Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

The deep continental biosphere is supported by chemolithoautotrophy and depends on rock-derived substrates for energy. The majority of microorganisms in these crustal environments are likely attached to mineral surfaces within rock fractures, making characterization of deep life challenging. To better understand both biogeochemical cycling and mineral-hosted microbial communities in the deep subsurface, we characterized naturally occurring mineral particulate and associated biomass collected from boreholes drilled into a 2.7 Ga banded iron formation within the southern Canadian Shield. Particulate mineralogy was characterized via X-ray diffraction and Fe X-ray absorption near edge structure (XANES) spectroscopy. The particulate in one borehole was identified as a mixture of hematite and quartz, while the other borehole contained a mixture of the iron sulfides mackinawite and greigite, suggesting an active sulfur cycle mediated by microbial activity. Carbon associated with the particulate was imaged via scanning transmission X-ray microscopy and characterized via C XANES spectroscopy. In both boreholes, the particulate was colonized by microbial cells; many samples contained abundant biofilm. The cells and biofilm were chemically distinct, with the C XANES spectra for the cells consisting primarily of a protein-like signal and the biofilm resembling a mixture of protein, saccharide, and lipid. In the borehole containing the sulfidic particulate, the abundance of cells and biofilm increased with sample depth. Mineral particulate found in boreholes, whether forming in situ or as a result of drilling and weathering, are a valuable way to access the deep subsurface without the contamination and disturbance caused by drilling new cores. To better understand the microbial community composition and function associated with the particulate-biofilm aggregates and surrounding groundwater, filtered-water and particulate samples were characterized via shotgun metagenomic sequencing. Metagenomic analyses showed that while microbial communities were distinct between boreholes, all communities contained the genetic potential for the oxidation and reduction of a variety of sulfur phases. This suggests that the biogeochemical cycling of S, potentially connected to Fe cycling in this iron-rich habitat, could be fueling life in deep crustal environments.