Microbial functional responses to water quality alterations in a landfill-impacted aquifer

Aquifers represent the largest accessible and fragile resource of freshwater on Earth. The municipal solid waste disposal in landfills, followed by the dispersion of leachate, is among the major threats for groundwater ecosystems, which harbour different microbial taxa generally adapted to low availability of organic carbon and nutrients. The aim of this study was to explore how groundwater quality can influence major microbial community characteristics, and particularly the functional diversity and metabolic responsiveness to a spectrum of organic carbon inputs. We hypothesized that the landfill-induced water quality changes will represent relevant stress factor for the ecological dynamics of groundwater microbial communities. A comprehensive study of a deep aquifer underlying a municipal solid waste landfill was performed in Central Italy across a network of twelve piezometers. Along with major physical-chemical parameters, the microbial community structure and the microbiological contamination (total coliforms and Escherichia coli), were assessed respectively by flow cytometry and the test Colilert-18. The BIOLOG EcoPlates(TM) assay was used to analyze the metabolic potential and the organic substrate utilization profiles. Three groundwater groups with distinct hydrogeochemical characteristics (pristine, mixed and altered conditions) were identified across the landfill area. Relatively higher total cell counts and the presence of E. coli under altered conditions suggested the occurrence of land-derived inputs. The kinetic analysis of the BIOLOG profiles showed that the aquatic microorganisms in samples from altered conditions were relatively more efficient in metabolizing a larger number of organic substrates, including complex structural molecules of xenobiotic origin. Our outcomes showed that the landfill-induced groundwater quality changes can directly affect the oxidation-reduction potential and the overall carbon cycle. Moreover, a multidisciplinary methodological approach is likely to represent a supportive tool to better characterize the impact of groundwater quality alterations, with potential consequences on the ecosystem functioning.