Anoxic marine microbe communities from the Eastern Tropical North Pacific were analyzed using culture-independent molecular methods. Community composition and associations were determined with pyrosequencing and phylotype analysis, followed by species distribution modeling and exploratory statistics. Real-time quantitative polymerase chain reaction (PCR) assays were used to quantify activity and distribution of key sulfur- and nitrogen-cycling groups within the oxygen minimum zone (OMZ). Strong correlations were found between putative sulfur-oxidizing groups and known nitrite-oxidizing bacteria. Quantitative PCR assays confirmed the activity of sulfate- reducing genes, strong evidence for cryptic sulfur cycling.