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In the mining-impacted Rio Tinto, Spain, Fe-cycling microorganisms influence the transport of heavy metals (HMs) into the Atlantic Ocean. However, it remains largely unknown how spatial and temporal hydrogeochemical gradients along the Rio Tinto shape the composition of Fe-cycling microbial communities and how this in turn affects HM mobility. Using a combination of DNA- and RNA-based 16S rRNA (gene) amplicon sequencing and hydrogeochemical analyses, we explored the impact of pH, Fe(III), Fe(II), and Cl2 on Fecycling microorganisms. We showed that the water column at the acidic (pH 2.2) middle course of the river was colonized by Fe(II) oxidizers affiliated with Acidithiobacillus and Leptospirillum. At the upper estuary, daily fluctuations of pH (2.7 to 3.7) and Cl2 (6.9 to 16.6 g/L) contributed to the establishment of a unique microbial community, including Fe(II) oxidizers belonging to Acidihalobacter, Marinobacter, and Mariprofundus, identified at this site. Furthermore, DNA- and RNA-based profiles of the benthic community suggested that acidophilic and neutrophilic Fe(II) oxidizers (e.g., Acidihalobacter, Marinobacter, and Mariprofundus), Fe(III) reducers (e.g., Thermoanaerobaculum), and sulfate-reducing bacteria drive the Fe cycle in the estuarine sediments. RNA-based relative abundances of Leptospirillum at the middle course as well as abundances of Acidihalobacter and Mariprofundus at the upper estuary were higher than DNA-based results, suggesting a potentially higher level of activity of these taxa. Based on our findings, we propose a model of how tidal water affects the composition and activity of the Fe-cycling taxa, playing an important role in the transport of HMs (e.g., As, Cd, Cr, and Pb) along the Rio Tinto. [ABSTRACT FROM AUTHOR]

Peña de Hierro, located in southwest Spain, encompasses the springs and headwaters for the Rio Tinto River that emerge above normal faults and has been mined for its rich sulfide ore since 2500 BC. The springs are typically characterized by an orange coloration, typical pH of ~2.33, and contain elevated concentrations of heavy metals that are produced by acid rock drainage (ARD). ARD is a natural phenomenon that results from chemolithoautotrophs metabolizing the sulfide ore. Mining has amplified the magnitude of the acidity and concentrations of heavy metals evidenced within sedimentary cores from the Huelva estuary. Acidity, redox state, hydrochemistry and isotopic analyses were examined for the purpose of characterizing the subsurface flows and determining the interconnectivity of the groundwaters. Previous studies have documented the geochemistry of the springs, dating a select few, yet many springs remain uncharacterized. Acidity presented spatial variability throughout the field area, caused by extensive sulfide interactions which generated and modified the pH. Redox exhibited a large range of values due to oxygen diffusivity though the fracture network. The surrounding geology is highly heterogeneous because of intensive deformation during the Variscan and Tertiary periods, and this heterogeneity is shown in the varied aqueous chemistry. Fractionation patterns observed in δ2H and δ18O values predominantly reflected enrichment by intensive evaporation and depletion in δ18O as a result of the proposed sulfatic-water model for Rio Tinto's hydrogeology. The analysis illustrates minimal hydrologic interconnectivity, evidenced by the extensive physical and chemical contrasts within such a small proximity. [ABSTRACT FROM AUTHOR]