Detoxification of Mercury by Methanobactin from Methylosinus trichosporium OB3b

Abstract

bMany methanotrophs have been shown to synthesize methanobactin, a novel biogenic copper-chelating agent or chalkophore. Methanobactin binds copper via two heterocyclic rings with associated enethiol groups. The structure of methanobactin suggests that it can bind other metals, including mercury. Here we report that methanobactin from Methylosinus trichosporium OB3b does indeed bind mercury when added as HgCl 2 and, in doing so, reduced toxicity associated with Hg(II) for both Alphaproteobacteria methanotrophs, including M. trichosporium OB3b, M. trichosporium OB3b ⌬mbnA (a mutant defective in methanobactin production), and Methylocystis sp. strain SB2, and a Gammaproteobacteria methanotroph, Methylomicrobium album BG8. Mercury binding by methanobactin was evident in both the presence and absence of copper, despite the fact that methanobactin had a much higher affinity for copper due to the rapid and irreversible binding of mercury by methanobactin. The formation of a gray precipitate suggested that Hg(II), after being bound by methanobactin, was reduced to Hg(0) but was not volatilized. Rather, mercury remained associated with methanobactin and was also found associated with methanotrophic biomass. It thus appears that although the mercury-methanobactin complex was cell associated, mercury was not removed from methanobactin. The amount of biomass-associated mercury in the presence of methanobactin from M. trichosporium OB3b was greatest for M. trichosporium wild-type strain OB3b and the ⌬mbnA mutant and least for M. album BG8, suggesting that methanotrophs may have selective methanobactin uptake systems that may be based on TonB-dependent transporters but that such uptake systems exhibit a degree of infidelity.

Methanobactin and the Link between Copper and Bacterial Methane Oxidation

“…Interestingly, such a protective effect was seen in a variety of methanotrophs when exposed to mercury and mb-OB3b. These results suggest that mb secreted by any methanotroph could serve to protect the broader microbial community from mercury toxicity (184).…”

“…Specifically, by strongly binding mercury, mbs may significantly alter its speciation and bioavailability, thereby reducing its toxicity. Indeed, growth studies showed that in the presence of mercury, added as mercuric chloride, methanotrophic growth was completely inhibited, but growth did occur when mb was also added (184). Interestingly, such a protective effect was seen in a variety of methanotrophs when exposed to mercury and mb-OB3b.…”

“…Indeed, addition of mb-OB3b enhances expression of mmoX in in cultures of both wild-type M. trichosporium OB3b and its mbnA::Gm r mutant (184,187), suggesting that mb may control gene expression by acting as a signaling molecule. Further, given that all known forms of mb have significant structural similarity, mbs may allow for cross-species communication.…”

“…Given this high export rate and the fact that mb can bind most transition and near-transition metals, it is likely that mb is able to influence metal mobilization and thus bioavailability of different metals in soil systems and that this affects the structure and functioning of microbial communities. Recent studies have shown that mb will bind both inorganic and organic forms of mercury (154) and thereby protect the host bacterium as well as other bacteria from mercury toxicity (184). However, additional studies are required to determine which metals are affected in their mobility by mbs and the effects of mbs on the soil microflora.…”

“…In addition to copper, mb-OB3b and mb-SB2 have been shown to bind a number of transition and near-transition metals (137,148,154,184) (Table 1). In general, the spectral properties following binding are unique for each metal and can be used in initial analysis of metal binding and in metal competition studies (137,154).…”