The effects of gold mining on microbiome

Mercury (Hg) contamination of freshwater ecosystems due to gold mining activities is a source of human health, environmental, and food security problems around the world. In Guyana, South America, it is estimated that 80 tons of mercury is expelled into the environment due to gold mining processes each year. Once in the aquatic environment, mercury undergoes methylation by microorganisms including sulfate reducing bacteria (SRB) iron reducing bacteria (IRB), firmicutes, and methanogens that have capabilities for mercury methylation. Methyl mercury (MeHg) is a neurotoxin, and it bioaccumulates and biomagnifies through the food chain leading to high MeHg concentrations in the tissues of fish ordinarily consumed by people. This places them at risk of MeHg poisoning and health related problems including ataxia, organ damage and birth defects. Although the effects of biomagnification and bioaccumulation are previously documented, further characterization of the microbiome existing in these aquatic systems is necessary to fully comprehend the nature of microbial mercury methylation. This study tested the following hypotheses. Hypothesis 1: Certain physiochemical and habitat characteristics will be significantly different between gold mined and non-mined sites. Hypothesis 2: The concentrations of gold (Au), arsenic (As), and sulfur (S) in sediments will be significantly different between gold mined and non-mined sites. Hypothesis 3: The concentration of Hg and MeHg in sediments at gold- mined sites will be higher than at non-mined sites. Hypothesis 4: The composition of the microbiome structure will be significantly different between gold mined and non-mined sites. Results indicate that certain physical (e.g., temperature, total dissolved solids (TDS), and turbidity), chemical (pH and electrical conductivity) and habitat (% macrophytes) were significantly different between gold mined and non-mined sites (Mann-Whitney U test, p-value<0.05); however, because of the complex nature of neotropical rivers, the significant difference in specific parameters cannot be explained simply due to presence of mining activity. Elemental analysis revealed that there is a higher concentration of Au, As, Hg and MeHg in soil sediments collected from gold-mined sites than found at non-mined sites (Mann-Whitney U test, p-value<0.05. Results also revealed significant differences in microbial community structure between mined and non-mined sites on various taxonomic levels. Proteobacteria were present in greater percent relative abundance (Kruskal Wallis test, p < 0.01) at mined sites (46.23%) than the non-mined sites (34.23%), while Actinobacteria were found significantly in greater abundance (Kruskal Wallis test, p < 0.01) at non-mined sites (34.39%) than the mined sites (25.78%). Additionally, known bacteria, including Geobacter and Desulfosporosinus with mercury methylation capability were found in higher abundance at mined sites (Kruskal Wallis test, p<0.05). In conclusion, the composition of microbiome was significantly different between mined and non-mined sites, and mined sites had higher abundance of confirmed mercury methylators, which are likely to be contributing to the production of MeHg at three gold-mined sites.