Sulfur Cycle: Major Pools Lithosphere holds largest amounts

Sulfur Cycle: Major Pools Lithosphere holds largest amounts

Sulfur Cycle: Major Pools Lithosphere holds largest amounts of Sulfur In terestrial enironments, SOM holds the greatest amounts of S Global S cycle Reservoir units: teragrams (1012 g) S; Flux units,

teragrams S/yr. Terrestrial S pools and transformations Physical Weathering release of sulfides (HS-) or sulfates (SO4-3) from minerals Biological transformations: aerobic sulfur-oxidizing bacteria sulfides are converted to sulfate

(SO4-2) sulfate is assimilated by plants and microbes anaerobic sulfate-reducing converted bacteria; sulfate to sulfides

aerobic or anaerobic Mineralization of organic S, release as either HS- or SO4-2 Volatile organic S compounds Assimilation biomass of mineral

S into S oxidation states S is highly redox active, used in energy generation as both e- donor and e- acceptor Microbial S transformations

S utilization by microbes Sulfur-oxidizing bacteria 5 groups of sulfur-oxidizing bacteria: anoxygenic phototrophs (e.g. Green and Purple Sulfur bacteria) morphologically conspicuous colorless sulfur bacteria (e.g. Thiospira), obligate autotrophic colorless sulfur bacteria (e.g. Thiobacillus), facultatively autotrophic colorless sulfur bacteria (e.g. Thiobacillus), sulfur dependent archaea (e.g. Thermococcus).

Species of Thiobacillus and substrates Sulfur oxidation S oxidation mediated by compounds as e- donors chemolithotrophs using reduced

Model reaction mediated by Thiobacillus thiooxidans is: HS- + O2 ---> SO4-2 + H+ G'o = - 46 kJ Microbes environment may be pH < 2 HS- oxidation can occur under anaerobic conditions. Thiobacillus denitrificans facultative anaerobe and couple S oxidation to respiratory denitrifcation. HS- oxidation is not mediated by oxygenases as is CH 4 and NH4 oxidation. S

Acidification accompanying S oxidation Environmental effects of S oxidation: Acid Mine Drainage Mine spoils, excavation material stockpiled

as wastes from mine Minerals in spoil piles often contain pyrite (FeS2)

Exposure to air accelerates S-oxidation; chemical and biological. Rain water leaching through piles is acidified (pH to <1), solubilizes metals Use of S-oxidizers in biocontrol Streptomyces scabies causative agent of potato scab disease S. scabies prefers neutral to slightly alkaline conditions, acid produced (e.g., pH 5) from S oxidation

inhibits growth of S. scabies but not the potato plants Dissimilatory Sulfate & S reduction Obligate anaerobes that use either H2 or organics as e- donors and S oxyanions or S as e- acceptors produce sulfides. Catabolism (energy metabolism) is shown in black; anabolism (cell synthesis) is shown in red

S oxidation states Reduction of sulfate or sulfide occurs via a number of intermediates. Unlike nitrate-reducing bacteria, S-reducers usually do not release intermediate oxidation states, but only the final product sulfide Diversity of Sulfate & S reducers Sulfate- or sulfurreducing microorganisms

are longestablished functional groups. They are not necessarily coherent from the viewpoint of modern molecular systematics Phylogenetic trees reflecting the relationships of groups of sulfate-reducing bacteria to other organisms on the basis of 16S rRNA sequences. (A) Overview showing the three domains of life:

(1), Eubacteria; (2), Archaebacteria; (3), Eukaryotes. Characteristics of sulfate-reducing bacteria (SRB) Bacteria and Archaea Differ in use of SO3, S2O3 as TEA e- donors coupled to S reduct. Bacterial genera identified by

the prefix "Desulfo SRB-Methanogen Competition/Syntrophism SRBs use the same e- donors as methanogens (H2, acetate) and by coupling these to sulfate reduction obtain higher energy yields than methanogens. chemolithotrophic growth H2 + SO4= + ---> HS- + H2O chemoorganotrophic growth lactate + SO4= ---> acetate + CO2 + H2O + HSHigh sulfate environments: SRB compete with may dominant over

methanogens. Low or no sulfate environments, SRB grow syntrophically with methanogens as may proton-reducers (producing H2 for interspecies H2 transfer) Interspecies H2 transfer methanogens are physiologically linked with proton-reducers. An example reaction mediated by the latter organisms is: acetate + H2O ---> CO2 + H2

G'o = +111 kJ high positive G'o makes this reaction unfavorable for supporting growth H2 produced is rapidly consumed at kept at a very low level the energetics become favorable. Methanogens consume H2 and making the reaction energetically favorable. This is an example of syntrophism SRB transformations of metals and chloroaromatics Metals

Reduce: Fe+3 (no growth) U+6 (no growth) Cr+6 (no growth) As+5 (growth) Methylate Hg Chloroaromatics Reductive dehalogenation of chlorobenzoates(growth)

Trace S gases in the atmosphere Low levels of S in atmosphere Most sulfur gases are rapidly returned (within days) to the land in rain and dry deposition Types and sources of S gases also SO4-2 MM DMS

DMDS Volatile S: terrestrial sources and sinks volatile S compounds produced by heterotrophic microbes during aerobic or anaerobic decomposition of S-containing compounds . Volatile S from aerobic and anaerobic decomposotion of S proteins Zein: a mixture of water insoluble

proteins that constitute about half of the protein in corn or 4-5 weight % of the corn. Gluten is composed of storage proteins, the prolamins, comprising monomeric gliadins and polymeric glutenins. Major products: MM, DMS, DMDS Phosphorus: Pools and cycles

Phosphorus cycle. Reservoir units, teragrams (1012 g) P; flux units, teragrams P/yr P in Terrestrial Environments P released into the mineral pool as phosphate Phosphate assimilated into/released from biomass without reduction or oxidation. Like nitrogen and sulfur, SOM contains the greatest amount of P (30-50% of the total) Mineral forms of P All known phosphate minerals are orthophosphates [anionic group is

PO43-] Over 150 species of phosphate minerals: Small amounts of mineral P in soil (ca. 1% of total) Organic forms of P SOM contains the greatest amount of P (30-50% of the total) The chemical nature of much of the organic P is unknown Up to 50% may be inostitol hexaphosphate

(phytic acid) P cycle lacks gaseous form Reduced forms of phosphorus: Phosphite, hypophosphite occur in bacteria; functions unknown Phosphine is produced natural environments, reacts rapidly in atmosphere, short half life (ca. 5-24 h) Trace amounts of P in atmosphere, predominant movement is from terrestrial environments to streams, lakes and oceans.

Fe and Mn Cycles Cycling revolves around the transition from oxidized insoluble forms (Fe+3 / Mn+4 )to reduced, soluble oxidation states (Fe+2/Mn+2) Fe Oxidation Ferrous iron (Fe+2) used as an electron donor linked with oxygen reduction High levels of Fe+2 are needed But: aerobic conditions, neutral pH iron is essentially all solid Fe +3

oxides Two adaptations for use of Fe+2 : Low pH and/or low O2 Fe Oxidation at low pH The pH effect on Fe+2 concentrations is reflected in the energy yield: Fe+2 + O2 + H+ ---> Fe+3 + H2O G'o (pH 7) = - 0.25 kJ Go (pH 0) = - 2.54 kJ Thiobacillus ferrooxidans, an acidophilic iron-oxidizer, pH optimum for growth of 2 to 3 Contribute to formation of acid mine drainage.

Thiobacillus-type [rods] in yellow floc from acid water Fe Oxidation at Neutral-Alkaline pH, Low Oxygen Levels Neutral-alkaline pH, Fe+2 concentrations increase with decreasing oxygen concentration. The "iron bacteria" (e.g., Gallionella, Leptothrix, Siderocapsa) have adapted to grow by oxidizing Fe+2 at low O2 concentrations (0.1 - 0.2 mg L-1). Low energy yields, microbes must oxidize large amounts of Fe+2 to sustain

growth. Small populations of iron bacteria generate a lot of Fe+3. Problem for the well water industry as the resulting FeOOH (hydroxyoxides) precipitates may clog wells. Light Micrographs of Iron bacteria Gallionella ferruginea [braidlike] in red floc from neutral water

Leptothrix cholodnii [sausagelike] in red floc from neutral water Leptothrix discophora fresh rounded holdfasts [doughnut-like], which are parts of the bacteria that attach to rocks or microscope slides, like those here on microscope slide left in neutral water riffle TEM Micrographs of Iron bacteria &iron deposits

Low magnification image of Gallionella and Leptothrix stalks and sheaths Gallionella stalk coated with nanometer-scale Fe(OH)3 and FeOOH aggregates

Light Micrographs: Iron bacteria and iron deposits Gallionella Note the twisted strands of iron oxide characteristic of this organism Wet mount, 400 X Gallionella Stained with crystal violet, 1000 X

Fe/Mn reduction: Biogeochemical Significance Fe(III) and Mn(IV) reduction affects cycling of iron and manganese fate of a variety of other trace metals and nutrients degradation of organic matter Fe(III)-reducers can outcompete sulfate-reducing and

methanogenic microorganisms for electron donors can limit production of sulfides and methane in submerged soils, aquatic sediments, and the subsurface IRB may be useful agents for the bioremediation of environments contaminated with organic and/or metal pollutants Fe/Mn-reducing microbes (FMRM) A wide phylogenetic diversity of microorganisms, (archaea and bacteria), are capable of dissimilatory Fe(III) reduction. Most microorganisms that reduce Fe(III) also can transfer

electrons to Mn(IV), reducing it to Mn(II). Two major groups, those that support growth by conserving energy from electron transfer to Fe(III) and Mn(IV) and those that do not. FMRM that Conserve Energy to Support Growth from Fe(III) and Mn(IV) Reduction Phylogenetically diverse Most cultured FMR are in the family Geobacteraceae in the delta -Proteobacteria:Proteobacteria:

Geobacter, Desulfuromonas , Desulfuromusa and Pelobacter Acetate is a primary electron donor Most Geobacteraceae also can use hydrogen Phylogenetic Diversity of Fe/Mn reducers Phylogenetic tree, based on 16S rDNA sequences, of

microorganisms known to conserve energy to support growth from Fe(III) reduction Microbes that conserve energy from Fe/Mn

reduction Micrographs of Fe/Mn-reducers Phase contrast micrographs of various organisms that conserve energy to support growth from Fe(III) reduction. Bar equals 5 mm, all micrographs at equivalent

magnification Pathways for electron donor production and use in Fe /Mn reduction Mechanisms for Electron Transfer to Fe(III) and Mn(IV) Mechanism(s) of electron transfer to insoluble Fe(III) and Mn(IV) are poorly understood. Possibilities include : Direct contact with and reduction of Fe(III) and Mn(IV) oxides Solubilization of Fe(III) and Mn(IV) oxides by chelators,

reduction of solubilized spec ies Indirect reduction mediated by extracellular electron shuttles (quinone groups in humics)

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