Oil symbiosis in cold seeps!

Cold seeps arise in waters rich in methane and/or sulphides which come from the geology of the ocean floor. Life exists in these deep sea hot-spots due to the activity of two types of chemoautotrophic bacteria; Sulphur oxidisers and Methane oxidisers. These bacteria are symbionts to tube worms and mussels and can be found densely packed within specialised tissues. Many cold seeps have been studied in the north of the Gulf of Mexico but not many in the south. A new type of cold seep was discovered recently which is covered with solid asphalt. Asphalt flows are formed by the seepage of hydrocarbons which settle and remain on the sea floor. This study aimed to characterise symbiotic fauna of this new site (Chapopote) using molecular methods and stable carbon isotopes analysis and comparing the results to those found in cold seeps in the north of the Gulf of Mexico.

They found that the symbiotic relationships are very similar (if not identical) to those found in the north. An Escarpia sp. tube worm was identified with two species of Bathymodiolus mussels, all of which had intracellular sulphur-oxidising symbionts that are found in association with the same species in the northern cold seeps. Both mussel species contained methane oxidising symbionts identical to those found in the north. Until now only methane and reduced sulphur compounds have been shown to power cold seep chemosynthetic symbioses. However, novel hydrocarbon-degrading bacteria belonging in the Cycloclasticus genus were found in close relation to the mussel Bathymodiolus hecherae which also had genes for the metabolism of aromatic compounds, suggesting the utilisation of heavy aromatic hydrocarbons in addition to sulphate oxidation.

This study was very interesting and links Grahams and Colins lecturers. It reinforces the opportunistic nature of ‘nature’ and provides the first evidence of hydrocarbon symbiosis in a cold seep environment. 

Raggi et al. (2012) Bacterial symbionts of Bathymodiolus mussels and Escarpia tubeworms from Chapopote, an asphalt seep in the southern Gulf of Mexico