Rhizobial and bradyrhizobial symbionts of mesquite from the Sonoran Desert: salt tolerance, facultative halophily and nitrate respiration

Abstract

Rhizobial symbionts were isolated from the surface (0–0.5 M) and phreatic (3.9–5.0 M) root environments of a mature mesquite woodland in the Sonoran Desert of Southern California, and from variable depths (0–12 m) of non-phreatic mesquite ecosystems in the Chihuahuan Desert of New Mexico. They were tested for their ability to tolerate high salinity, and respire NO 3 − as mechanisms of free-living survival. Sixteen of 25 isolates were grown in yeast-extract mannitol (YEM) broth at NaCl concentrations of 2 (basal concentration), 100, 300, 500 and 600 mM, and their specific growth rates, cell dry weight and lag times were determined. Twenty of the 25 isolates were also grown in YEM broth under anaerobic conditions with or without 10 mM KNO 3. Three categories of NaCl salinity responses were observed: (1) eight isolates showed decreased specific growth rates at NaCl concentrations of 100, 300 and 500 mM, but they nevertheless remained viable at 500 mM NaCl concentration; (2) the specific growth rate of six isolates increased significantly at 100 and 300 mM NaCl; and (3) specific growth rates of two isolates were significantly greater than the base-rate at all concentrations of NaCl. Five of 11 of the Bradyrhizobium isolates tested respired NO 3 −, but showed no growth. Seven Rhizobium isolates, three from the deep (3.9–5 m) phreatic rhizobial community, and four from the surface community denitrified NO 3 − but only the isolates from the phreatic community displayed anaerobic growth. Long-term interactions between rhizobial and bradyrhizobial communities and the surface and phreatic root environments of the mature Sonoran Desert mesquite woodland appear to have selected for strains of NO 3 − respiring rhizobia, general salt tolerance of both rhizobial and bradyrhizobial symbionts, and strains of weak facultative halophilic bradyrhizobia. These survival characteristics of mesquite rhizobia may be important regarding mesquite's establishment and long-term productivity in marginal desert soils, and may provide novel types of rhizobia for food crops growing in harsh environments.