Diversity, relative abundance and metabolic potential of bacterial endosymbionts in three Bathymodiolus mussel species from cold seeps in the Gulf of Mexico

Abstract

Cold seeps in the Gulf of Mexico are often dominated by mussels of the genus Bathymodiolus that harbour symbiotic bacteria in their gills. In this study, we analysed symbiont diversity, abundance and metabolic potential in three mussel species from the northern Gulf of Mexico: Bathymodiolus heckerae from the West Florida Escarpment, Bathymodiolus brooksi from Atwater Valley and Alaminos Canyon, and 'Bathymodiolus' childressi, which co-occurs with B. brooksi in Alaminos Canyon. Comparative 16S rRNA sequence analysis confirmed a single methanotroph-related symbiont in 'B.' childressi and a dual symbiosis with a methanotroph- and thiotroph-related symbiont in B. brooksi. A previously unknown diversity of four co-occurring symbionts was discovered in B. heckerae: a methanotroph, two phylogenetically distinct thiotrophs and a methylotroph-related phylotype not previously described from any marine invertebrate symbiosis. A gene characteristic of methane-oxidzing bacteria, pmoA, was identified in all three mussel species confirming the methanotrophic potential of their symbionts. Stable isotope analyses of lipids and whole tissue also confirmed the importance of methanotrophy in the carbon nutrition of all of the mussels. Analyses of absolute and relative symbiont abundance in B. heckerae and B. brooksi using fluorescence in situ hybridization (FISH) and rRNA slot blot hybridization indicated a clear dominance of methanotrophic over thiotrophic symbionts in their gill tissues. A site-dependent variability in total symbiont abundance was observed in B. brooksi, with specimens from Alaminos Canyon harbouring much lower densities than those from Atwater Valley. This shows that symbiont abundance is not species-specific but can vary considerably between populations.

Several deep-sea mussels and their associated symbionts are able to live both on wood and on whale falls

“…C and D display a sister-group relationship that is also observed for their associated bacteria and (ii) subclusters of the A. crypta species complex display the same dominant bacterial lineages. A similar close relationship between sister species of mussels and their symbionts was reported for Bathymodiolus azoricus and Bathymodiolus puteoserpentis, Bathymodiolus heckerae and B. aff heckerae, and Bathymodiolus brevior and Bathymodiolus septemdierum (Duperron et al 2006(Duperron et al , 2007. However, the phylogenetic congruence is not observed at deeper phylogenetic nodes ( Won et al 2008).…”

“…A dataset including all our 16S rRNA sequences and sequences from thioautotrophic symbionts of mussels from literature was built (electronic supplementary material 3). Trees were rooted on the methanotrophic symbiont of Bathymodiolus brooksi (Duperron et al 2007).…”

Changing redox conditions at cold seeps as revealed by authigenic carbonates from Alaminos Canyon, northern Gulf of Mexico

“…Jahnke et al, 1999). The 16S rRNA results, however, clustered close to mussel gill symbionts (Niemann et al, 2006;Duperron et al, 2007), which were assigned to γ-proteobacteria (Methanococcales), representing most likely Type I/X methanotrophs (Distel et al, 1995).…”

“…The former compound was so far only found in the cultured aerobic methanotrophs Methylococcus capsulatus (Bird et al, 1971;Bouvier et al, 1976) and Methylosphaera hansonii (Schouten et al, 2000), both belonging to the genus Methylococcales, as well as in endosymbiotic methanotrophs of the same genus (e.g. Jahnke et al, 1995;Duperron et al, 2007).…”

“…Jahnke et al, 1995). However, Duperron et al (2007) reported that these gill symbionts are γ-proteobacteria, which consequently represent Type I or X methanotrophs. Unfortunately, only fatty acids were analyzed in this study, but not sterols or hopanoids.…”