CARBON TRANSFORMATIONS BY ATTACHED BACTERIAL-POPULATIONS IN GRANITIC GROUNDWATER FROM DEEP CRYSTALLINE BED-ROCK OF THE STRIPA RESEARCH MINE

This paper presents and compares the assimilation rates of CO2 and lac tate, and the lactate respiration rates, of attached bacterial populat ions growing in slowly flowing groundwater (1-3 mm s(-1)) from deep cr ystalline bed-rock of the Stripa research mine, Sweden. The bacteria s tudied grew in anoxic, high-ph (9-10) and low-redox artesian groundwat er 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 at tach to and grow on sterile glass microscope slides in laminar-flow re aders connected to the flowing groundwater. Total numbers of bacteria were counted by acridine orange direct counts. The bacteria grew slowl y, with doubling times of 34 d at 10 degrees C for the 812-820 m popul ation, 23 d for the 970-1240 m population at 10 degrees C and 16 d for this population at 20 degrees C. Numbers of attached bacteria reached between 10(6) and 10(7) bacteria cm(-2). The populations at the two l evels of the borehole were different in physiology as well as in phylo geny and reflected the heterogeneity between the sampling levels. The earlier proposed presence of sulphate-reducing bacteria could not be c onfirmed. This is discussed in relation to results from 16S rRNA gene sequencing studies. The CO2 assimilation rates (as mol CO2 cm(-2) h(-1 ), using liquid scintillation techniques) increased with depth and tem perature. The quotients calculated for inorganic/organic carbon utiliz ation were between 0.07 and 0.25, indicating that autotrophy could not support the levels of growth observed and that heterotrophy was the d ominant carbon transformation process for growth of the studied popula tions. The Stripa bacteria could further be seen not only to assimilat e but also to catabolize lactate and release CO2 which added to the in dications of a heterotrophic dominance in the Stripa environment.