Chemolithoautotrophic iron-oxidizing bacteria (FeOB) play an essential role in the global iron cycle. Thus far, the majority of marine FeOB have been associated with a novel class, the Zetaproteobacteria, within the phylum Proteobacteria. Marine iron-oxidizing microbial communities have been found at volcanically active seamounts, crustal spreading centers, and in coastal waters. One conspicuous absence of FeOB is at hydrothermal systems associated with the slow-crustal spreading center along the Mid-Atlantic Ridge (MAR). From October to November 2012 samples were collected from rust-colored mats at three well-known hydrothermal vent systems at the MAR (Rainbow, TAG, and Snakepit) using the ROV Jason II. Samples were interrogated for community composition uning 16S amplicon pyrosequencing.