Microbial Diversity and Biogeochemistry of a Shallow Pristine Canadian Shield Groundwater System

An investigation was conducted to assess the microbial diversity and biogeochemistry of a pristine shallow glaciofluvial groundwater system on the Canadian Shield. Vadose zone sands were very fine-grained, consisting mostly of iron oxide-coated quartz. Below the water table in the saturated zone, iron oxide-coating on quartz grains were absent, owing presumably to reductive dissolution by Fe(III)-reducing bacteria and chemical weathering, respectively. Groundwater pH was circumneutral at 6.4 + 0.1, and Eh was slightly reducing at 349 +/- 15mV compared to local surface waters at 417 +/- 21mV; the total dissolved solids concentration in groundwater was 45.2 +/- 6.9mg/L. A total of 269 clones were sequenced and compiled into a 16S rRNA gene library, and representative operational taxonomic units (OTU) were retrieved from basic local alignment search tool (BLAST) analyses at the species-level across 7 phyla, including Acidobacteria, Actinobacteria, Bacteroidetes, Chlorobi, Firmicutes, Planctomycetes, and Proteobacteria. Aerobic, anaerobic, and facultative anaerobic heterotrophs were predominant among the OTU. Lithoautotrophic ammonium- and nitrite-oxidizers were especially prominent, as were diazotrophs, nitrate- and nitrite-reducers. Representative OTU also included Fe(II)-oxidizers and Fe(III)-reducers, whereas those associated with sulfur cycling were rare. These observations suggest that there is considerable potential for biogeochemical cycling of nitrogen and iron within shallow glaciofluvial groundwater systems on the Canadian Shield.