Inorganic N Assimilation and Ammonium Pools in a Deep-Sea Mussel Containing Methanotrophic Endosymbionts.

Abstract

Undescribed mussels (seep mytilid 1a) harboring methanotrophic endosymbionts exhibit high biomass around hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico. These mussels assimilate ammonium and nitrate present at high concentrations in their environment. Pathways of assimilation were investigated by enzyme activity measurements and 15N tracer experiments. Glutamine synthetase was detected in all freshly collected mussels tested. Nitrate reductase activity was not always observed. Exposure to 15NH3 resulted in the appearance of millimolar concentrations of 15NH3 within the symbiont-containing tissues. The concentration of internal 15NH3 was several times higher than in the medium and correlated with 15NH3 assimilation rate. These results indicate that exogenous 15NH3 was taken up into a large internal pool before it was assimilated. Our results do not indicate the extent to which ammonium pools were within the host or symbiont or whether ammonium assimilation was facilitated by either partner exclusively. The observation of elevated internal ammonium concentrations is inconsistent with the "depletion-diffusion" mechanism of nutrient uptake proposed for algal-invertebrate symbioses and is suggestive of active ammonium uptake mechanisms across the host surface. Exposure to 15NO3- also resulted in the appearance of 15NH3, with internal 15NH3 concentration correlated with 15NO3- assimilation rate. This result indicates that 15NO3- was reduced more rapidly than it was assimilated and that 15NH3 derived from 15NO3- may also enter an internal ammonium pool. Assimilation of nitrate in the presence of millimolar concentrations of internal ammonium and reduction of 15NO3- in excess of assimilation is consistent with the functioning of dissimilatory nitrate reduction pathways with ammonium as a major endproduct. Such a mechanism may operate in other chemosynthetic symbioses that exhibit dissimilatory nitrate reduction.