Nitrogen fixation at rates of up to 7.3 µg N2 fixed g dry wt−1 h−1 is associated with the marine macroalga Codium fragile. A bacterium, identified as an Azotobacter, isolated from Codium, is capable of fixing atmospheric nitrogen in pure culture under aerobic conditions. Scanning electron micrographs showed dense populations of rod‐shaped bacteria, presumed to be Azotobacter, on the surface of Codium.
When Codium photosynthesis is decreased by light shading, N2 fixation drops proportionally, suggesting that fixation is coupled to the release of dissolved compounds by Codium. Addition of glucose to seawater increases N2 fixation, indicating that the agent of fixation is a heterotroph.
In a field survey, an inverse relationship was found between N2 fixation and the concentration of combined N compounds in seawater. At total available N (NO2−, NO3−, NH4+, urea) concentrations greater than 10 µg‐atoms liter−1, no fixation was associated with Codium. At Nobska Beach the total nitrogen input through N2 fixation in summer was 2.0 mg m−2 day−1 for water columns ranging between 1 and 3 m, and 0.47 in the 3–5‐m stratum.
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Laboratory experiments were conducted to determine the effect of salinity on embryonic development and hatching in Neobenedenia melleni, a monogenean ectoparasite of seawater‐cultured (37 ppt) Florida red tilapia (Oreochromis urolepis hornorum × O. mossambicus). Eggs, collected from adult monogeneans at 37 ppt, were exposed to different salinities (0, 6, 12, 18, 24, 30 and 37 ppt) for 48, 72 or 96 h. Varying degrees of post‐treatment development and hatching occurred when natural seawater conditions were restored. Hatching success generally declined with decreasing salinity and increasing duration of exposure. Under all durations of exposure, hatching success remained relatively high (≥ 69.6%) at salinities of 24 ppt and above, but declined markedly (≤ 32.5%) at lower salinities. Hatching did not occur in eggs exposed to fresh water (0 ppt) for 72 and 96 h. The most effective treatments in preventing hatching, other than prolonged exposure to fresh water, were 96 h exposures to low‐salinity brackish water. Percent hatch after 96 h exposure to 6, 12 and 18 ppt was 5.5, 11.9 and 5.8%, respectively.
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