Pectin decomposition and associated nitrogen fixation by mixed cultures of Azospirillum and Bacillus species

Cocultures of different Azospirillum species with Bacillus polymyxa or Bacillus subtilis allow the efficient utilization of pectin as carbon and energy sources for nitrogen fixation. The nitrogenase activity obtained with cocultures was as high as 30–80 nmol C2H4 h−1 mL−1, a much higher value than that obtained with pure cultures of either Azospirillum (up to 13 nmol C2H4 h−1 mL−1) or B. polymyxa (up to 2 nmol C2H4 h−1 mL−1) alone. To establish to what extent each partner contributed to nitrogenase activity, acetylene reduction was assayed as a function of time and it was also measured on Azospirillum cultivated in the cultures filtrates of the Bacillus. The results suggested that the nitrogenase activity was mostly produced by Azospirillum. The nitrogenase activity occurred at the expense of the degradation and fermentation products of the pectin. The new pectinolytic species, Azospirillum irakense, utilized both degradation and fermentation products of pectin, whereas the nonpectinolytic strains (Azospirillum brasilense, Azospirillum lipoferum, Azospirillum amazonense) utilized only the fermentation products of pectin, including acetic and succinic acids. These cocultures can be considered as metabolic associations, where the Bacillus produces degradation and fermentation products of pectin, which can be used by Azospirillum species. Key words: cocultures, nitrogen fixation, pectin degradation, Azospirillum, Bacillus, metabolic association.