Abstract
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•Biofilms formed in drainage from an acid-neutralised mine site.•Cu- and Zn- rich biofilms formed on a time-scale of months.•Bio-precipitates are uniquely free of Fe and Mn.•Neutrophilic microorganisms are adapted to high metal loads.•Biofilms provide substrate for metal attenuation from waters.•Microorganisms induce metallic micro-fossilisation.
Biofilms enriched in copper and zinc have formed at the aqueous-air interface and in subaqueous environments at the abandoned Mount Chalmers volcanic-hosted massive sulphide mine in Queensland, Australia. Remedial activity on site has limited acid mine drainage and associated iron-rich discharges, and surface waters leave the site at pH ~ 7. Unlike in many other oxygenated metal-sulphide environments, (bio)precipitates at this site conspicuously lack iron and manganese. Hence, this site provides an opportunity to observe the interactions of microorganisms with other metals, unobscured by the usually dominant Fe and Mn processes. Small subunit rRNA gene sequencing analyses suggest that biofilms are dominated by neutrophilic species of moss, diatoms, fungi and rotifera, as well as minor cyanobacteria and photosynthesising microorganisms adapted to high metal loads (e.g., up to ~ 8 mg/L Zn and ~ 2 mg/L Cu) in surface waters. Microbial communities formed metal-bearing biofilms on a time-scale of months. Investigations of these biofilms using scanning electron microscopy with electron dispersive x-ray spectroscopy and synchrotron-based X-ray fluorescence microscopy indicate that biofilms are intimately associated with metals, especially Cu and Zn, with Cu concentrations of up to ~ 20 wt%. Microorganisms on site help to catalyse dissolution and precipitation processes, although abiotic precipitation and anthropogenic intervention (raised pH) are the main geochemical drivers of remediation. Our observations suggest that microorganisms catalysed metal accumulation processes by (1) providing a substrate to which metals can attenuate during wetting and drying cycles and (2) micro-fossilisation induced by microorganism-metal interactions. This site provides a unique view of the impacts of rehabilitation processes on the microbial community, and vice versa. However, small numbers of bacteria that can contribute to acid mine drainage remain in the biome, ready to proliferate should remediation activities wane. Processes identified at this site are also relevant for potentially economic, enhanced metal extraction from legacy mine sites.