SUMMARY: The incorporation of CO and assimilation of introduced organic compounds by bacterial populations in deep groundwater from fractured crystalline bedrock has been studied. Three depth horizons of the subvertical borehole V2 in the Stripa mine, Sweden, 799-807 m, 812-820 m and 970-1240 m, were sampled. The groundwaters, obtained from fracture systems without close hydraulic connections, were anoxic and had the following physicochemical characteristics: pH values of 9·5, 9·4 and 10·2; values of + 205, + 199 and—3 mV; sulphide, 0, 106 and 233 μM; CO , 158, 50 and 57 μ; CH, 245, 170 and 290 μl l; and N, 25, 31 and 25 ml l. Biofilm reactors, each containing a series of parallel glass surfaces, were connected to the groundwaters issuing from these depth horizons at flows of approximately 1×10 m s during two periods of two and four months. There were from 1·8×10 to 1·2×10 bacteria per ml groundwater and from 1·2×10 to 7·1×10 bacteria per cm of colonized test surface. These results imply that the populations of attached bacteria are several orders of magnitude greater than those of unattached bacteria in bedrock fractures with flowing groundwater. The incorporation of CO, [C]formate, [U-C]lactate, [U-C]glucose and -[4,5-H]leucine by the bacterial populations was demonstrated using microautoradiographic and liquid scintillation counting techniques. The measured CO incorporation reflected the production of organic carbon from CO. Incorporation of formate followed that of CO and indicated the presence of bacteria able to substitute formate for CO, e.g. methanogenic bacteria. The presence of sulphate-reducing bacteria is suggested by the observed incorporation of lactate by up to 74% of the bacterial populations. The recorded uptake of glucose indicates the presence of heterotrophic bacteria other than sulphate-reducing bacteria. Up to 99% of the populations incorporated leucine, showing that major fractions of the populations were viable.


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