Assimilation of CO2 and introduced organic compounds by bacterial communities in groundwater from southeastern Sweden deep crystalline bedrock

The nutritional responses of unattached and attached bacterial communities were studied in groundwater from 3 sampling depths, i.e., 830–841 m, 910–921 m, and 999–1,078 m, of the subvertical borehole KLX01 at the Laxemar study area in SE Sweden. The salinity profile of the groundwater in this borehole is homogeneous. There were negative redox potentials (Eh) in the waters (–220 to –270 mV) and they contained sulfide, hydrogen, and methane. Biofilm reactors with hydrophilic glass surfaces were connected to the flowing groundwaters from each of the 3 depths with flow rates of approximately 3 x 10^–3 m sec^–1 over 19 days. There were 0.15 to 0.68 × 10⁵ unattached bacteria ml^–1 groundwater and 0.94 to 1.2 × 10⁵ attached bacteria cm^–2 on the surfaces. The assimilations of ¹⁴CO₂, ¹⁴C-formate, 1,2,3-³H-acetate, U-¹⁴C-lactate, U-¹⁴C-glucose, and L-4,5-³H-leucine by the communities were demonstrated with microautoradiographic and liquid scintillation counting techniques. There were significant assimilations of CO₂ by all communities, except for the unattached bacteria at the 910–921 m depth, indicating in situ production of organic carbon from carbonate. Assimilation of formate was detected in two communities, indicating the presence of bacteria able to substitute CO₂ with formate. Acetate, lactate, and glucose assimilations demonstrated the presence of heterotrophic bacteria. The assimilation of lactate by the attached bacteria dominated over acetate and glucose at all depths. Leucine was assimilated by 20 to 98% of the communities, which showed that major portions of the communities studied were viable. The results indicate that the attached communities at the 830–841 m and 910–921 m depths were in more metabolically active states than the unattached bacteria. Incubation in air compared with N₂ indicated that portions of the studied communities were obligate anaerobes, as their ability to assimilate the added compounds was sensitive to oxygen. The results show that the use of several different compounds reduces the risk for false conclusions about the viability and the metabolic activity of the deep groundwater communities.Offprint requests to: K. Pedersen.