The earliest microbial cells evolved in an anoxic ocean (Martin and Muller, 1998), and many argue that the earliest Eukarya arose in the absence of oxygen as well. The microbe-mediated biogeochemical processes taking place below oxic/anoxic interfaces are of undeniable importance and interest, and the Cariaco Basin off the coast of Venezuela is a superb model ecosystem in this regard. It is the world’s largest body of truly marine anoxic waters and has been almost continuously anoxic for at least the last 2M years (Schubert, 1982). Consequently, the Basin became the site of many investigations, including a long-term biogeochemical time series initiated in 1995 (Muller-Karger et al., 2001; Taylor et al., 2001) and an NSF-funded Microbial Observatory established in 2004. This Observatory was a multi-year, multi-investigator international effort. Its principal goals were: to combine molecular and cell-based approaches to survey microbial Eukarya among geochemically diverse habitats in the Basin; to analyze the community structure and its dynamics across time and space; to discover novel organisms, and to gain insights into what governs their distribution dispersal and biogeography.