Microbial community analysis of a deep-sea volcanic and hydrothermal system at Vailulu'u Seamount yielded 89 new organisms and three detailed 16S-rRNA gene clone libraries (one rock and two microbial mats). Proteobacterial communities dominate in most environments, but important differences are found between microbial mats from distinctly different geochemical environments and for the rock surface. Many cultured organisms are metabolically and functionally diverse, displaying at least two of the tested functions: heterotrophy, Fe(II) and Mn(II) oxidation, and siderophore-production. Metabolic versatility of microorganisms is suggested as an important trait allowing diverse populations of bacteria to adapt to these environments.