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Abstract:
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Nitrogen -fixation is an essential biochemical reaction involving the reduction of inert , atmospheric dinitrogen (N2 ) into biochemically accessible ammonia (NH3 ) . Organisms that are capable of this process are collectively called “diazotrophs” and are ubiquitous in marine and terrestrial environments . Despite the wide distribution , little is known about the biological nature of the diverse groups of diazotrophs . This study was designed to address the influence of light and nutrients on nitrogen fixation and growth in several marine diazotrophic symbioses collectively termed “Diatom -Diazotroph Associations (DDAs ) .” The organisms of interest included the diatoms Hemiaulus haukii Grunow and Hemiaulus membranaceus Cleve , and their diazotrophic endosymbiont Richelia intracellularis Schmidt . The study included acetylene reduction assays , growth rate , and nutrient analysis experiments on both associations in order to better understand the similarities and differences within and between the two DDAs . The results indicate distinct differences in nitrogen fixation rates within and between the species . In the nitrogen addition experiment , the “no added nitrogen” treatment had the highest N2 -fixation rate (N2 -fixmax = 7 .43 x 10 -5 nmols N2 heterocyst -1min -1 ) , followed by the added nitrate treatment (N2 -fixmax = 6 .49 x 10 -5 nmols N2 heterocyst -1min -1 ) and the added ammonium treatment (N2 -fixmax = 3 .79 x 10 -5 nmols N2 heterocyst -1min -1 ) . The maximum growth rate occurred in the “added ammonium” treatment (0 .42 divisions day -1 ) , which had a higher percentage of asymbiotic cells than the two other treatments . The maximum recorded rate of N2 -fixation for H . haukii was 7 .43 x 10 -5 nmol N2 heterocyst -1min -1 and the maximum value of N2 -fixation for H . membranaceus was 1 .88 x 10 -4 nmol N2 heterocyst -1min -1 . The maximum growth rate for H . haukii was 0 .99 divisions day -1 , and 1 .06 divisions day -1 for H . membranaceus . Growth followed light saturation kinetics in H . haukii with a compensation light intensity (IC ) of 10 µmol quanta m -2sec -1 and saturation light intensity (IK ) of 100 µmol quanta m -2sec -1 . H . haukii and H .membranaceus expressed light saturation kinetics in N2 -fixation . N2 -fixation was generally limited to the light period , with no evidence of a morning or evening enhancement . The DDAs grew solely on N2 -fixation and did not use nitrate . This study contributes to current knowledge of DDAs and their role in global marine nitrogen fixation . |