Abstract
Summary Chemosynthetic life was recently discovered at C hapopote, an asphalt hydrocarbon seep in the southern G ulf of M exico. Preliminary morphological analyses indicated that one tubeworm and two mussel species colonize C hapopote. Our molecular analyses identified the tubeworm as E scarpia sp., and the mussels as B athymodiolus heckerae and B . brooksi . Comparative 16 S rRNA analysis and FISH showed that all three species harbour intracellular sulfur‐oxidizing symbionts highly similar or identical to those found in the same host species from northern G ulf of M exico ( nGoM ). The mussels also harbour methane‐oxidizing symbionts, and these shared highly similar to identical 16 S rRNA sequences to their nGoM conspecifics. We discovered a novel symbiont in B . heckerae , which is closely related to hydrocarbon‐degrading bacteria of the genus C ycloclasticus . In B . heckerae , we found key genes for the use of aromatic compounds, and its stable carbon isotope values were consistently higher than B . brooksi , indicating that the novel symbiont might use isotopically heavy aromatic hydrocarbons from the asphalt seep. This discovery is particularly intriguing because until now only methane and reduced sulfur compounds have been shown to power cold‐seep chemosynthetic symbioses. The abundant hydrocarbons available at C hapopote would provide these mussel symbioses with a rich source of nutrition.