An Asphalt Hydrocarbon Seep: a new energy source?

Chapopote is a recently discovered asphalt hydrocarbon seep (Appendix I), in the Southern Gulf of Mexico (3000m). It is different from all other hydrocarbon seeps due to the presence of solid asphalt, which contains a large component of aromatic compounds as well as hydrocarbons. Molecular morphological analyses indicated that one tubeworm (Escarpia sp.) and two mussel species (Bathymodiolus heckerae and B. brooksi) colonise Chapopote. Up until now only methane and reduced sulphur compounds have been shown to power cold- seep chemosynthetic assemblages, this study took the first steps to uncover how asphalt seep organisms metabolise and what type of symbionts they harbour.

Comparative 16S rRNA analysis and FISH analyses was used to distinguish which symbionts each organism had. Escarpia sp. has thiotrophic (sulphur- oxidising) symbionts, which is not surprising as all other vestimentiferan tubeworms investigated to date have these. Thiotrophic symbionts were also found in the mussels, as well as methane- oxidising ones (Figure 1). All of these symbionts found are highly similar or identical to those found in the same host species from northern Gulf of Mexico (nGoM). This shows that closely related chemosynthetic fauna can colonise both cold seeps in the northern and the Chapopote asphalt seep in the southern Gulf of Mexico.

Clone libraries of 16rRNA for the two Bathymodiolus species gave light to two other symbiont phylotypes, previously unknown to associate with these mussels. By using FISH, B. heckerae was shown to harbour symbionts closely related to the genus Cycloclasticus, which are specialists for aromatic hydrocarbon degradation (Figure 1). It was also found that they had key genes for the use of aromatic compounds and its stable carbon isotope values were consistently higher than B. brooksi. This indicates that this novel symbiont, first discovered in this study, might use isotopically heavy aromatic hydrocarbons from the asphalt seep as a carbon and energy source, as all previously cultured Cycloclasticus species can used PAHs in this fashion.

B. brooksi had Psychromonas in the gill tissues (Figure 1), however it was rarely found which made it hard to determine whether they occurred in or outside of the bateriocytes and also symbiotic members of this genus are currently not known. This made it unclear whether Psychromonas is a symbiont that regularly occurs in all host species, or if it is only occasionally associated with B. brooksi gill tissue. This is a potential area to be looked at closer, as if the presence of Psychromonas was not a coincidence, the implications to why it is there and what it does could have important significance.

Intriguingly, host-derived lipids in B. heckerae are isotopically heavier than those from B. brooksi, which could be explained by contribution of hydrocarbons to the host for nutrition through the PAH- degrading Cycloclasticus symbiont. However, these values should be interpreted with caution as the fixation of isotopically heavy CO₂ by the thiotrophic symbiont from Cycloclasticus respiration could also result in such values. On future expeditions to Chapopote, isotope- labelling experiments could be used to investigate this hypothesis by analysing the flow of carbon from aromatic compounds in the B. heckerae symbiosis.

Its not proved in this study that B. heckerae uses PAHs, but if so this would be the first description of an animal symbiont that can degrade environmental hydrocarbon pollutants. It would also be a significant discovery as currently only methane and sulphide have been shown to power chemosynthetic symbioses at cold seeps.

Figure 1:  a) Bathymodiolus brooksi and B. heckerae mussels with Escarpia sp. tubeworms on asphaltic sediments at Chapopote. B) Tubeworms. C) Symbionts (arrows) in cross-section through the trophosome of an Escarpia tubeworm with symbiont- specific FISH probe. D)+E) Different shell morphology of d) B. brooksi and e) B. heckerae. F - I) Symbionts in gill tissues of F- B, heckerae G- B. brooksi H- B. heckerae I- B. brooksi: methanotrophic (green), thiotrophic (pink) Cycloctasticus/ Psychromomnas (yellow).

Raggi, L., Schubotz, F., Hinrichs, K. U., Dubilier, N., & Petersen, J. M. (2013). Bacterial symbionts of Bathymodiolus mussels and Escarpia tubeworms from Chapopote, an asphalt seep in the southern Gulf of Mexico. Environmental microbiology, 15(7), 1969-1987.

Can be found: http://onlinelibrary.wiley.com/doi/10.1111/1462-2920.12051/full

Appendix I: Asphalt flows are formed by the seepage of hydrocarbons heavier than water, which remain on the seafloor providing a novel substrate for the colonisation of chemosynthetic fauna, and a rich localized source of hydrocarbons.

Asphalt contains a complex mix of hydrocarbons, including a large proportion of aromatic compounds (Corbett, 1969).