A decade-long study of the Cariaco Basin has demonstrated that the suboxic interface in this system is temporally and spatially variable and that it is the home for a diverse and active microbial community. The thickness and structure of the suboxic zone appears to be primarily controlled by the physics of the basin, with oxygen containing intrusions occurring from both eastern and western sills. The microbial populations, which include both heterotrophic and chemoautotrophic micro-organisms, are actively growing at rates which cannot be supported by vertical transport of oxidants or reductants. Our studies suggest that the microbial loop depends on cycling of intermediate oxidation state sulfur species (sulfite, thiosulfate and elemental S) possibly coupled with cycling of metal oxides. Production of these species is maintained by the active intrusion process and by transport from land of iron and metal oxides during rain events.