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

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Abstract
A perfusion method for assaying nitrogenase activity (acetylene reduction) in marine sediments was developed. The method was used to assay sediment cores from Spartina alterniflora (salt marsh), Zostera marina (sea grass), and Thalassia 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 the Spartina community ranged from 0.15 to 0.60 nmol of C2H4 · 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 C2H4 production. Initial values obtained by using the perfusion method were 0.66 nmol of C2H4 · g of dry sediment−1 · h−1 for sediments from Zostera communities and 0.70 nmol of C2H4 · g of dry sediment−1 · h−1 for sediments from Thalassia communities. In all cases, rates determined by simultaneous slurry assays were lower than those determined by the perfusion method.