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Abstract

Abstract Samples were collected from organically polluted and unpolluted groundwater of sandy and gravelly deposits. After filtration onto polycarbonate filters (0.2 µm pore size) the number of respiring bacteria was recorded by microscopically counting cells containing red INT-formazan spots, which characterize respiring bacteria. The total number of bacteria was simultaneously recorded by epifluorescence microscopy after staining with acridine orange. The number of respiring bacteria in the groundwater samples (55-490 x 10³/cm³) is within the range of values for other aquatic biotopes, but as the total number of bacteria in groundwater was usually higher, the proportion of respiring groundwater bacteria (0.66-7.4%) was lower. Mainly larger bacteria, rods, and bacteria on particles could be identified as being active, whereas hardly any respiratory activity could be detected among small cocci and free interstitial bacteria. If the supply of dissolved organic matter (DOM) is adequate, the biomass of respiring bacteria correlates well with oxygen concentration, but there is no direct correlation between DOM concentration in groundwater and active bacterial biomass. Nor could any relationship be observed between the biomass of total and respiring bacteria, or between the quantity of respiring bacteria and heterotrophic bacterial activity.