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A perfusion method for assaying nitrogenase activity (acetylene reduction) in marine sediments was developed. The method was used to assay sediment cores fromSpartina alterniflora (salt marsh),Zostera marina (sea grass), andThalassia testudinum (sea grass) communities, and the results were compared with those of conventional sealed-flask assays. Rates of ethylene production increased progressively with time in the perfusion assays, reaching plateau values of 2 to 3 nmol  g of dry sediment−1  h−1 by 10 to 20 h. Depletion of interstitial NH4 + was implicated in this stimulation of nitrogenase activity. Initial acetylene reduction rates determined by the perfusion assay of cores from theSpartina community ranged from 0.15 to 0.60 nmol of C2 H4  g of dry sediment−1  h−1 . These rates were similar to those for sediments assayed in sealed flasks without seawater when determined over linear periods of C2 H4 production. Initial values obtained by using the perfusion method were 0.66 nmol of C2 H4  g of dry sediment−1  h−1 for sediments fromZostera communities and 0.70 nmol of C2 H4  g of dry sediment−1  h−1 for sediments fromThalassia communities. In all cases, rates determined by simultaneous slurry assays were lower than those determined by the perfusion method.

applied and environmental microbiology

Perfusion Method for Assaying Microbial Activities in Sediments: Applicability to Studies of N 2 Fixation by C 2 H 2 Reduction