Bacterial tolerance to heavy metals (HM) poses a significant risk to ecosystems and human health. This study evaluated tolerance to HM of 24 bacterial isolates from two deep aquatic environments in southern Brazil: the Guarani Aquifer System (GAS) and the Rio Grande Cone (RGC). GAS wells were in a region of intense agricultural activity, with water levels of iron, selenium and aluminum above regulatory limits. RGC is a deep-sea environment considered pristine, although presenting evidence of human impact. Bacillus, Lysinibacillus, Pseudomonas and Paenibacillus isolates were subjected to HM minimum inhibitory concentration tests, biofilm formation, scanning electron microscopy (SEM) and analysis of metal-related genes. 58% of GAS isolates were more tolerant to (FeSO4·7H2O), 75% to (NH4Al(SO4)2·12H2O), and 67% to (Na2SeO) than reference-comparative strains (RSs). RGC isolates were tolerant to ((CH3COO)2Hg) in low concentrations but equaled the RSs in (CuCl2·2H2O) and (NiCl2·6H2O) tolerance. Biofilm formation was intense in GAS isolates after 24 h-exposure to sodium selenite, while those from RGC showed weak biofilms under mercury (II) acetate. SEM revealed potential selenium nanoparticles and morphological alterations in GAS isolates, also evidencing external particles with high mercury content in marine bacteria. Three isolates from each environment presented genes related to selenium incorporation or mercury reduction. Results indicated that HM may interfere in structure and dynamics of microbial communities of these ecosystems, highlighting the relevance of these microbial phenotypic traits from unexplored (whereas impacted) aquatic environments as markers of water pollution and toxicity, and/or for bioremediation purposes, in line with the One Health principle.
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