Distribution and metabolic diversity of microorganisms in deep igneous rock aquifers of Finland

Groundwater samples from 200- to 950-m depths in four igneous rock sites in Finland were investigated for different metabolic groups of microorganisms , and the data were compared with the available geochemical record. Samples were collected with a pressurized groundwater sampling system developed fo r gas and microbiological sampling. Two of the sites had groundwater that w as fresh, with <0.2 g/l dissolved solids, whereas that at the two other sit es was a much more saline, reaching a maximum of 24 g/l dissolved solids, T he groundwater contained gases, 33 to 340 ml/l, with nitrogen or methane do minating.. Total cell numbers were 10(5) to 10(6) cells/ml, which is typica l for deep igneous rock aquifers. Growth media were designed to mimic the a ctual groundwater chemistry at each sampling point and used for most probab le number enumeration of methanogens, acetogens, sulfate-reducing bacteria (SRB), and iron-reducing bacteria (IRB). SRB pre dominated in sites where i ron sulfide fracture-filling minerals are common. IRE were the main populat ion in one site where iron sulfide fracture minerals are not present, brit iron hydroxide fracture minerals predominate. Fracture-filling minerals wer e a better indicator of microbial populations than was groundwater chemistr y;. Low numbers of autotrophic methanogens were cultured. One of several po ssible interpretations of stable isotope data suggested that most of the de tected methane is thermogenic, which would correlate with few active methan ogens. However ave concluded other interpretations were also possible.