Embedded deep within northern Minnesota’s expansive Iron Range, the Soudan Iron Mine transects massive veins of hematite and Archaean (2.7 Gy) banded iron formations, reaching a depth of 713 m (2,341 ft) below the surface. Now an actively maintained State Park serving to preserve Minnesota’s oldest and deepest underground mine, Soudan provides year-round access to the deep terrestrial subsurface and the unique microbial communities it hosts.
Before mining ceased in 1962, vertical and horizontal exploratory cores were taken at the mine’s lowest level (Level 27, 713 m depth). Today, these boreholes act as conduits for anoxic, low flow, saline groundwater in an otherwise dry mine. Calcium chloride brines emanate from seeps with ionic strengths up to three times saltier than seawater, low oxidation-reduction potentials, circumneutral pH, and low concentrations of organic electron donors. Despite carbon limitation, anoxic brines contain reduced metals and continuously evolve methane gas, raising fundamental questions about the microbes responsible for primary production and carbon flux in this subsurface ecosystem.
Our project takes a shotgun metagenomics approach to address fundamental questions about the biogeochemical roles of Soudan’s microbial life and how these processes structure subsurface food webs. Having taken samples from several different borehole seeps on Level 27, we specifically focused on understanding how redox potential constrains the metabolism of microbial communities, identifying the microorganisms responsible by leveraging cutting edge, post-assembly genome reconstruction methods, and building conceptual metabolic food webs of subsurface communities based on previous ribosomal sequencing efforts and physiological insights gained from genome annotation.