Two new vent-systems on the Mid-Cayman Rise each exhibit novel geologic settings and distinctively hydrogen-rich vent fluid compositions. We have determined and compared the chemistry, energy landscape, abundance, community composition, diversity, and function of microbes in venting fluids from both sites: Piccard, the world?s deepest vent site-site, hosted in mafic rocks, and Von Damm, an adjacent, ultramafic-influenced system.