Nitrogen Fixation (Acetylene Reduction) by Rhizosphere Sediments of the Eelgrass Zostera marina

Abstract

Nitrogen fixation (acetylene reduction) was consistently and immediately detectable in rhizosphere sediments of the eelgrass Zostera marina L. collected from several stations and at various times of the year Nitrogenase activity was detected down to 12 cm with the major fraction occurring in the 0 to 6 cm segment. Nitrate and NH,+ (100 to 200 &M) inhibited nitrogenase activity, while glucose (1 mM) accelerated rates of C,H, ~reductlon Much of the nitrogenase activity appears to be associated with sulfate-respiring bacteria. During the summer, rates of C,H, reduction to 10 cm averaged about 1.5 to 2.5 nmol C,H, X ~ m-~ h-' (0 l to 0.2 nmol x E] dry s e d-' h-'). T h ~ s could account for from 3 to 28 % of the net nitrogen demand of the plant. W h ~ l e supplying a substantial fraction of the nitrogen required by eelgrass, rhizosphere N, f ~ x a t ~ o n In Z. marina communities may represr,nt a lesser inpul when compared to the tropical seagrass Thalassia test~ldjnum Information on the magnitude of other nitrogen transformations is needed to evaluate fully the importance of N, fixirtion in these systems.