Contaminants from a former Croatian coal sludge dictate the structure of microbiota in the estuarine (Rasa Bay) sediment and soil

Croatian Rasa coal had been mined for nearly 400 years. The release of hazardous trace elements (HTEs) and toxic organic pollutants (TOPs) into the local environment by coal mining, preparation, and combustion activities has resulted in pollution. In this study, the diversity and composition of microbial communities in estuarine sediment and soil samples as well as community function responses to the pollutants were investigated for the first time. Samples were collected from an estuarine site (north Adriatic Sea) that had been contaminated by coal sludge (wastewater from coal washing). The results showed that following 60 years of natural attenuation, the location is still heavily polluted by polycyclic aromatic hydrocarbons (PAHs) and HTEs. Microbial analyses have shown that high concentrations of PAHs have reduced the diversity and richness of microbial communities at the site. PICRUSt analysis also suggests that the native communities in this area contain microbes capable of degradation of PAHs and sulfur-containing organic compounds. Notably, the microorganisms that can degrade these substances may have been enriched as a result of the presence of organic pollutants. The results of this study revealed an adverse, long-term impact of coal waste pollution on the microbial community structure and function in the brackish aquatic ecosystem. This study could provide a basis for the monitoring and restoration of ecosystems impacted by coal mining activities in light of the expected decommission of a large number of coal plants on a global scale in the coming years due to growing global climate change concerns.