This paper presents and compares the assimilation rates of CO and lactate, and the lactate respiration rates, of attached bacterial populations growing in slowly flowing groundwater (1-3 mm s) from deep crystalline bed-rock of the Stripa research mine, Sweden. The bacteria studied grew in anoxic, high-pH (9-10) and low-redox artesian groundwater flowing up through tubing from two levels of a borehole designated V2, 812-820 m and 970-1240 m below ground. Bacteria were allowed to attach to and grow on sterile glass microscope slides in laminar-flow reactors connected to the flowing groundwater. Total numbers of bacteria were counted by acridine orange direct counts. The bacteria grew slowly, with doubling times of 34 d at 10 °C for the 812-820 m population, 23 d for the 970-1240 m population at 10 °C and 16 d for this population at 20 °C. Numbers of attached bacteria reached between 10 and 10 bacteria cm. The populations at the two levels of the borehole were different in physiology as well as in phylogeny and reflected the heterogeneity between the sampling levels. The earlier proposed presence of sulphate-reducing bacteria could not be confirmed. This is discussed in relation to results from 16S rRNA gene sequencing studies. The CO assimilation rates (as mol CO cm h, using liquid scintillation techniques) increased with depth and temperature. The quotients calculated for inorganic/organic carbon utilization were between 0.07 and 0.25, indicating that autotrophy could not support the levels of growth observed and that heterotrophy was the dominant carbon transformation process for growth of the studied populations. The Stripa bacteria could further be seen not only to assimilate but also to catabolize lactate and release CO, which added to the indications of a heterotrophic dominance in the Stripa environment.


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