Bacterial community composition and its relationship to oxygen were investigated in the non-sulfidic shelf waters exposed to seasonal upwelling and oxygen deficiency off central Chile. Using 16S rRNA gene clone libraries, terminal restriction fragment length polymorphism (T-RFLP) and iTAG sequencing analyses of nearly 4 years of monthly sampling through the water column (5-80 m depth), we found a clear partitioning of community composition that could be attributed to dissolved oxygen (Spearman correlation coefficient = 0.578) in comparison to other environmental variables, such as light, fluorescence, temperature, salinity, microbial abundance or nutrients. Bacteroidetes (orders Sphingobacteriales and Flavobacteriales), SAR11 (subclades Ia - Candidatus Pelagibacter ubique and II), an unclassified group of Alphaproteobacteria, and SAR86 dominated in waters containing dissolved oxygen concentrations >= 70 mu M. In contrast, taxonomic groups associated with capabilities of either oxidative or reductive cycling of inorganic sulfur dominated in waters with dissolved oxygen from similar to 70 mu M to undetectable levels. The dominant groups were Arctic96BD-19, SUP05 and SAR324 (sulfur oxidation) and Desulfobacterales (sulfate reduction). Desulfobacterales are known to come primarily from sulfidic, nitrate/nitrite-depleted waters and sediments. Their presence, therefore, provides taxonomic evidence for pelagic dissimilatory sulfate reduction within oxygen-depleted coastal environments.