Effect of oxygen on substrate utilization for nitrogen fixation and growth in Frankia spp.

Abstract

Frankia, the actinomycete partner in the nitrogenfixing symbiosis of certain woody non-legumes, has been shown to fix nitrogen in pure culture under aerobic conditions. The sensitivity of in vivo nitrogen-fixation (acetylene reduction) to oxygen tension in the gas phase was measured in short-term assays with two Frankia isolates designated ARI3 and CcI3. The carbon source utilized had an effect on the optimum O₂ concentration for acetylene reduction. Cells utilizing an organic acid, e.g., propionate or pyruvate had maximum nitrogenase activity at an oxygen concentration of 15 to 20%. In contrast, cells respiring a sugar, e.g., trehalose or glucose, or endogenous reserves (glycogen or trehalose) had maximum acetylene reduction activity at 5 to 10% in the gas phase. Oxygen uptake kinetics showed that respiration in vesicle-containing cells utilizing trehalose had a biphasic response to oxygen concentration with a diffusion limited component at oxygen concentrations of 20 μM to more than 300 μM. These results suggested that trehalose was oxidized in the vesicles as well as in the vegetative hyphae. Oxygen concentration also had an effect on the trehalose-supported growth of cells (non nitrogenfixing, [⁺NH₄Cl]). Cells grown with 5–10% O₂ in the gas phase had a doubling time approximately half those grown with 20% O₂ (atmospheric). Propionate-grown cells showed similar growth rates at the two oxygen tensions, and grew faster (almost 2x) than the trehalose cells at 5–10% O₂. Trehalose also supported approximately 40% lower rates of oxygen uptake than propionate in vesicle-containing cells.