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

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Abstract
N fixation (acetylene reduction) was consistently and immediately detectable in rhizosphere sediments of the eelgrass Z. 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 0-6 cm segment. Nitrate and NH4+ (100-200 .mu.M) inhibited nitrogenase activity, while glucose (1 mM) accelerated rates of C2H2 reduction. Much of the nitrogenase activity apparently is associated with sulfate-respiring bacteria. During the summer, rates of C2H2 reduction to 10 cm averaged .apprx. 1.5-2.5 nmol C2H4 .times. cm-2 h-1 (0.1-0.2 nmol .times. g dry sed-1 h-1). This could account for 3-28% of the net N demand of the plant. While supplying a substantial fraction of the N required by eelgrass, rhizosphere N2 fixation in Z. marina communities may represent a lesser input when compared to the tropical seagrass Thalassia testudinum. Information on the magnitude of other N transformations is needed to evaluate fully the importance of N2 fixation in these systems.