Microbial Geochemistry of the Acidic Saline Pit Lake of Brunita Mine (La Union, SE Spain)

We present the first study of a unique acidic lake formed in the Brunita open pit (La Union mines, Cartagena, SE Spain). This pit lake exhibits chemical characteristics typical of AMD, such as low pH (pH 2.2-5.0) and high iron content (500-6400 mg/L total Fe). It also has some of the highest sulfate concentrations reported to date in pit lakes (26,000-38,400 mg/LSO42- and transition metals like Mn (up to 2000 mg/L), Zn (500 mg/L), or Cu (250 mg/L). In addition, we found abnormally high concentrations of salt-forming ions (e.g. 5500 mg/L Mg, 750-1300 mg/L Cl, and 300-630 mg/L Na). The resulting high salinity (58 parts per thousand) at the bottom creates a meromictic lake despite the lake's low relative depth (9%), with an anoxic, reducing monimolimnion isolated from the oxygenated mixolimnion. In the monimolimnion, we observed decreased metal concentrations (e.g. Cu, Zn, Cd, Cr, Pb, Th). We hypothesize that these metals are being removed by interaction with biogenic H2S and subsequent precipitation as metal sulfides. Scanning electron microscopy shows sub-micron, spherical particles of ZnS in close association with cocci and rod-like bacteria. Analysis of the microbial community composition through 16S rRNA gene amplicon sequencing revealed different genera of sulfate-reducing bacteria (SRB) in the monimolimnion, includingDesulfobacca,Desulfomonile,Desulfurispora, andDesulfosporosinus. Their apparent ability to reduce sulfate and selectively precipitate potentially toxic metals, and their resistance to this lake's extreme geochemical conditions, makes these bacteria of great interest for biotechnological applications (e.g. bioremediation and biomining).