Biomethylation of Arsenic in Contaminated Soils - Environmental Contamination: Health Risks and Ecological Restoration - page 236

Whereas.bacteria.under.aerobic.conditions.reduction.steps.are.mediated.by.
glutathione.and.other.thiol-containing.compounds,.anaerobic.bacteria.may.
use.methylcobalamin.as.the.electron.donor.(Krautler.1990,.Stupperich.1993).
Biomethylation. of. As. is. thought. to. be. a. detoxiication. process. as. the.
methylated.As. products. are. less. toxic. than. the.inorganic. As. species,. espe-
cially. those. most. poisonous. forms. of. arsenious. acid. (or. its. salts. or. esters).
(Michalke. et. al.. 2000,. Bissen. and. Frimmel. 2003).. This. is. because. inorganic.
As.(III).compounds.have.a.high.afinity.to.sulfhydryl.groups.in.proteins.and.
can.cause.deactivation.to.enzymes..And.inorganic.As.(V).can.compete.with.
phosphate. in. cell. reactions,. uncouple. oxidative. phosphorylation,. and. then.
impede.the.formation.of.high-energy.adenosine.triphosphate.bonds.(Bissen.
and.Frimmel.2003).
Woolson. (1983). summarized. past. research. on. As. biomethylation. and.
concluded. methylation. of. As. is. a. widely. occurring. natural. process,. which.
accounts.for.signiicant.As.losses.from.soil.
Microbial Genetic Controls and Energetics
Interestingly,.bacteria.have.developed.mechanisms.to.overcome.the.toxicity.
of.arsenite.and.arsenate..This.well.may.be.why.they.have.evolved.to.perform.
As.redox.reactions.and.volatilization.
Microorganisms.can.avoid.the.build.up.of.intracellular.arsenicals.by.either.
inhibiting. uptake. or. actively. exporting. these. ions. (Rosenberg. et. al.. 1977,.
Willsky. et. al.. 1980,. Moblev. and. Rosen. 1982,. Silver. and. Keacch. 1982,. Silver.
et.al..1989)..The.export.mechanisms.are.probably.the.most.important.means.
by.which.microorganisms.can.resist.As.toxicity.by.the.reduction.of.arsenate.
to.arsenite.which.is.eliminated.from.cells.by.a.speciic.eflux.pump.(Silver.
1996)..Secondarily,.they.may.avoid.toxicity.by.chemical.modiication.of.As.to.
relatively.less.toxic.forms.
One.reduction.mechanism.is.the.conversion.of.arsenate.to.arsenite.which.
in. turn. is. exported. out. of.the. bacterial. cells.. This. reaction. is. controlled. by.
the. ars .operon.gene.as.a.regulatory.repressor,.(ArsR),.an.arsenate.reductase.
(ArsC),.and.an.arsenite.eflux.pump.(ArsB).(Xu.et.al..1998)..Silver.(1998).and.
Suzuki.et.al..(1998).have.reported.that.other.genes.in.the. ars .operon.extended.
the.arrangement.of.this.operon.to.be. arsRDABC ..ArsA.is.an.ATPase.that.can.
provide. ArsB. energy. while. efluxing. arsenite. out. of. cells.. ArsD. is. a. found.
to. be. a. As. chaperone. for. the. arsAB. transporter.. It. helps. transport. arsenite.
to.ArsA.and.thus.increases.the.afinity.of.ArsA.for.arsenite.(Lin.et.al..2007).
Surprisingly,.there.is.strong.evidence.that.this.reduction.is.more.common.
under.aerobic.than.anaerobic.conditions..Macur.et.al..(2001).in.column.stud-
ies. of. mining. tailings. showed. that. only. As. (V)-reducing. microorganisms.
could.be.cultivated.from.aerobic.soils.(Eh.of.400.mV).and.none.from.anaero-
bic.columns..Analysis.of.16SrDNA.genes.(As.reducing.genes).from.isolates.
that. reduced. As. (V). to. As. (III). under. pure. culture. conditions. had. genes.
identical. to. that. found. in. gene. analysis. of. the. total. populations. from. the.