Abstract
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.
Highlights
Arsenic (As) is a metalloid toxic to organisms including humans
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
It is clear that As transformation is largely driven by microorganisms in soils in addition to chemical properties of the soil, the rate and direction of As transformation depend on the abundance and structures of microbial communities and the expression of functional genes
Summary
Arsenic exists primarily as inorganic arsenate (As(V)) and arsenite (As(III)) in paddy soil [15]. No. monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and trimethylarsine (TMA) which may have been derived from microbial and algal biomethylation could be present in paddy soils as a minor component [14,16]. The behavior and fate of As in soil environment depend largely on its speciation, different As species vary greatly in their toxicity to organisms. Inorganic As species As(V) and As(III) are generally considered to be more toxic than organic species for many organisms [4]. Arsenite is more mobile and toxic than As(V) [17] because it is neutral under normal environmental conditions and poor adsorption on soil particles [18]. Trimethylarsine (TMA) is volatilizable and thought to be nontoxic [19,20]
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