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 for gas and microbiological sampling. Two of the sites had groundwater that was fresh, with <0.2 g/l dissolved solids, whereas that at the two other sites was a much more saline, reaching a maximum of 24 g/l dissolved solids, The groundwater contained gases, 33 to 340 ml/l, with nitrogen or methane dominating.. Total cell numbers were 10(5) to 10(6) cells/ml, which is typical for deep igneous rock aquifers. Growth media were designed to mimic the actual groundwater chemistry at each sampling point and used for most probable number enumeration of methanogens, acetogens, sulfate-reducing bacteria (SRB), and iron-reducing bacteria (IRB). SRB pre dominated in sites where iron sulfide fracture-filling minerals are common. IRE were the main population in one site where iron sulfide fracture minerals are not present, brit iron hydroxide fracture minerals predominate. Fracture-filling minerals were a better indicator of microbial populations than was groundwater chemistry;. Low numbers of autotrophic methanogens were cultured. One of several possible interpretations of stable isotope data suggested that most of the detected methane is thermogenic, which would correlate with few active methanogens. However ave concluded other interpretations were also possible.