribution No. 4296.

See also: Ammonification; Anthropospheric and Anthropogenic Impact on the Biosphere; Autotrophs; Bioaccumulation; Biodiversity; Biogeochemical Approaches to Environmental Risk Assessment; Biogeochemical Models; Biomagnification; Carbon Cycle; Classification and Regression Trees; Climate Change 1: Short-Term Dynamics; Constructed Wetlands, Subsurface Flow; Constructed Wetlands, Surface Flow; Coral Reefs; Decomposition and Mineralization; Denitrification; Ecological Health Indicators; Ecosystem Ecology; Ecotoxicology: The Focal Topics; Energy Flows in the Biosphere; Estuaries; Global Change Impacts on the Biosphere; Global Warming Potential and the Net Carbon Balance; Mangrove Wetlands; Matter and Matter Flows in the Biosphere; Methane in the Atmosphere;

Microbial Cycles; Microbial Ecological Processes: Aerobic/Anaerobic; Microbial Ecology; Microbial Models; Natural Wetlands; Nitrification; Nitrogen Cycle; Oxygen Cycle; Rhizosphere Ecology; Riparian Wetlands; Salinity; Salt Marshes; Soil Ecology.

Further Reading

Fenchel T and Blackburn TH (1979) Bacteria and Mineral Cycling.

London: Academic Press. Hines M (1996) Emission of sulfur gases from wetlands. In: Adams DD, Crill PM, and Seitzinger SP (eds.) Mitteilungen derIVL, Vol. 25:

Cycling of Reduced Gases in the Hydrosphere, pp. 153-161. Stuttgart: Science Publishers. IvanovMV (1981)Global biogeochemical sulfur cycle. In: Likens GE (ed.) Some Perspectives of the Major Biogeochemical Cycles, ch. 4. Chichester, UK: Wiley. Jorgensen BB (1988) Ecology of the sulfur cycle: Oxidative pathways in sediments. In: Cole JA and Ferguson SJ (eds.) The Nitrogen and Sulfur Cyles, pp. 31-63. Cambridge: Cambridge University Press. Loka Bharathi PA (2004) Synergy in sulfur cycle: The biogeochemical significance of sulfate reducing bacteria in syntrophic associations. In: Ramaiah NN (ed.) Marine Microbiology Facets and Opportunities, pp. 39-51. Panaji, India: National Institute of Oceanography. Madigan MT, Martinko JM, and Brock PJ (1997) Brock Biology of

Microorganisms, 8th edn. Upper Saddle River, NJ: Prentice-Hall. Van Dover CL (2000) The Ecology of Deep-Sea Hydrothermal Vents, Princeton, NJ: Princeton University Press, pp. 115-226.

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