RELATIONSHIPS BETWEEN NITROGENASE ACTIVITY, STABILITY, AND THE RATIO OF COMPONENTS IN MIXED CULTURES OF DIAZOTROPHIC BACTERIA

Seventy diazotrophic bacterial associations were obtained from samples of rhizosphere soil. Experiments were conducted with two binary cultu res, A and E (A: Bacillus sp. A2 + Pseudomonas sp. A4; E: Bacillus sp. E3 + Erwinia sp. E6) exhibiting a pronounced ''associative effect'': the nitrogenase activity of the associations exceeded the sum of the a ctivities of the components 2- to 10-fold. The highest nitrogen-fixing activity of the associations corresponded to a strictly defined ratio of their components, which was individual for each association. This component ratio, corresponding to the maximum nitrogenase activity, wa s established in the associations cultivated for a long time, which su ggests that autoregulation of diazotrophic microbial communities is di rected toward increasing nitrogen-fixing activity. The results of 50-d ay cultivation confirmed the stability of interspecies relations in th e associations studied: after a period of fluctuation, the contents of the components returned to their initial levels. The associations of diazotrophs were especially stable under conditions of nitrogen fixati on. Resistance to antibiotics (streptomycin and rifampicin) significan tly diminished the nitrogenase activity of the associations but not of the pure cultures of the components.