Anaerobic Reductive Biomethylation by Shewanella: A Bioremediation Pathway for Arsenic and Metalloids

Anaerobic reductive biomethylation presents an alternative strategy for the bioremediation of toxic metals and metalloids. Various microorganisms, including Shewanella species, perform this process under both aerobic and anaerobic conditions in diverse environments such as metal waste deposits, sewage sludge, and alluvial soils [35-44]. The resulting methylated metal compounds exhibit altered solubility, volatility, and toxicity compared to their inorganic forms [45-47]. For instance, volatile methylated species like (CH3)2Se (dimethyl selenide) and (CH3)2Se2 (dimethyl diselenide) can efflux from water into the atmosphere, effectively removing them from a contaminated site [44,47]. These biomethylation reactions involve the enzymatic transfer of methyl groups through stepwise processes, yielding both partially methylated nonvolatile species and fully methylated volatile compounds. Specifically, Shewanella species generate methylated derivatives of arsenic (As), selenium (Se), tellurium (Te), and iodine (I) by first reducing oxyanions like arsenate and selenite, then methylating the reduced forms anaerobically [48-55].

Reductive Methylation of Arsenic by Shewanella. Arsenic contamination often stems from the agricultural use of organoarsenical herbicides and feed additives. This element exists primarily in four oxidation states: arsenate (As(V)), arsenite (As(III)), elemental arsenic (As(0)), and arsenide (As(-III)). The water-soluble As(V) and As(III) are most relevant environmentally, with As(V) dominating in aerobic settings and As(III) in anaerobic ones. The model organism Shewanella sp. strain ANA-3 possesses two distinct As(V) reductases for managing arsenic: one for respiratory energy generation (encoded by the arr genes) and another for detoxification (encoded by the ars genes) [53]. These systems allow the strain to tolerate high (micromolar) levels of inorganic arsenic, with the broadly expressed ars detoxification system being particularly crucial for survival in high-arsenic environments [56].

The Arr respiratory system is a periplasmic heterodimer composed of ArrA and ArrB subunits and is expressed only under anaerobic conditions, being repressed by oxygen and nitrate [57]. Regulation of these systems is complex and responsive to arsenic speciation and concentration. The arr system is activated by very low (nanomolar) concentrations of As(III), while the ars system requires much higher (micromolar) levels. Conversely, As(V) induces the arr system at low concentrations. This intricate regulation involves the ArsR family protein ArsR2 as the primary As(III)-sensitive regulator and requires coordination with global regulators like the cAMP-CRP complex, linking central metabolism to arsenic stress responses [58].

The pathway for microbial arsenic methylation involves the initial reduction of As(V) to As(III), followed by the oxidative addition of methyl groups [59]. This generates a series of methylated species with the general formula (CH3)nAsH3-n, including methyl arsenite (MMA), dimethyl arsenate (DMA-V), dimethyl arsenite (DMA-III), and trimethyl arsine oxide (TMAO) as major volatile compounds [60]. The methylation is catalyzed by As(III) methyltransferases (ArsM), a conserved group of enzymes that use S-adenosyl methionine (SAM) as a methyl donor and require three strictly conserved cysteine residues for catalytic function [61]. Shewanella oneidensis MR-1 performs this partial methylation, transforming inorganic arsenic into less toxic metabolites like DMA-III [55]. Although the specific methyltransferases in Shewanella are yet to be fully characterized, genomic analyses reveal the presence of homologous ArsM genes, indicating a potential pathway for arsenic biomethylation (Figure 1).

Figure 1. E'₀ values of electron acceptors respired by Shewanella oneidensis span nearly the entire continuum of redox potentials encountered by bacteria in nature