Microbial communities in deep Canadian shield groundwaters—an in situ biofilm experiment

Microbial biofilm communities were cultivated on stainless steel and polypropylene surfaces within brine‐filled exploration boreholes in crystalline rocks of the Canadian Shield at Kidd Creek Mine (Timmins, Ontario) and Copper Cliff South Mine (Sudbury, Ontario) at depths of 1402 m and 1219 m, respectively. The calcium‐sodium‐chloride brines were acidic (pH 3.5–4.8) and had temperatures of 23.4°C to 18.6°C. Direct microscopic counts using DAPI (4’,6‐diamidino‐2‐phenylindole) epifluorescent staining revealed coccoid‐shaped, vibroid, rod‐shaped, and filamentous bacteria in Copper Cliff South biofilms. The same morphological types were observed at Kidd Creek with the addition of a group of large (2–3 μm in length) vibroid to rod‐shaped bacteria. Average total counts were 9 ±7 × 106 bacteria/cm2 for Copper Cliff South biofilms and 5 ± 1 × 106 bacteria/cm2 for Kidd Creek biofilms with a mean biomass value of 131 ± 35 μg carbohydrate/cm2. Transmission electron microscopy revealed slightly vibroid, rod‐shaped, gram‐negative cells (up to 2.3 μm in length) at Kidd Creek Mine. At Copper Cliff South Mine, small gram‐negative coccoid‐shaped bacteria were observed (<0.5 μm in diameter). Many of the bacteria contained large polyhydroxybutyrate storage inclusions and were commonly mineralized by amorphous precipitates exhibiting a typical iron oxide morphology. The accumulation of these iron oxide precipitates in the biofilms suggests that Fe2+‐oxidizing bacteria are present. Copper Cliff South ground‐waters contained sufficient concentrations of sulfate and nitrate to support sulfate‐reducing and denitrifying bacteria, whereas Kidd Creek groundwaters did not. These results show that diverse populations of biofilm‐forming bacteria exist in deep groundwaters at Copper Cliff South and Kidd Creek Mines.