Determination of Nitrogenase Activity in Aquatic Samples Using the Acetylene Reduction Procedure

In many aquatic systems, N2 fixation is an important component of the nitrogen (N) cycle. Nitrogenase is a molybdenumenzyme complex found in phylogenetically diverse prokaryotes that is eubacteria and archebacteria, of physiologies ranging from aerobic heterotrophs, aerobic and anaerobic phototrophs to strict anaerobes. The acetylene (C2H2) reduction method, first described by W. D. P. Stewart and R. W. F. Hardy provided a convenient, very sensitive, relatively simple and inexpensive means to determine nitrogenase activity. The C2H2 reduction method provides, by its nature, an indirect estimate of N2 fixation. Considering the potential problems, within the biological and physical assay system chosen, one should make direct comparisons of acetylene reduction and N2 fixation to derive empirical conversion factors. Experimental manipulation of many of the factors can provide useful information concerning the physiological composition of natural populations of diazotrophs and the factors limiting activity.