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Lithosphere holds largest amounts of Sulfur

In terrestrial environments, SOM holds the greatest amounts of S

Sulfur Cycle: Major Pools

Global S cycle

Reservoirunits:teragrams(1012 g) S;

Flux units,teragramsS/yr.

Physical Weatheringrelease of sulfides (HS-) or sulfates(SO4

-3) from minerals

Biological transformations:aerobicsulfur-oxidizing bacteriasulfides are converted to sulfate(SO4

-2) sulfate is assimilated byplants and microbes

anaerobicsulfate-reduc ing bacteria; sulfateconverted to sulfides

aerobic or anaerobicMineralization of organic S, release aseither HS- or SO4

-2

Volatile organic S compounds

Assimilation of mineral S intobiomass

Terrestrial S pools and transformations

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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

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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).

Sulfur-oxidizing bacteria

Species of Thiobacillus and substrates

Sulfur oxidation

S oxidation mediated by chemolithotrophs using reduced S compoundsas e- donors

Model reaction mediated by Thiobacillus thiooxidans is:

HS- + O2 ---> SO4-2 + H+ ΔG'o = - 46 kJ

Microbes’ environment may be pH < 2

HS- oxidation can occur under anaerobic conditions.

Thiobacillus denitrificans facultative anaerobe and couple S oxidationto respiratory denitrifcation.

HS- oxidation is not mediated by oxygenases as is CH4 and NH4oxidation.

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S oxidation pathway

Oxygen is not directlyinvolved in S oxidationpathway

Two pathways exist foroxidation of sulfite tosulfate

APS = adenosine 5-phosphosulfate

AMP = adenosine 5-monophosphate

Acidification accompanying S oxidation

Environmental effects of S oxidation: Acid Mine Drainage

Mine spoils, material stockpiled as wastes from mine excavation

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

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Streptomyces scabies causative agent of potatoscab disease

S. scabies prefers neutral to slightly alkalineconditions,

acid produced (e.g., pH 5) from S oxidation inhibitsgrowth of S. scabies but not the potato plants

Use of S-oxidizers in biocontrol

Obligate anaerobes that use either H2 or organics as e- donors and Soxyanions or S as e- acceptors produce sulfides.

Dissimilatory Sulfate & S reduction

Catabolism (energymetabolism) is shown in black;anabolism (cell synthesis) isshown 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 releaseintermediate oxidation states, but only the final product sulfide

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Diversity of Sulfate & S reducers

Phylogenetic trees reflecting the relationships of groups of sulfate-reducing bacteria to otherorganisms on the basis of 16S rRNA sequences. (A) Overview showing the three domains of life:(1), Eubacteria; (2), Archaebacteria; (3), Eukaryotes.

Sulfate- or sulfur-reducingmicroorganismsare long-establishedfunctional groups.They are notnecessarilycoherent from theviewpoint ofmodern molecularsystematics

Characteristics ofsulfate-reducing bacteria

(SRB)

Bacteria and Archaea

Differ in use of SO3, S2O3 as TEA e- donors coupled to S reduct.

Bacterial genera identified bythe prefix "Desulfo”

SRB-Methanogen Competition/Syntrophism

SRBs use the same e- donors as methanogens (H2, acetate) and bycoupling these to sulfate reduction obtain higher energy yields thanmethanogens.

chemolithotrophic growthH2 + SO4

= + ---> HS- + H2O

chemoorganotrophic growthlactate + SO4

= ---> acetate + CO2 + H2O + HS-

High sulfate environments: SRB compete with may dominant overmethanogens.

Low or no sulfate environments, SRB grow syntrophically withmethanogens as may proton-reducers (producing H2 for interspecies H2transfer)

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InterspeciesH2 transfer

SRB may grow as “proton-reducers” that are physiologically linked tomethanogens in hydrogen transfer

high positive ΔG'o makes this reaction unfavorable for supporting growth

H2 produced is rapidly consumed at kept at a very low level the energeticsbecome favorable.

Methanogens consume H2 and making the reaction energetically favorable. Thisis 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 inrain and dry deposition

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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 duringaerobic or anaerobic decomposition of S-containing compounds

.

Volatile S from aerobic and anaerobic decomposotion of Sproteins

Gluten is composed of storageproteins, the prolamins,comprising monomeric gliadinsand polymeric glutenins.

Zein: a mixture of water insolubleproteins that constitute about halfof the protein in corn or 4-5 weight% of the corn.

Major products: MM, DMS, DMDS

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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 isPO43-]

Over 150 species of phosphate minerals:

Small amounts of mineral P in soil (ca. 1% of total)

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Organic forms of P

SOM contains the greatest amount of P (30-50% of the total)

The chemical nature of much of the organicP 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 inatmosphere, short half life (ca. 5-24 h)

Trace amounts of P in atmosphere, predominant movement is fromterrestrial environments to streams, lakes and oceans.

Fe and Mn Cycles

Cycling revolves around the transition from oxidizedinsoluble forms (Fe+3 / Mn+4 )to reduced, solubleoxidation states (Fe+2/Mn+2)

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Ferrous iron (Fe+2) used as an electron donor linked with oxygenreduction

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

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 ΔG'o (pH 7) = - 0.25 kJ

ΔGo (pH 0) = - 2.54 kJ

Acidithiobacillus ferrooxidans, an acidophilic iron-oxidizer, pH optimum for growth of 2to 3

Contribute to formation of acid mine drainage.

Thiobacillus-type [rods] inyellow floc from acid water

Fe Oxidation at Neutral-Alkaline pH, Low Oxygen Levels

Neutral-alkaline pH, Fe+2 concentrations increase with decreasing oxygenconcentration.

The "iron bacteria" (e.g., Gallionella, Leptothrix, Siderocapsa) have adapted togrow 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 sustaingrowth.

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.

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Gallionellaferruginea [braid-like] in red flocfrom neutral water

Leptothrix cholodnii [sausage-like] in red floc from neutralwater

Leptothrix discophora fresh roundedholdfasts [doughnut-like], which are parts ofthe bacteria that attach to rocks ormicroscope slides, like those here onmicroscope slide left in neutral water riffle

Light Micrographs of Iron bacteria

TEM Micrographs of Iron bacteria &iron deposits

Gallionella stalk coatedwith nanometer-scaleFe(OH)3 and FeOOHaggregates

Low magnificationimage ofGallionella andLeptothrix stalksand sheaths

GallionellaNote the twisted strands ofiron oxide characteristic ofthis organism

Wet mount, 400 X

Gallionella

Stained withcrystal violet, 1000 X

Light Micrographs: Iron bacteria and iron deposits

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Fe(III) and Mn(IV) reduction affectscycling of iron and manganesefate of a variety of other trace metals and nutrientsdegradation of organic matter

Fe(III)-reducers can outcompete sulfate-reducing and methanogenicmicroorganisms 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 environmentscontaminated with organic and/or metal pollutants

Fe/Mn reduction: Biogeochemical Significance

A wide phylogenetic diversity of microorganisms, (archaea andbacteria), are capable of dissimilatory Fe(III) reduction.

Most microorganisms that reduce Fe(III) also can transfer electronsto Mn(IV), reducing it to Mn(II).

Two major groups, those that support growth by conservingenergy from electron transfer to Fe(III) and Mn(IV) and those thatdo not.

Fe/Mn-reducing microbes (FMRM)

FMRM that Conserve Energy to Support Growth from Fe(III) andMn(IV) Reduction

Phylogenetically diverse

Most cultured FMR are in the family Geobacteraceae in thedelta δ-Proteobacteria:

Geobacter, Desulfuromonas , Desulfuromusa and Pelobacter

Acetate is a primary electron donor

Most Geobacteraceae also can use hydrogen

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Phylogenetic Diversity of Fe/Mn reducers

Phylogenetictree, based on16S rDNAsequences, ofmicroorganismsknown toconserve energyto supportgrowth fromFe(III) reduction

Microbes thatconserve

energy fromFe/Mn

reduction

Micrographs of Fe/Mn-reducers

Phase contrast micrographsof various organisms thatconserve energy to supportgrowth from Fe(III)reduction. Bar equals 5 mm,all micrographs at equivalentmagnification

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Pathways for electron donor production and use in Fe /Mnreduction

Mechanisms for Electron Transfer to Fe(III) and Mn(IV)

Mechanism(s) of electron transfer to insoluble Fe(III) and Mn(IV) arepoorly 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, reductionof solubilized spec ies

Indirect reduction mediated by extracellular electron shuttles(quinone groups in humics)

Mechanisms for Electron Transfer to Fe(III) and Mn(IV)

Identification of AQDS/humate-reductionmutants. a, Genetic screen used to isolatemutants defective in AQDS reduction.Because AQDS is clear when oxidizedand orange when reduced to AHDS,mutants were identified as those whichremained clear. b, The ADQS-reductiondefective mutants were tested for theirability to grow on and reduce humicsubstances as measured indirectly by therelease of Fe(II) after reacting the reducedhumate with Fe(III). The wild-type (opencircles) rapidly reduced humate, whereasthe mutant (open squares) did not. Datashown are representative of triplicatecultures, with fliled circles showing anuninoculated control

http://www.nature.com/nature/journal/v405/n6782/fig_tab/405094a0_F1.html