Abstract

<jats:p> Basalt aquifers are common in many parts of Canada and the United States; however, little is known about their microbial ecology. Microbial populations were characterized at a basalt aquifer with 7 m of clay and silt in the vadose zone and a saturated zone extending from 7 to 500 m. Planktonic total counts and viable counts were 1.7 × 10<jats:sup>5</jats:sup> and 2.3 × 10<jats:sup>4</jats:sup> cells/mL, while sessile total counts and viable counts were 1.9 × 10<jats:sup>7</jats:sup> and 1.7 × 10<jats:sup>5</jats:sup> cells/g, respectively. Some of the physical and chemical habitat conditions measured were pH (7.1), E<jats:sub>h</jats:sub> (135 mV), dissolved O<jats:sub>2</jats:sub> (4.1 mg/L), temperature (10 °C), NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> (4 μM), NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> (253 μM), Fe (0.17 μM), and Mn (36 nM). Variation in physical and chemical variables correlated with depth and the region of the aquifer sampled. One hundred forty-nine random isolates of the dominant culturable bacteria from groundwater and subsurface cores were submitted to phenotypic analysis on the basis of antibiotic responses. The resultant dominant bacteria were Gram-negative, mesophilic heterotrophs. Cluster analysis revealed at least 14 discernible clusters containing representatives from the genera Pseudomonas, Bacillus, Acinetobacter, Arthrobacter, Micrococcus, and Clavibacter. Centrotype representatives were selected from each cluster and tentatively identified.Key words: basalt aquifer bacterial ecology. </jats:p>