Indirect Redox Transformations of Iron, Copper, and Chromium Catalyzed by Extremely Acidophilic Bacteria

Abstract

Experiments were carried out to examine redox transformations of copper and chromium by acidophilic bacteria (Acidithiobacillus, Leptospirillum, and Acidiphilium), and also of iron (III) reduction by Acidithiobacillus spp. under aerobic conditions. Reduction of iron (III) was found with all five species of Acidithiobacillus tested, grown aerobically on elemental sulfur. Cultures maintained at pH 1.0 for protracted periods displayed increasing propensity for aerobic iron (III) reduction, which was observed with cell-free culture liquors as well as those containing bacteria. At. caldus grown on hydrogen also reduced iron (III) under aerobic conditions, confirming that the unknown metabolite(s) responsible for iron (III) reduction were not (exclusively) sulfur intermediates. Reduction of copper (II) by aerobic cultures of sulfur-grown Acidithiobacillus spp. showed similar trends to iron (III) reduction in being more pronounced as culture pH declined, and occurring in both the presence and absence of cells. Cultures of Acidithiobacillus grown anaerobically on hydrogen only reduced copper (II) when iron (III) (which was also reduced) was also included; identical results were found with Acidiphilium cryptum grown micro-aerobically on glucose. Harvested biomass of hydrogen-grown At. ferridurans oxidized iron (II) but not copper (I), and copper (I) was only oxidized by growing cultures of Acidithiobacillus spp. when iron (II) was also included. The data confirmed that oxidation and reduction of copper were both mediated by acidophilic bacteria indirectly, via iron (II) and iron (III). No oxidation of chromium (III) by acidophilic bacteria was observed even when, in the case of Leptospirillum spp., the redox potential of oxidized cultures exceeded +900 mV. Cultures of At. ferridurans and A. cryptum reduced chromium (VI), though only when iron (III) was also present, confirming an indirect mechanism and contradicting an earlier report of direct chromium reduction by A. cryptum. Measurements of redox potentials of iron, copper and chromium couples in acidic, sulfate-containing liquors showed that these differed from situations where metals are not complexed by inorganic ligands, and supported the current observations of indirect copper oxido-reduction and chromium reduction mediated by acidophilic bacteria. The implications of these results for both industrial applications of acidophiles and for exobiology are discussed.

Bioreductive Dissolution as a Pretreatment for Recalcitrant Rare-Earth Phosphate Minerals Associated with Lateritic Ores

“…While it has long been known that A. thiooxidans can reduce ferric iron [27], later work showed that this acidophile cannot grow by ferric iron respiration [28]. Reduction of ferric iron in aerobically grown cultures has been shown to be widespread amongst the acidithiobacilli [29], and ongoing reduction of soluble ferric iron by Acidithiobacillus caldus has recently been shown to sustain the growth of Leptospirillum ferriphilum in mixed cultures [30]. The fact that ferric iron can be reduced under aerobic conditions extends the potential for the bioprocessing of minerals such as goethite.…”

Do microbes control the formation of giant copper deposits?

“…The direct relationship observed between microbial aggregates and covellite in the cementation zone of the Las Cruces deposit and the in situ detection of abundant sulfatereducing bacteria strongly suggest that the current formation of the high-grade copper mineralization is controlled by microbial metabolism, with the covellite precipitating due to the increase of microbially mediated reduced sulfur. Precipitation of the copper sulfides also requires aqueous copper to be reduced from Cu 2+ to Cu + , a process that could be biologically mediated by some chemolithotrophic gammaproteobacteria related to Acidithiobacillus ferrooxidans at low pH under aerobic conditions (Johnson et al, 2017). Alternatively, in anaerobic environments, aqueous Fe 2+ is a rapid abiogenic reductant for copper under highly variable conditions (Matocha et al, 2005).…”

Acidithiobacillus ferrianus sp. nov.: an ancestral extremely acidophilic and facultatively anaerobic chemolithoautotroph

“…The online version of this article (https ://doi.org/10.1007/s0079 2-020-01157 -1) contains supplementary material, which is available to authorized users. growth by ferric iron respiration appears to be restricted to those species that also oxidize iron (Hallberg et al 2001;Johnson et al 2017). Some acidithiobacilli can also use hydrogen as sole electron donor, but while this appears to be a common trait for all strains of some species (A. ferrooxidans and A. ferridurans) this is not the case for strains of other species (A. ferrivorans, A. thiooxidans and A. caldus) and has not been observed in any strain of A. ferriphilus (Hedrich and Johnson 2013b;Falagán and Johnson 2016).…”