A comparison of carbon source utilization for growth and nitrogenase activity in two Frankia isolates

Abstract

The carbon source requirements for the growth and nitrogen fixation of two morphologically distinct Frankia isolates were examined. Isolate ArI3 (from Alnus rubra) grew well on propionate, malate, acetate, and trehalose, and isolate CcI2 (from Casuarina cunninghamiana) grew best on pyruvate, acetate, and propionate. In general, the same carbon sources that supported growth supported both the development of vesicles and nitrogenase activity in long-term induction experiments in both isolates. However, ArI3 cultures induced on proprionate had 7 to 26 times the activity of other carbon sources and ArI3 cultures induced on acetate did not develop any detectable acetylene reduction. In a parallel set of experiments, cultures of both isolates were induced for nitrogenase activity on propionate and the resulting nitrogen fixing cultures were washed free of the organic acid by centrifugation. The washed cultures were incubated in the presence of various carbon sources to determine the ability of a particular substrate to supply energy directly for nitrogen fixation when vesicles and nitrogenase were already present. As was observed in the long-term induction experiments, pyruvate, propionate, and acetate supported the greatest activity in CcI2. Succinate and malate supported the greatest activity in ArI3, and propionate had very little stimulation of acetylene reduction. The reason for the lack of stimulation by propionate for washed cells of ArI3 was unclear but may have been due to toxic concentrations of the organic acid. In an attempt to compare the carbon utilization of ArI3 in pure culture with that in the alder symbiosis, oxygen uptake in the presence of various carbon sources of vesicles clusters isolate from Alnus rubra nodules inoculated with ArI3 was compared with the oxygen uptake of nitrogen-fixing pure cultures of ArI3. The oxygen uptake of the isolated vesicle clusters was stimulated by sucrose, trehalose, and glucose, but not by a variety of organic acids. In comparison, nitrogen-fixing pure cultures of ArI3 readily oxidized sugars and organic acids.