Nitrogen Fixation in an Estuarine Environment: The Waccasassa on the Florida Gulf Coast1

Abstract

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.

Dinitrogen fixation by Azotobacter chroococcum and its isocitrate dehydrogenase activity: A quick method for screening nitrogen fixing strains.

“…In tropical soils, for example, which are often characteristically poor in nitrogen, the input of a few kg of nitrogen per ha per year may be as important to the maintenance of the ecosystem, as are the additions of very much higher quantities of nitrogen by legumes in intensively cultivated agriculture land. There have been several reports which indicate that nitrogen fixation in tropics could be increased if possible carbon energy sources were added to the reaction system (30)(31)(32). The results of this investigation demonstrate that the efficient dinitrogen fixing strains of Azotobacter chroococcum require carbon for growth and they increased their rate of fixation when various carbon sources were included in the medium.…”

Nitrogen fixation associated with seagrasses1

“…Several workers have observed nitrogen fixation in the sediments that could contribute to this requirement. Brooks et al ( 1971) estimated the contribution of nitrogen fixation to the nitrogen supply in the sediments of an estuary in Florida. Using their mean of 3.07 ng N fixed (g sediment-hr)-l, a sediment dry weight of 0.455 g crnb3 and a zone 3 cm thick per square meter we calculate the contribution by nitrogen fixation to be 1 mg N ( m2-day)-l. Bunt et al ( 1971) made in situ assays of sediments in sparse Thalassia beds in Card Sound, Florida, and found an average rate of fixation of 0.37 mg N( m2-day) -l.…”

“…Unfortunately the differences in results in nitrogen fixation experiments in seagrasses may be attributable to techniques. In our experiments plants were collected from their natural environment and subjected to experiment within a few hours ( at the most) ; in addition the duration of experiments was short, as recommended by Brooks et al 1971, and incubations were done in a seawater bath at ambient temperatures under oxic and anoxic conditions. These procedures were an attempt to reproduce the natural conditions and minimize the effects of containers, temperature, etc.…”

Nitrogen fixation in Flax Pond: A Long Island salt marsh1

“…Autotrophic nitrogen fixation is correlated with the presence of blue-green algae in the surface waters of certain lakes ( Dugdale and Dugdale 1962;Stewart et al 1967;Brezonik 1973) and in the surface waters of the Sargasso Sea (Dugdale et al 1961(Dugdale et al , 1964 , Heterotrophic bacterial nitrogen fixation occurs in anoxic lake waters (Brezonik and Harper 1969) and in lake and estuarine sediments (Brooks et al 1971;Howard et al 1970;Keirn and Brezonik 1971). Highest rates of nitrogen fixation appear to occur at the interfaces of aerobic 1 Research carried out at Brookhaven National Laboratory partially supported by the U.S. Atomic Energy Commission and by National Science Foundation grant and anaerobic zones (Stewart 1969), and it seemed reasonable to explore the possibility that nitrogen fixation was associated with the surface of the sediments in Flax Pond, as well as with blue-green algal mats.…”