Microbially Mediated Methylation of Arsenic in the Arsenic-Rich Soils and Sediments of Jianghan Plain

Abstract

Almost nothing is known about the activities and diversities of microbial communities involved in As methylation in arsenic-rich shallow and deep sediments; the correlations between As biomethylation and environmental parameters also remain to be elucidated. To address these issues, we collected 9 arsenic-rich soil/sediment samples from the depths of 1, 30, 65, 95, 114, 135, 175, 200, and 223 m in Jianghan Plain, China. We used microcosm assays to determine the As-methylating activities of the microbial communities in the samples. To exclude false negative results, we amended the microcosms with 0.2 mM As(III) and 20.0 mM lactate. The results indicated that the microbial communities in all of the samples significantly catalyzed arsenic methylation. The arsM genes were detectable from all the samples with the exception of 175 m, and 90 different arsM genes were identified. All of these genes code for new or new-type ArsM proteins, suggesting that new As-methylating microorganisms are widely distributed in the samples from shallow to deep sediments. To determine whether microbial biomethylation of As occurs in the sediments under natural geochemical conditions, we conducted microcosm assays without exogenous As and carbons. After 80.0 days of incubation, approximately 4.5–15.5 μg/L DMAsV were detected in all of the microcosms with the exception of that from 30 m, and 2.0–9.0 μg/L MMAsV were detected in the microcosms of 65, 114, 135, 175, 200, and 223 m; moreover, approximately 18.7–151.5 μg/L soluble As(V) were detected from the nine sediment samples. This suggests that approximately 5.3, 0, 8.1, 28.9, 18.0, 8.7, 13.8, 10.2, and 14.9% of total dissolved As were methylated by the microbial communities in the sediment samples from 1, 30, 65, 95, 114, 135, 175, 200, and 223 m, respectively. The concentrations of biogenic DMAsV show significant positive correlations with the depths of sediments, and negative correlations with the environmental NH4+ and NaCl concentrations, but show no significant correlations with other environmental parameters, such as NO3-, SO42+, TOC, TON, Fe, Sb, Cu, K, Ca, Mg, Mn, and Al. This work helps to better understand the biogeochemical cycles of arsenic in arsenic-rich shallow and deep sediments.

Variability in Arsenic Methylation Efficiency across Aerobic and Anaerobic Microorganisms

“…To pinpoint active As methylators, the studies correlated the abundance of arsMharboring microorganisms to the As content in soil or other soil geochemical characteristics. 31,32 The microbial communities in flooded rice paddy soils are distributed in oxic environments, surface soil and rhizosphere, as well as in the bulk, anoxic soil. 33 To date, the isolation of active As methylators from soil has succeeded only for aerobic bacteria 34−36 while, the organisms actively methylating As in anoxic soils remain unknown.…”

“…Efforts to identify the key microbial mediators of As methylation have involved metagenomic as well as gene amplification approaches. − These studies have shown that the phylogenetic diversity of arsM genes from paddy soil microbial communities is extensive. To pinpoint active As methylators, the studies correlated the abundance of arsM -harboring microorganisms to the As content in soil or other soil geochemical characteristics. , The microbial communities in flooded rice paddy soils are distributed in oxic environments, surface soil and rhizosphere, as well as in the bulk, anoxic soil . To date, the isolation of active As methylators from soil has succeeded only for aerobic bacteria − while, the organisms actively methylating As in anoxic soils remain unknown.…”

Significance of Shewanella Species for the Phytoavailability and Toxicity of Arsenic—A Review

“…Having described the key role of Shewanella species in the facilitation of direct or indirect mobilisation of environmental As species, its potential remediation role could be best perceived from its contribution to the detoxification of some As species and/or sequestration of the mobilised As through flavin-mediated iron plaque formation [104][105][106]. These various processes are explained in detail in the upcoming sections of this article.…”

“…Additionally, the bacteria involved have the potential to dramatically alter the geochemistry of the surrounding Fe mineral-containing As in groundwater and sediments, resulting in As contamination of drinking water sources, diseases, poisoning, and human health disruption [39,133,134]. Numerous bacterial strains were identified to be involved in the reduction and transformation of Fe, particularly in the As(V)-bearing Fe(III) mineral assemblage [104,135,136]. These bacteria include dissimilatory iron-reducing bacteria (DIRB) and dissimilatory arsenic-reducing bacteria (DARB), comprising Shewanella oneidensis MR-1 and Geobacter metallireducens GS-15 and Geobacter sp.…”

“…Increasing evidence suggests that the methylation process functions as an additional system that is critical in the detoxification of As(III) [104], and it can also serve in the bioremediation of As [48]. The process involves adding a methyl group to As(III) and subsequent detoxification of methylated products [75,81,105].…”