Arsenic(As) is a metalloid toxic to organisms including humans.Arsenic in rice represents a significant exposure pathway for the general population,particularly for those subsisting on rice.Arsenic transformation,namely reduction,oxidation and methylation,in soil-rice systems has fundamental impacts on its mobility and toxicity.In addition to soil chemical properties(pH,Eh,metallic oxides,organic matter),microorganisms play critical roles in As transformation and mobility in paddy soil,such as through ArsM(As(III) S-adenosylmethyltransferase) and interactions with iron oxides or organic matters.Arsenic species in paddy soil directly influence As speciation in rice grain because the methylated As species in rice are mainly derived from microbial methylation in paddy soil.This paper aims to provide an overview on the status of the knowledge and gaps on the chemical aspects of As transformation in soil-rice system in conjunction with microbial ecology and functional genes.In addition,potential pathways(manipulation of microorganisms in paddy soil and genetic engineering) to decrease total As and/or inorganic As in rice grain are proposed.
In Rhodopseudomonas palustris,an arsM gene,encoding bacterial and archaeal homologues of the mammalian Cyt19 As(III) S-adenosylmethionine methytransferase,was regulated by arsenicals.An expression of arsM was introduced into strains for the methylation of arsenic.When arsM was expressed in Sphingomonas desiccabilis and Bacillus idriensis,it had 10 folds increase of methyled arsenic gas compared to wild type in aqueous system.In soil system,about 2.2%–4.5% of arsenic was removed by biovolatilization during 30 days.This study demonstrated that arsenic could be removed through volatilization from the contaminated soil by bacteria which have arsM gene expressed.These results showed that it is possible to use microorganisms expressing arsM as an inexpensive,efficient strategy for arsenic bioremediation from contaminated water and soil.
