Seasonal changes in rates of nitrogen-fixation (AR) and numbers of potential nitrogen-fixing bacteria associated with excised roots of aquatic angiosperms

Abstract

Seasonal changes in nitrogen-fixation and numbers of potential nitrogen-fixing bacteria associated with excised roots of angiosperms from Balgavies Loch, a eutrophic loch in eastern Scotland, were studied by the acetylene reduction method. The acetylene reduction activity (ARA) and numbers of bacteria fluctuated seasonally in each plant and from one site to another. Significant rates of ARA were associated more with two C 3 grasses, Phalaris arundinacea L. and Phragmites australis (Cav.) Trin. than with the non-grasses, Juncus effusus L. and Polygonum amphibium L. Correlations were not always obtained between rates of ARA and numbers of nitrogen-fixing bacteria. Optimum pC 2H 2 for nitrogenase activity was variable. Thus, while a pC 2H 2 of 0.06 atm appeared adequate for Polygonum, the optimum for Phalaris seemed to lie somewhere between 0.10 and 0.15 atm. The rates of ARA of Phragmites and Juncus, on the other hand, increased with increases in pC 2H 2 between 0.01 and 0.20 atm. These increases were, however, very small, between 0.10 and 0.20 atm. As has been reported by other workers, the ARA of the angiosperms was both O 2 sensitive and O 2 dependent, with maximum rates being obtained at pO 2 between 0.01 and 0.05 atm. Lag periods of 3–5 h preceded the onset of the evolution of ethylene but these were eliminated by pre-incubating the samples for 6 h either in O 2 (pO 2 0.20 atm) or under anaerobic conditions. The ARA was depressed but not completely inhibited by ammonium and nitrate-nitrogen at concentrations of more than 2000 μg N l −1. This depression suggests that the acetylene reduction activity is by the nitrogenase enzyme system.