“Epsilonproteobacteria, it turns out, are one of the predominant groups of extremophiles in marine systems. In one environmental DNA sample taken from a hydrothermal vent, Epsilonproteobacteria represented nearly 50% of the inferred diversity (Sogin et al., 2006).”
Christopher Taylor, the curator of the Catalogue of Organisms (a regular read of mine), has an interesting post on Epsilonproteobacteria. I would just like to add that even the hydrothermal vent chemoautotrophic snail Alviniconcha hessleri contains Epsilonproteobacteria in its gills. Curiously, this has so far only been found in Indian Ocean populations. It may also end up that Alviniconcha is split into more than one species based on molecular data with some potential morphological differences.