Microbial Energetics and Redox Conditions within a Groundwater Flow Field of a Pristine Oligotrophic Aquifer

English Abstract:

Shallow uncontaminated pristine aquifers are veritable energy deserts that comprise an extreme environment for microbial life. In these systems, natural communities of microorganisms are constrained to various extents by the availability of energy yielding electron donors, as well as terminal electron acceptors, that are essential to the survival of microbial life. This study examines spatial relationships between aquifer hydrogeology and the distributions of redox active chemical species essential for microbial metabolism. The study site at Meilleur’s Bay is located in the Grenville Province of the Canadian Shield, 10 km west of Deep River in the region of Laurentian Hills, Ontario, Canada. Precambrian monzonitic gneiss in the area is overlain by unconsolidated sediments of late-glacial and post-glacial origin. Till and course gravel grade into fluvial sand, which was deposited during the drainage of the proto-Great Lakes between 6,000 to 11,200 years ago. The aquifer was evaluated on the basis of common metabolic redox indicator species. Sequential zones of O₂ reduction, NO₃^- reduction, Fe³⁺ reduction, oxygen-dependent Fe²⁺ oxidation, and nitrate-dependent Fe²⁺ oxidation were identified. The zonation is consistent with 16S rDNA profiles that reveal the presence of a metabolically diverse microbial community in the aquifer. In spatial terms, the distribution of metabolic redox zones corresponds to variations in the groundwater flow field that regulate the availability of electron donors and acceptors.