Culture-dependent comparison of microbial diversity in deep granitic groundwater from two sites considered for a Swedish final repository of spent nuclear fuel

Site selection for a spent nuclear fuel (SNF) repository required analysis of microbial abundance and diversity at two Swedish sites, Forsmark and Laxemar-Simpevarp. Information about sulphate-reducing bacteria (SRB) was required, as sulphide could corrode copper SNF canisters. Total number of cells (TNC) and ATP were analysed, and plate counts and most probable number (MPN) analyses were conducted using eight media based on different electron donors and acceptors for specific microorganism physiological groups. Groundwater chemical composition and E(h) were analysed; sampling depths were 112-978 m below sea level. TNC was 5.5 × 10(3) to 4.7 × 10(5) cells mL(-1), correlating with ATP concentrations. Culturability in TNC percentage was 0.01-35.9, averaging 5.12. Culturable numbers varied greatly between sample positions and uncorrelated with depth. SRB were found in 29 samples and were below detection in three; the MPN of SRB correlated negatively with E(h), as did the MPN of acetogens. Data indicated that microbial sulphate reduction was ongoing in many sampled aquifers; published stable isotope data and modelling results supported this observation. The sites did not differ significantly, but the large data range suggested that analysis of more samples would enable detailed evaluation of microbial processes and their relationship with geochemical information.