Biomethylation of Arsenic in Contaminated Soils - Environmental Contamination: Health Risks and Ecological Restoration - page 237

soils.in.the.aerobic.columns..Isolates.that.could.reduce.As.(V).under.aerobic.
conditions. included. Caulobacter, Sphingomonas, . and. Rhizobium -like. isolates;.
and. more. speciically. Pseudomonas luorescens, Pseudomonas aeruginosa ,. and.
Sphingomonas echinoids .(Macur.et.al..2001).
The. second. reduction. reaction. is. dissimilatory. reduction. which. occurs.
under.anaerobic. conditions. where.arsenate.is.a.terminal.electron.acceptor..
In.this.case,.As.respiration.reduction.is.done.by.membrane-bound.arsenate.
reductase.arrAB.(Macy.et.al..2000).
Arsenite.can.be.methylated.directly.or.oxidized.to.the.less.toxic.arsenate.
prior. to. methylation. (Oehme. 1972,. Coddington. 1986,. Ishinishi. et. al.. 1986)..
Bacteria. gain. energy. during. the. arsenite. oxidation. process. where. arsenite.
serves.as.an.electron.donor,.together.with.the.reduction.of.oxygen.or.nitrate.
(Anderson.et.al..1992,.Bryan.et.al..2009).
Bacteria.that.accomplish.oxidation.have.the. aox .gene.that.codes.for.arse-
nite.oxidase.(a.DMSO.reductase).(Ellis.et.al..2001),.which.converts.arsenite.to.
arsenate.(Anderson.et.al..1992)..AoxAB.is.a.heterodimeric.enzyme.composed.
of. a. large. subunit. (AoxB). and. small. subunit. (AoxA).. This. enzyme. consists.
of.a.monomer.of.86.kDa.containing.one.molybdenum,.ive.or.six.irons,.and.
inorganic.sulide..(Anderson.et.al..1992).
These. results. would. suggest. that. soils. potentially. contain. both. As. (V).
reducing. and. As. (III). oxidizing. microorganisms.. Indeed,. this. was. shown.
by. Macur. et. al.. (2000). who. used. both. culturing. and. molecular. analysis.
(16SrDNA. gene. sequences). and. found. both. As. (III). oxidizing. ( P. luore-
scens ,. Variovorax paradoxus ,. Agrobacterium tumefaciens ). and. As. (V). reducing.
( Flavobacterium heparinum, Agrobacterium tumefaciens, Microbacterium . sp.,.
Arthrobacter aurescens ,.and. Arthbacter .sp.).bacteria.in.soils.contaminated.with.
As.from.geothermal.activity.
As. mentioned. in. the. “methylation. reactions. and. pathways”. section,. the.
general.methylation.pathway(s).is.a.sequence.of.reactions.where.the.reduc-
tion.of.the.pentavalent.form.of.As.is.followed.by.a.methyl.group.that.is.added.
oxidatively. (Dombrowski. et. al.. 2005).. This. in. turn. results. in. increasingly.
methylated.As.species. (methyl.arsenite,.MMA;.dimethyl.arsenate,.DMA-V;.
dimethyl.arsenite,.DMA-III;.and.trimethyl.arsine.oxide,.MAO)..The.methyla-
tion.reactions.do.require.SAM.as.the.source.of.methyl.groups..All.the.SAM-
dependent. methylation. steps. are. controlled. by. a. distinct. gene. (arsM). that.
has.been.found.in.more.than.120.prokaryotic.species.(and.some.archaea).and.
further.characterized.in. Rhodopseudomonas palustris .(Qin.et.al..2006).
FactorsAffectingArsenicBiomethylation
Arsenic. biomethylation. is. a. microbial. process. and. production. of. methy-
larsines. requires. a. combination. of. enzyme. activities. and. energy. input.