Use of an ATP assay to determine viable microbial biomass in Fennoscandian Shield groundwater from depths of 3–1000 m

Bioluminescent biomass in pure cultures and in 94 samples of shallow and deep Fennoscandian Shield groundwater was analysed using a commercial ATP assay, and the results were compared with microscopic counts and counts based on cultivation methods. The assay appeared robust and reliable and had a detection range that covered all samples analysed. The detection limit in groundwater was determined to 2 × 103 cells ml− 1. ATP concentrations were found to correlate with the microscopic counts and with the volume and metabolic status of the investigated pure culture and groundwater cells. The results suggested that bacterial populations in deep groundwater vary significantly in size, and that metabolic activity is a function of prevailing environmental conditions. In cases in which analysis of the total and viable number of cells produced very low numbers, suggesting that the detected cells were of low viability, ATP analysis of the ratio of ATP to the total number of cells was able to verify such an interpretation of the obtained data.