Nitrogen fixation has been detected by the acetylene reduction method in the sediments of the Waccasassa estuary, a shallow embayment on the Florida Gulf Coast. Fixation rates in the range 1.6-15.0 ng C2H4/g sediment-hr were found within the top 2-5cm stratum of sediments. Expressed in terms of equivalent nitrogen fixed, the range was 0.64-6.0 ng N/g-hr.Much lower rates (0.03-0.40 ng GHa/g-hr) were found at greater depths in the sediment, and no fixation was observed in the flocculent unconsolidated l-2 cm at the sediment surface.All evidence indicates that the reduction of acetylene to ethylene is a biological phenomenon, directly related to the activity of nitrogen-fixing organisms in the sediments. Nitrogen-free media produced growths of Gram-positive spore-forming rods from sediments under an Nz atmosphere.A pure culture similar to Clostridium sp. was isolated on nitrogen-free media from Waccasassa sediments and was shown capable of nitrogen fixation by the acetylene reduction method.

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