Use of Microbial Biofilms to Assess Groundwater Quality in Karstic Ecosystems

Karstic aquifers are particularly vulnerable to contaminations but, unlike surface waters, biological indicators of their groundwater quality are lacking. We propose a methodology based on microbial biofilms developed on artificial substrates (clay beads) to evaluate groundwater quality. Clay beads were incubated every 2 months over 18 months in three karstic stations characterised by contrasting nutrient and organic matter inputs from their catchments. After every 2-month incubation, microbial biofilms on clay beads were analysed for biomass, hydrolytic and dehydrogenase activities, and prokaryotic community structure. NH₄ ⁺, NO₃ ^-, PO₄ ^3- and dissolved organic carbon (DOC) concentrations were also measured in groundwater during the experiment. Biofilm biomass and activities were not positively correlated with nutrient and DOC concentrations in groundwater probably due to biofilm growth inhibition by antibiotics in the station having the highest nutrient and DOC concentrations. In contrast, the diversity of prokaryotes in biofilms was positively correlated with nutrient and DOC availability. In the studied heterotrophic karstic stations, the quantity of resources originating from the catchments determined the diversity but also the prokaryotic community structures of biofilms. We selected several microbial taxa as potential indicators of groundwater quality. The next step will be to test their applicability in other karstic ecosystems.