Seafloor hydrothermal activity dominates heat and chemical exchange between seawater and ocean rocks, however the extent to which diffuse venting contributes to these processes is poorly constrained. The low-temperature, ultra-diffuse venting and extensive iron mats at the FeMO Deep (~0.2°C anomaly) and Shinkai Deep (~6°C anomaly) sites on the flanks of Loihi Seamount provides a unique opportunity to study Fe, C, N, and S cycling in modern analogues to ancient seafloor hematitic formations that are preserved in the rock record. While aerobic microbial activity near the mat surface is responsible for much of the iron deposition, these microaerophiles are limited to the uppermost suboxic zone. Most of the mat thickness is characterized by anoxic conditions, and we hypothesize that anaerobic metabolisms dominate in these zones. Here we propose preliminary studies to characterize this cryptic anaerobic community. We will use isotope-tracer incubation experiments, functional gene analyses and a silver sulfide capture technique to quantify S-, N-, and C-based microbial metabolisms in the anoxic mat interior. Coupled to our ongoing geochemical and bioenergetic efforts, this work will lead to a coupled microbial/geochemical model at FeMO Deep style mats that elucidates the role these systems play in global biogeochemical cycles.