Abstract
Two microbial mats found inside two old (gold and uranium) mines in Zloty Stok and Kowary located in SW Poland seem to form a natural barrier that traps heavy metals leaking from dewatering systems. We performed complex physiological and metagenomic analyses to determine which microorganisms are the main driving agents responsible for self-purification of the mine waters and identify metabolic processes responsible for the observed features. SEM and energy dispersive X-ray microanalysis showed accumulation of heavy metals on the mat surface, whereas, sorption experiments showed that neither microbial mats were completely saturated with heavy metals present in the mine waters, indicating that they have a large potential to absorb significant quantities of metal. The metagenomic analysis revealed that Methylococcaceae and Methylophilaceae families were the most abundant in both communities, moreover, it strongly suggest that backbones of both mats were formed by filamentous bacteria, such as Leptothrix, Thiothrix, and Beggiatoa. The Kowary bacterial community was enriched with the Helicobacteraceae family, whereas the Zloty Stok community consist mainly of Sphingomonadaceae, Rhodobacteraceae, and Caulobacteraceae families. Functional (culture-based) and metagenome (sequence-based) analyses showed that bacteria involved in immobilization of heavy metals, rather than those engaged in mobilization, were the main driving force within the analyzed communities. In turn, a comparison of functional genes revealed that the biofilm formation and heavy metal resistance (HMR) functions are more desirable in microorganisms engaged in water purification than the ability to utilize heavy metals in the respiratory process (oxidation-reduction). These findings provide insight on the activity of bacteria leading, from biofilm formation to self-purification, of mine waters contaminated with heavy metals.
Highlights
Our studies suggested that the biofilm formation and heavy metal resistance (HMR) functions are more desirable in microorganisms engaged in self-purification of contaminated waters than the ability to transform heavy metals in respiratory processes
This study provide comprehensive insight into biodiversity, biofilm formation and metabolic activity of two microbial mats forming a natural barrier trapping heavy metals leaking from closed mines dewatering systems located in Zloty Stok and Kowary (SW Poland)
The biofilm formation and HMR functions are more desirable in microorganisms engaged in self-purification of contaminated waters than the ability to transform heavy metals in respiratory processes
Summary
Among the different environments where microbial mats have been described, mine waters, especially those contaminated with heavy metals, are of special interest (Drewniak et al, 2012; Mendez-Garcia et al, 2014), such communities are usually described in the context of acid mine drainage (AMD), as microorganisms are often responsible for the formation of these effluents, which constitute a well-known environmental problem (Hogsden and Harding, 2012). Numerous studies apply metagenomic approach to investigate the phylogenetic and functional properties of microorganisms involved in AMD development (Chen et al, 2016; Huang et al, 2016) as well as metal-microbes interactions and biogeochemical pathways. The Zloty Stok goldmine (ZS), and Kowary uranium mine (KOW) located in South-Western Poland are interesting locations to study the metal-microbe interactions at the ecosystem level, as complex microbial mats communities were observed in the bottom sediments of both slightly acidic, and neutral mine waters with elevated concentrations of heavy metals
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