Distribution and activity of bacteria in deep granitic groundwaters of southeastern sweden

Abstract

This study investigated the distribution of bacteria in groundwater from 16 different levels in five boreholes in granite bedrock down to a maximum of 860 m. Enrichment cultures were used to assay the groups of bacteria present. Autoradiographic studies with(14)C- or(3)H-labeled formate, methanol, acetate, lactate, glucose, sodium bicarbonate, leucine, glutamine, thymidine, orN-acetyl-glucosamine were used to obtain information about bacteria active in substrate uptake. The biofilm formation potential was studied in one borehole. The chemical environment in the groundwater was anaerobic with an Eh between -112 and -383 mV, a pH usually around 8, and a temperature range of 10.2 to 20.5°C, depending on the depth. The organic content ranged between <0.5 and 9.5 mg total organic carbon liter(-1). Carbon dioxide, hydrogen, hydrogen sulfide, and methane were present in the water. The nitrate, nitrite, and phosphate concentrations were close to, or below, the detection limits, while there were detectable amounts of NH4 (+) in the range of 4 to 330 μg liter(-1). The average total number of bacteria was 2.6×10(5) bacteria ml(-1), as determined with an acridine organge direct-count (AODC) technique. The average number of bacteria that grew on a medium with 1.5 g liter(-1) of organic substrate was 7.7×10(3) colony-forming units (CFU) ml(-1). The majority of these were facultatively anaerobic, gram-negative, nonfermenting heterotrophs. Enrichment cultures indicated the presence of anaerobic bacteria capable of growth on C-1 compounds and hydrogen, presumably methanogenic bacteria. Most probable number assays with sulfate and lactate revealed up to 5.6×10(4) viable sulfate-reducing bacteria per ml. A biofilm development experiment indicated an active attached microbial population. Active substrate uptake could not be registered with the bulk water populations, except for an uptake of leucine not associated with growth. The bulk water microbial cells in deep groundwater may be inactive cells detached from active biofilms on the rock surface.