Symbioses between marine invertebrates and their chemoautotrophic and methanotrophic symbionts are now known to exist in a variety of habitats where reduced chemical species are present. The utilization of chemical energy and reliance on C(1) compounds by these symbioses are well documented. Much less is known about their metabolism of nitrogen. Earlier work has shown that the tissues of organisms in these associations are depleted of N compared with those of other marine organisms, indicating that local sources of nitrogen are assimilated and that novel mechanisms of nitrogen metabolism may be involved. Although these symbioses have access to rich sources of ammonium (NH(4) and NH(3)) and/or nitrate, several investigators have proposed that N(2) fixation may account for some of these isotope values. Here we report that [N]ammonium and, to a lesser degree, [N]nitrate are assimilated into organic compounds by Solemya reidi, a gutless clam containing S-oxidizing bacteria, and seep mussel Ia, an undescribed mytilid containing methanotrophic bacteria. In contrast, Riftia pachyptila, the giant hydrothermal vent tube worm symbiotic with S-oxidizing bacteria, assimilated nitrate but not exogenous ammonium. The rates of assimilation of these sources are sufficient to at least partially support C(1) compound metabolism. N(2) assimilation was not exhibited by the symbionts tested.
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