RESEARCH OBJECTIVE: To delineate the interactions between a newly discovered mussel and its methanotrophic symbionts in order to reach a more complete understanding of the intact symbiosis. ACCOMPLISHMENTS: Since the discovery and demonstration of the symbiosis between methanotrophic symbionts and a deep-sea mytilid in 1986, our efforts have been directed at determining the interactions between the symbionts and hosts which determine the properties of the association. Of particular concern has

more »

... een the role of each partner in determining which external metabolites are taken up and utilized by the association and what the role of each partner is in the uptake and metabolism of these metabolites. We have been particularly concerned with the C and N sources for the symbionts and the intact association. We have investigated the biology of this symbiosis within the context of our developing model of mussel/chemoautotroph symbioses in general as being relatively unspecialized with regard to hosts' control of the symbionts' environment. For example, unlike other groups with chemoautotrophic endosymbionts, Bathymodioluslike mussels can have either sulfur-oxidizing or methanotrophic symbionts or in one recently discovered case both simultaneously. There appears to be relatively little morphological or physiological specialization associated with these symbioses in these mussels and the mussels retain the ability to utilize particulate food as well as organic material from the symbionts. However, the distributions of these mussels and the stable C isotope ratios of their tissues make it clear that the symbionts are the primary source of food for the animals under most conditions. While it is apparent that CH 4 is the major source of carbon for the methanotrophic mussels which we studied, the nitrogen source is not so obvious. We have investigated four possible sources: Particulate organic N, dissolved organic N, NH4 + , and direct fixation of N2. The first two sources (particulate and dissolved), can also serve as carbon sources. These mussels' environment is rich in ammonium and free amino acids unlike the habitats of typical photoautotrophic symbioses. 9 1