Variability in Arsenic Methylation Efficiency across Aerobic and Anaerobic Microorganisms

Abstract

Microbially-mediated methylation of arsenic (As) plays an important role in the As biogeochemical cycle, particularly in rice paddy soils where methylated As, generated microbially, is translocated into rice grains. The presence of the arsenite (As­(III)) methyltransferase gene (arsM) in soil microbes has been used as an indication of their capacity for As methylation. Here, we evaluate the ability of seven microorganisms encoding active ArsM enzymes to methylate As. Amongst those, only the aerobic species were efficient methylators. The anaerobic microorganisms presented high resistance to As exposure, presumably through their efficient As­(III) efflux, but methylated As poorly. The only exception were methanogens, for which efficient As methylation was seemingly an artifact of membrane disruption. Deletion of an efflux pump gene (acr3) in one of the anaerobes, Clostridium pasteurianum, rendered the strain sensitive to As and capable of more efficiently methylating As. Our results led to the following conclusions: (i) encoding a functional ArsM enzyme does not guarantee that a microorganism will actively drive As methylation in the presence of the metalloid and (ii) there is an inverse relationship between efficient microbial As efflux and its methylation, because the former prevents the intracellular accumulation of As.

Meta-omics-aided isolation of an elusive anaerobic arsenic-methylating soil bacterium

“…Both cultures exhibited As methylation, reaching an efficiency of transformation of the initial As(III) of 27.7% and 19.5% for the EA and TSB cultures, respectively (Figures S1 and S2 and SI). The TSB and EA microbiomes compare favorably to previous studies of anaerobic enrichments [16] and single-strain cultures [13, 14, 19], and represent the most efficient anoxic As-methylating microbial cultures reported.…”

“…Although the expression of ArsI and ArsH, catalysing oxygen-dependent MMAs(III)-resistance mechanisms (Figure 3), is difficult to reconcile with anoxic conditions, it is conceivable that these proteins are capable of additional functions in the absence of O 2 . Up until now, the lack of available anaerobic microbial isolates able to methylate As in vitro [22] precluded the investigation of the hypotheses raised above. This work represents the first study applying a combination of three meta-omic techniques in order to characterize As metabolism in microbial communities and to perform meta-omics-aided isolation of a microorganism [84][85].…”

“…All experiments were performed in biological triplicates. Sampling for soluble As species, determination of As speciation and total As concentration were performed as described in [13] using an Agilent 8900 ICP-QQQ instrument coupled to an HPLC 1260 Infinity II (Agilent Technologies, CA, USA). Instrument settings in Table S1.…”

“…An additional complexity is the evidence for the fortuitous methylation of As upon cell lysis and release of methyltransferases. This occurrence was suggested by considering the methanogen Methanosarcina mazei for which As methylation was initiated only when cell viability decreased [13]. Thus, presence of As methylation for cultures incubated beyond the exponential phase may only be an experimental artefact.…”

Interactions with Arsenic: Mechanisms of Toxicity and Cellular Resistance in Eukaryotic Microorganisms

“…A second prokaryotic As biomethylation pathway has been elucidated generating TMA(III) in several anaerobic bacteria (for instance, Clostridium collagenovorans, Desulfovibrio vulgaris, and Desulfovibrio gigas), and arsine in the archaea Methanobacterium formicium, as end products of As methylation. Experimental data from aerobic and anaerobic prokaryotic species, that commonly inhabit soil environments, have concluded that encoding a functional ArsM enzyme does not guarantee that a microorganism will actively drive As methylation in the presence of the metalloid [97]. Besides the microbial arsenic methylation, humans and other animals, which possess the enzyme AS3MT, can contribute to methylarsenicals generation in the environment.…”