Porous-rock and fracture associated microbial communities involved in nitrate reduction in the karstified and fractured aquifer of the ‘Bronnbachquelle’ catchment (SW Germany)

• Applicant: Professorin Dr. Sara Kleindienst, Ph.D.
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 517714013

Karstified, fractured subsurface aquifers are important sources of drinking water. However, the extensive use of agricultural fertilizers has led to nitrate infiltration into this environment. The consumption of nitrate-containing drinking water could lead to health problems, and the European Union has determined that nitrogen concentrations in drinking water must be less than 50 mg/L to be potable. However, in agricultural regions, nitrate concentrations often exceed this threshold. For example, nitrate concentrations in the Ammer River catchment (Germany) have been up to 60 mg/L, whereas discharge areas only contained 1 mg/L of nitrate. This observed gradient suggests intensive denitrification. In subsurface oligotrophic environments, microorganisms can couple nitrate reduction to iron/sulfur oxidation, e.g., using pyrite (FeS2) within the rock. Determining spatial hot spots where microbes play an important role in denitrification within the subsurface is challenging and, thus, it is unknown if denitrification occurs only in fractures or also in the porous rock matrix. To reveal hot spots of subsurface microbial activity, we retrieved rock cores from 70 m depths – the saturated zone of the catchment of the “Bronnbachquelle” karstic spring, which is a tributary of the River Neckar (Germany). The unique borehole without casing was used to establish a groundwater well. For this project, the first goal is to identify the taxonomy and functional capabilities of denitrifying microorganisms inhabiting the carbonate rock obtained from the ‘Bronnbachquelle’ catchment. The second goal is to characterize the microbial colonization of pyrite-containing artificial and natural porous rock matrixes and the rate of pyrite oxidation using laboratory experiments. For this in-vitro study, microorganisms that have previously been enriched from carbonate rocks and are capable of coupled nitrate-reduction to (iron/sulfur)-oxidization will be used. The third goal is to evaluate the rate of in situ pyrite oxidation within the artificial and natural porous rock matrixes. This will be achieved by long-term incubation of pyrite-containing microbial trapping devises (MTDs; including rock matrixes) in the newly-installed groundwater well, and by monitoring the changes in composition and functions of microbial communities colonizing the MTDs. The combined results of field and laboratory work will allow us to reveal: i) the environmental niches of nitrate-reducing microorganisms in the aquifer of the ‘Bronnbachquelle’ catchment, ii) the key microorganisms responsible for nitrate turnover, iii) the metabolic characteristics and functions of nitrate-reducing microorganisms, iv) the potential of subsurface denitrifiers to colonize the porous rock matrix, v) the factors affecting the efficiency of porous- and fracture-associated denitrification, and v) the rate of pyrite oxidation within the porous rock matrix.

• DFG Programme: Research Grants