Mineral raw materials are essential to support ongoing technological advances, such as energy production from renewable sources. Phytomining constitutes a non-destructive technology for the recovery of metals which could be potentially applied to obtaining scarce (rare earth elements (REEs), tungsten (W)) or valuable (gold (Au)) elements. Soil microbial processes involved in metal mobility and availability to plants represent a central role in the phytoextraction efficiency of metals such as Ni, but their involvement in the case of rare elements is unclear. In this context, the objectives of the present work were to (a) identify, in areas of geological interest, plant species which possess the ability to extract and accumulate trace elements and (b) isolate and characterize the rhizosphere bacterial communities associated with this vegetation and elucidate bacterial mechanisms involved in the geochemical cycling of strategic elements, with the view to developing phytomining techniques for the recovery of strategic elements.

Exploratory studies were conducted in two abandoned opencast mines in NW Spain. The mines were dedicated to the extraction of either, tungsten (W) and copper (Cu) in San Finx (Galicia), or gold (Au) and molybdenum (Mo) in Salave (Asturias). Samples of the dominant vegetation, rocks, bulk and rhizosphere soil were collected in different locations of each mine area. Trace element concentrations in soils and in the aboveground plant biomass were determined by ICP-MS. Rhizosphere bacteria were isolated from the soil and screened for W and Mo resistance, ability to produce organic acids, siderophores, biosurfactants, IAA and phosphate solubilisation capacity. Batch culture assays using selected bacterial strains and ground rock, or soil, were performed to evaluate their weathering capacities. Significant concentrations of the REEs La, Ce and Y, as well as Mo, were detected in both mine areas, which were for some plant species reflected in their aboveground tissues. Salix spp., Erica spp. (in San Finx) and Pteridium spp. (in Salave) harboured a higher number of bacterial isolates with ability to produce siderophores and surfactants, whereas the tolerance to high concentrations of W and Mo was generalised amongst most bacterial isolates. Furthermore, the growth and activity of different bacterial strains induced the mobilisation of different critical or valuable elements after their incubation with soil from Salave or San Finx.

To conclude, the prospection of abandoned mining areas could allow the identification of plant species with capacity to accumulate rare elements, further our understanding of the microbial communities inhabiting these soils and identify beneficial bacterial strains with application in phytomining techniques.

We acknowledge the financial support of the Spanish Ministerio de Economía, Industria y Competitividad (Mineco) and FEDER (CTM2015-66439-R).