Sa. Haveman et al., Distribution and metabolic diversity of microorganisms in deep igneous rock aquifers of Finland, GEOMICROB J, 16(4), 1999, pp. 277-294
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.