Journal of Conference Abstracts

Symbiotic Mussels from the Mid-Atlantic Ridge: Adaptations to Trophic Resources

Aline Fiala-Médioni U.P.M.C., Observatoire Oceanologique, B.P. 51, 66651 Banyuls-sur-Mer, France

afiala@arago.univ-perp.fr

Colleen Cavanaugh Biolabs, Havard University, 16 Divinity Ave., Cambridge, MA 02138, USA

Paul Dando School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Gwynedd, LL59 5EY, UK

Cindy Van Dover Institute of Marine Science, University of Alaska, Fairbanks, AK 99775, USA

Along with the bresiliid shrimps, mytilids are the main organisms associated with Atlantic vent sites. Two new Bathymodiolus have been found: the first one, Bathymodiolus puteoserpentis, (Von Cosel et al., 1994) is present in the Snake Pit site (23°N) at 3500 m and the second, undescribed, is the dominant species found in the Azores at both Lucky Strike (37°18'N) at 1700 m and Menez Gwen sites (37°50'N) at 840 m.

This new B. sp. from Azores represents an additional exemple of thio/chemo symbiosis as described in B. puteoserpentis. Transmission electron microscope observations showed that the great majority of the gill cells contained two distinct morphological types of symbionts. The first type were large (mean diameter: 1.25 µm ) coccoid-shaped bacteria which resemble type I metanotrophs with stalked intracytoplasmic membranes. The second type were smaller (mean diameter: 0.35 µm) coccoids or short rods which were similar to the sulfide-oxidizing symbionts observed in B. thermophilus. Dividing stages of both symbionts were observed. As in other mytilids, the bacteria were found in vacuoles, delimited by a peribacterial membrane, in the apical part of the bacteriocytes which was in close contact with the water.

Methanol dehydrogenase activities have been detected; a mean value of 1.56 nmoles/mg protein/min. (S.D. 1.36; n=9) was found in Bathymosiolus near the Azores. Activities of RuBisco (mean value of 0.128 ± 0.116 nmoles/mg protein/min; n=17) as well as sulfur metabolising enzymes: sulfate adenylyltransferase (mean value of 32.2± 25.3 nmoles/mg protein/min; n=9) and adenylylsulfate reductase (mean value of 83.8 ± 53.7 mmoles/mg protein/min; n=9) were also measured in the gill of Azores mussel. This indicates that both sulfur-oxidizing and methane-oxidizing metabolisms could be active, allowing the species a greater flexibility in the colonization of different environments.

The high level of enzyme activities, as well as the abundance of the symbionts, support the hypothesis that the endosymbionts could make a significant contribution to the host nutrition. Different isotopic values of ¹³C were obtained in the two mussels: B. puteoserpentis had a ¹³C (-32.7 to -35.6‰) similar to that in other thiotrophic organisms while higher values (- 24.1‰)were found in the Azores mussel possibly reflecting a higher input from methane found at the sites near Azores. The hypothesis of the predominance of metanotrophic metabolism in this is also in agreement with preliminary data showing concentrations of methane as high as 2.2 mmoles/kg when sulfide concentrations are lower than 2 nmoles/kg. Ecological conditions (i.e. respective concentrations in sulfide and methane) may regulate a balance between physiological activities of the different symbionts associated with these mussels.

References

Cavanaugh, C., Wirsen, C.O. & Jannasch, H.W., Applied Env. Microb. 58, 3799-3803 (1992).

Charlou, J.-L., Donval, J.P., Knoery, J., Fouquet, Y., Costa, I., Lourenço, N., Ondréas, H., Pelle H., Segonzac, M. & Tivey, M., EGU,Terra Abstracts 7, 206 (1995).

Colodner, D., Lin, J. et al., Eos, Trans., AGU 74, 99 (1993).

Distel, D.L., Lee, H.K.W. & Cavanaugh, C.M., Proc. Natl. Acad. Sci. USA 92, 9598-9602 (1995).

Donval, J.P., Charlou, J.-L., Radford Knoery, J., Fouquet, Y., Costa, I., Lourenço N., Ondréas H., Pelle, H., Segonzac, M. & Tivey, M., Eos, Trans., AGU 75, 309 (1994).

Ivanoff et al., In: Biochemistry of Global Change. Radioactive trace gas, 505-520 (1993).

Van Dover, C., In: Hydrothermal Vents and Processes, Geol. Soc. Spec. Pub. N° 87, 257-294 (1995).

Von Cosel, R., Metivier, B. & Hashimoto, J., Veliger 37, 374-392 (1994).