Microbial community structure with trends in methylation gene diversity and abundance in mercury-contaminated rice paddy soils in Guizhou, China

Abstract

Paddy soils from mercury (Hg)-contaminated rice fields in Guizhou, China were studied with respect to total mercury (THg) and methylmercury (MeHg) concentrations as well as Bacterial and Archaeal community composition. Total Hg (0.25-990 μg g-1) and MeHg (1.3-30.5 ng g-1) varied between samples. Pyrosequencing (454 FLX) of the hypervariable v1-v3 regions of the 16S rRNA genes showed that Proteobacteria, Actinobacteria, Chloroflexi, Acidobacteria, Euryarchaeota, and Crenarchaeota were dominant in all samples. The Bacterial α-diversity was higher in samples with relatively Low THg and MeHg and decreased with increasing THg and MeHg concentrations. In contrast, Archaeal α-diversity increased with increasing of MeHg concentrations but did not correlate with changes in THg concentrations. Overall, the methylation gene hgcAB copy number increased with both increasing THg and MeHg concentrations. The microbial communities at High THg and High MeHg appear to be adapted by species that are both Hg resistant and carry hgcAB genes for MeHg production. The relatively high abundance of both sulfate-reducing δ-Proteobacteria and methanogenic Archaea, as well as their positive correlations with increasing THg and MeHg concentrations, suggests that these microorganisms are the primary Hg-methylators in the rice paddy soils in Guizhou, China.

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Similarity, in sediments collected in a river impacted by effluents from a chlor‐alkali plant, data suggested that physico‐chemistry varied significantly among reservoirs, while functional gene activities, including hgcA , were very similar and did not correlate with MMHg concentrations (Bravo et al ). In contrast, in Hg‐contaminated paddy soils, the hgcAB copy number increased with both increasing THg and MMHg concentrations (Vishnivetskaya et al ).…”

“…Despite the differences in primer pairs used in the studies mentioned earlier (Table ), until now, data globally suggested that in hgcA + δ‐Proteobacteria communities are abundant in surface sediments, but in some sites hgcA + methanogens and other hgcA + uncultivated groups are prevalent (Christensen et al , ; Vishnivetskaya et al ; Bravo et al ; Jones et al ). Among δ‐Proteobacteria , syntrophs and Geobacter spp.…”

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria

“…Letters that are not shared between treatments indicate a significant difference according to the Tukey HSD test (p < .05) The trends observed for biotic Hg 0 production were in line with our predictions. The most oxidized sulphur source, sulphate, led to lower Hg 0 production compared to the thiosulphate and μM cysteine treatments, which provided cells with relatively more reduced sulphur [e.g., S redox states for individual S atoms in thiosulphate are −1 and + 5 (Vairavamurthy et al, 1993) and S redox state in cysteine is −2 vs. +6 in sulphate] (Figure 3a). Although cells did not grow in the bioreactor, we suspect that the higher Hg 0 production in the presence of thiosulphate and cysteine was tied to the lower metabolic costs of assimilating more reduced sulphur sources, as supported by our growth experiments (Figure 2b).…”

“…For instance, in rice paddies, sulphur can become a limiting nutrient and sulphate amendments are employed to maintain agricultural productivity (Blair & Lefroy, 1998). Such amendments would supply sulphate-reducing bacteria (SRB) that often dominate Hg methylation communities (Ma, Du, Wang, & Sun, 2017;Su, Chang, Hsi, & Lin, 2016;Vishnivetskaya et al, 2018) with ample substrates for anaerobic respiration, which can subsequently increase MeHg production (Jeremiason et al, 2006;Yu, Reinfelder, Hines, & Barkay, 2018). Reduced sulphurbearing molecules such as sulphide (Pham et al, 2014;Ticknor, Kucharzyk, Porter, Deshusses, & Hsu-Kim, 2015) and cysteine (Lin, Lu, Liang, & Gu, 2015;Schaefer & Morel, 2009) generated by sulphur metabolism can also act as ligands affecting Hg bioavailability to Hg methylators.…”

An improvedhgcABprimer set and direct high-throughput sequencing expand Hg-methylator diversity in nature

“…Rice paddy soils were collected during the rice growing season from Yanwuping in Guizhou province, China. Rice paddy fields were 2-14.5 km downstream from a historical Hg mining site, with HgT levels ranging from 0.83-990 mg kg -1 and MeHg from 0.0038-0.021 mg kg -1 (Vishnivetskaya et al, 2018).…”