Sulphur cycle

M. Sc., Microbiology Study Material (Semester-I, General Microbiology)
Department of Microbiology, School of Bioscience,
Periyar University, Salem.

1. SULPHUR CYCLE

2. Sulphur cycle
Microorganismscontributegreatlytothesulfurcycle.
Photosyntheticmicroorganismstransformsulfurbyusing
sulfideasanelectronsource,allowingThiobacillusand
similarchemolithoautotrophicgenera.
Incontrast,whensulfatediffusesintoreducedhabitats,it
providesanopportunityfordifferentgroupsof
microorganismstocarryoutsulfatereduction.
Forexample,whenausableorganicreductantispresent,
Desulfovibriousessulfateasanoxidant.
Thisuseofsulfateasanexternalelectronacceptorto
formsulfide,whichaccumulatesintheenvironment,isan
exampleofadissimilatoryreductionprocessand
anaerobicrespiration.

3. Incomparison,thereductionofsulfateforuseinaminoacidand
proteinbiosynthesis.
Othermicroorganismshavebeenfoundtocarryoutdissimilatory
elementalsulfurreduction.
TheseincludeDesulfuromonas,thermophilicarchaeaandalso
cyanobacteriainhypersalinesediments.
Sulfiteisanothercriticalintermediatethatcanbereducedtosulfide
byawidevarietyofmicroorganisms,includingAlteromonasand
Clostridium,aswellasDesulfovibrioandDesulfotomaculum.

4. Inadditiontotheveryimportantphotolithotrophicsulfur
oxidizerssuchasChromatiumandChlorobium,which
functionunderstrictanaerobicconditionsindeepwater
columns,alargeandvariedgroupofbacteriacarryout
aerobicanoxygenicphotosynthesis.
Theseaerobicanoxygenicphototrophsuse
bacteriochlorophyllaandcarotenoidpigmentsandare
foundinmarineandfreshwaterenvironments;theyare
oftencomponentsofmicrobialmatcommunities.
ImportantgeneraincludeErythromonas,Roseococcus,
Porphyrobacter,andRoseobacter.

5. “Minor”compoundsinthesulfurcycleplaymajorrolesinbiology.
Anexcellentexampleisdimethylsulfoniopropionate(DMSP),
whichisusedbybacterioplankton(floatingbacteria)asasulfur
sourceforproteinsynthesis,andwhichistransformedto
dimethylsulfide(DMS),avolatilesulfurformthatcanaffect
atmosphericprocesses.
WhenpHandoxidation-reductionconditionsarefavorable,
severalkeytransformationsinthesulfurcyclealsooccurasthe
resultofchemicalreactionsintheabsenceofmicroorganisms.
Animportantexampleofsuchanabioticprocessistheoxidation
ofsulfidetoelementalsulfur.
ThistakesplacerapidlyataneutralpH,withahalf-lifeof
approximately10minutesforsulfideatroomtemperature.

7. Iron Cycle
Theironcycleincludesseveraldifferentgenerathatcarryoutiron
oxidations,transformingferrousion(Fe2)toferricion(Fe3).
Thiobacillusferrooxidanscarriesoutthisprocessunderacidic
conditions,GallionellaisactiveunderneutralpHconditions,and
Sulfolobusfunctionsunderacidic,thermophilicconditions.
Muchoftheearlierliteraturesuggestedthatadditionalgeneracould
oxidizeiron,includingSphaerotilusandLeptothrix.
Thesetwogeneraarestilltermed“ironbacteria”bymanynon
microbiologists.
Confusionabouttheroleofthesegeneraresultedfromtheoccurrence
ofthechemicaloxidationofferrousiontoferricion(forminginsoluble
ironprecipitates)atneutralpHvalues,wheremicroorganismsalso
growonorganicsubstrates.Thesemicroorganismsarenowclassified
aschemoheterotrophs.

8. RecentlymicrobeshavebeenfoundthatoxidizeFe2using
nitrateasanelectronacceptor.
Thisprocessoccursinaquaticsedimentswithdepressedlevels
ofoxygenandmaybeanotherroutebywhichlargezonesof
oxidizedironhaveaccumulatedinenvironmentswithlower
oxygenlevels.
Ironreductionoccursunderanaerobicconditionsresultingin
theaccumulationofferrousion.
Althoughmanymicroorganismscanreducesmallamountsof
ironduringtheirmetabolism,mostironreductioniscarried
outbyspecializediron-respiringmicroorganismssuchas
Geobactermetallireducens,Geobactersulfurreducens,
Ferribacteriumlimneticum,andShewanellaputrefaciens,
whichcanobtainenergyforgrowthonorganicmatterusing
ferricironasanoxidant.

9. Inadditiontotheserelativelysimplereductionstoferrous
ion,somemagnetotacticbacteriasuchasAquaspirillum
magnetotacticumvalenceironoxidemineralmagnetite
(Fe3O4)andconstructintracellularmagneticcompasses.
Furthermore,dissimilatoryironreducingbacteria
accumulatemagnetiteasanextracellularproduct.
Magnetitehasbeendetectedinsediments,whereitis
presentinparticlessimilartothosefoundinbacteria,
indicatingalongertermcontributionofbacteriatoiron
cyclingprocesses.

10. Genesformagnetitesynthesishavebeenclonedintoother
organisms,creatingnewmagneticallysensitivemicroorganisms.
Magnetotacticbacteriaarenowdescribedasmagneto-aerotactic
bacteria,duetotheirusingmagneticfieldstomigratetothe
positioninabogorswampwheretheoxygenlevelisbestsuited
fortheirfunctioning.
Inthelastdecadenewmicroorganismshavebeendiscoveredthat
useferrousionasanelectrondonorinanoxygenicphotosynthesis.
Thus,withproductionofferricioninlightedanaerobiczonesby
iron-oxidizingbacteria,thestageissetforsubsequent
chemotrophic-basedironreduction,suchasbyGeobacterand
Shewanella,creatingastrictlyanaerobicoxidation/reductioncycle
foriron.