AUTHOR=Johnson D. Barrie , Hedrich Sabrina , Pakostova Eva 

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

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.00211

DOI=10.3389/fmicb.2017.00211

ISSN=1664-302X

ABSTRACT=<p>Experiments were carried out to examine redox transformations of copper and chromium by acidophilic bacteria (<italic>Acidithiobacillus, Leptospirillum</italic>, and <italic>Acidiphilium</italic>), and also of iron (III) reduction by <italic>Acidithiobacillus</italic> spp. under aerobic conditions. Reduction of iron (III) was found with all five species of <italic>Acidithiobacillus</italic> 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. <italic>At. caldus</italic> 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 <italic>Acidithiobacillus</italic> 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 <italic>Acidithiobacillus</italic> grown anaerobically on hydrogen only reduced copper (II) when iron (III) (which was also reduced) was also included; identical results were found with <italic>Acidiphilium cryptum</italic> grown micro-aerobically on glucose. Harvested biomass of hydrogen-grown <italic>At. ferridurans</italic> oxidized iron (II) but not copper (I), and copper (I) was only oxidized by growing cultures of <italic>Acidithiobacillus</italic> 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 <italic>Leptospirillum</italic> spp., the redox potential of oxidized cultures exceeded +900 mV. Cultures of <italic>At. ferridurans</italic> and <italic>A. cryptum</italic> 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 <italic>A. cryptum</italic>. 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.</p>