MEPS 123:137-148 (1995)  -  doi:10.3354/meps123137

Undescribed deep-sea mussels containing methanotrophic endosymbionts (seep mytilid Ia) are found at high densities around hydrocarbon seeps of the Gulf of Mexico where methane, nitrate and ammonium are present at high concentrations. In this study we investigated assimilation of ammonium, nitrate, glycine and methane using ¹⁵N- and ¹³C-labelled tracers to determine sites of assimilation, rates of assimilation, and the chemical form in which assimilated C and N appears. We then investigated the interaction between inorganic nitrogen assimilation and methane assimilation to assess whether they are directly dependent on each other. ¹⁵NO₃^-, ¹⁵NH₃ and ¹³CH₄ were assimilated primarily into the gills where the bacteria are located, with negligible incorporation into symbiont-free tissue. In contrast,¹³C¹⁵N-glycine was assimilated equally into gill and symbiont-free tissue. These results indicate that inorganic N is assimilated in the gill tissue. The bulk of ¹³C and ¹⁵N label from methane and ammonium incubations was in the 80% EtOH soluble fraction, suggesting that the primary assimilation product was low-molecular-weight metabolites. Some ¹³C was incorporated into carbon storage products. Mussels that assimilated excess ¹³CH₄ converted 35% of ¹³CH₄ into an EtOH insoluble form. Negligible ¹⁵N label was observed in this fraction. Inorganic N assimilation and methane assimilation were not tightly coupled. N assimilation was not affected by absence of methane or inhibition of methanotrophy. Methane assimilation was not stimulated by increased N assimilation. Seep mytilids were capable of luxury consumption of inorganic N, i.e. C/N assimilation ratios below the average C/N ratio of mussel tissues (4.2). We believe that luxury consumption is supported by C reserves resulting in part from methanotrophy. From relations between CH₄ and N source concentration and assimilation rate, we estimated the environmental conditions that result in balanced C/N assimilation. From this analysis we predict that at environmentally realistic methane and inorganic N concentrations seep mytilids assimilate excess CH₄.

Seep mytilid Ia . Methanotrophy . Chemoautotrophy . Nitrogen assimilation . Ammonium . Nitrate . Methane . Hydrocarbon seep