Microbial Arsenic Methylation in Soil and Rice Rhizosphere
- Publication date
- 2013
- Publisher
Abstract
Methylated arsenic
(As) species are a common constituent of rice
grains accounting for 10–90% of the total As. Recent studies
have shown that higher plants are unlikely to methylate As in vivo
suggesting that As methylation is a microbial mediated process that
occurs in soils prior to plant uptake. In this study, we designed
primers according to the conserved essential amino acids and structural
motifs of arsenite <i>S</i>-adenosylmethionine methyltransferase
(ArsM). We report for the first time the successful amplification
of the prokaryotic <i>arsM</i> gene in 14 tested soils with
wide ranging As concentrations. The abundance and diversity of the <i>arsM</i> gene in the rice rhizosphere soil and roots were analyzed
using the designed primers. Results showed that microbes containing <i>arsM</i> genes were phylogenetically diverse, as revealed by
the clone library and terminal restriction fragment length polymorphism
(T-RFLP) analysis, and were branched into various phyla. Concentration
of methylated As species in the soil solution was elevated in the
rhizosphere soil and also by the addition of rice straw into the paddy
soil, corresponding to the elevated abundance of the <i>arsM</i> gene in the soil. These results, together with evidence of horizontal
gene transfer (HGT) of the <i>arsM</i> gene, suggest the
genes encoding ArsM in soils are widespread. These findings demonstrate
why most rice, when compared with other cereals, contains unusually
high concentrations of methylated As species