The Cariaco Basin is the world’s largest truly marine permanently anoxic basin. It exhibits pronounced vertical chemical and biological gradients controlled by physical transport of oxygen downward and reduced compounds upward countered by biological demands. Typically, oxygen concentrations decrease from saturation at the surface to 0 between 250 and 350 m and waters below remain anoxic and sulfidic to the basin’s floor (≈ 1,400 m) over timescales of centuries to millennia. This water column is characterized by pronounced and predictable vertical layering of microbial communities and productivit.  Significant enrichments in abundances of prokaryotes, protozoans and viruses and prokaryotic productivity are routinely observed in the sulfidic waters immediately underlying the oxic/anoxic interface. The basin is  characterized by strong seasonal  and vertical fluctuations in organic carbon production and in carbon flux to the basin. This is due to seasonal upwelling events that occur annually between January and May. As a result of increased nutrient supply, both primary productivity and POC flux tend to be much higher between January and May than between June and December. Peaks of chemoautotrophic production are below the suboxic/anoxic interface of the basin. The Cariaco Basin is sampled in collaboration with S. Epstein (NEU, Boston), V. Edgcomb (Woods Hole Oceanographic Institution, Woods Hole), G.T. Taylor (Stony Brook, NY) and R. Varela (Estacion de Investigaciónes Mariñas de Margarita, Venezuela).