Biogeochemical control of potential toxic elements mobility in mining sediments: The role of Arthrobacter-mediated Mn precipitation in the Mar Menor alluvial plain.

This study deciphers the complex biogeochemical interactions governing metal mobility within a deep soil core from the Cartagena-La Unión mining district (SE Spain), a Mediterranean semiarid area under severe environmental stress. We integrated geochemical, mineralogical, and microbiological analyses to elucidate the vertical distribution and fate of Potentially Toxic Elements (PTEs) across the alluvial plain. Results reveal extreme accumulations of Lead (Pb, ∼4 wt%) and Zinc (Zn, ∼3 wt%) throughout the 3-m core. A critical geochemical horizon was identified at 260 cm, characterized by peak concentrations of Calcium (17%), Sulfur (10%), and Manganese (2%). While physicochemical gradients partially dictate these patterns, phylogenetic sequencing revealed a predominant presence of the Arthrobacter genus at these depths. This suggests that microbial-mediated Mn oxidation acts as a biological engine for secondary mineral precipitation, effectively conditioning the partitioning of metals in the subsurface. Furthermore, the identification of marine-derived DNA sequences in deep terrestrial layers provides novel biological evidence of the historical shoreline fluctuations and the broader geographical extent of the Mar Menor Lagoon. Our findings demonstrate that microbial signatures not only serve as essential proxies for understanding geochemical evolution but also offer critical insights into the long-term stabilization of metals in mining-impacted alluvial systems.