Comparison of the Nitrogen-15 Uptake and Acetylene Reduction Methods for Estimating the Rates of Nitrogen Fixation by Freshwater Blue-Green Algae

The relationship of acetylene reduction to nitrogen-15 uptake was investigated using blue-green algal populations in three lakes in the Experimental Lakes Area, northwestern Ontario. Nitrogen fixation rates, as estimated with both techniques, were compared and acetylene to nitrogen ratios determined. Lake ratios ranged from 6.3 to 9.1 moles of acetylene reduced per mole of nitrogen fixed varying from sample to sample and also with the method of calculation. Explanations of the discrepancies between theoretical and empirical ratios are discussed; these include hypotheses of excretion of assimilated nitrogen-15 labeled material and interference from nitrogenase-mediated hydrogen production.