A COMPARISON OF METHODS FOR DETERMINING RATES OF ACETYLENE-REDUCTION (NITROGEN-FIXATION) BY HETEROTROPHIC BACTERIA IN SEAGRASS SEDIMENT

Three methods for determining rates of acetylene reduction in sediments with seagrass were compared. The simplest method was to make a slurry in a glass jar. A perfusion method, in which water in the sediment was replaced by slow diffusion for 1 h with seawater containing acetylene gave similar results when rates for a 6-h incubation period were integrated over the whole core. A third method was developed for studies of plant-bacteria interactions in which acetylene was allowed to diffuse through the lacunae of the seagrass to the roots and rhizomes. Rates of acetylene reduction, determined with the third method, were lower in the deep sediment where few roots penetrated; over a 12-cm deep core, total rates were not significantly different from those obtained by the other methods. This third method has the advantage that there was no disturbance of bacteria or O2 and nutrient gradients in the sediment. All rates were linear for at least 6 h and there was no lag period before ethylene was produced. Gas spaces above the water were not present or were minimal; ethylene was sampled directly from the interstitial water.