Light dependant growth and nitrogen fixation rates in the Hemiaulus haukii and Hemiaulus membranaceus diatom-diazotroph associations

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.