In recent years, our group has been studying the microbial communities of the water column associated with the oxygen minimum zone (OMZ) off northern Chile and Peru, trying to understand who the main players are in the OMZ biogeochemical cycles and which metabolic strategies they are using. For that, we are combining flow cytometry, molecular techniques and biogeochemical approaches. Most of our work, so far, has focused on microbes capable of doing oxygenic photosynthesis, such as the cyanobacteria, and those involved in the nitrogen cycle, such as the denitrifying, nitrifying and anammox bacteria. We have also started to look at the more general microbial abundance and diversity. In each case, we are finding that the OMZ holds distinct microbial communities, either from a phylogenetic or a functional perspective. In this presentation we will give a summary of our main results.