Anaerobic growth and denitrification among different serogroups of soybean rhizobia

Abstract

We screened soybean rhizobia originating from three germplasm collections for the ability to grow anaerobically in the presence of NO3- and for differences in final product formation from anaerobic NO3- metabolism. Denitrification abilities of selected strains as free-living bacteria and as bacteroids were compared. Anaerobic growth in the presence of NO3- was observed in 270 of 321 strains of soybean rhizobia. All strains belonging to the 135 serogroup did not grow anaerobically in the presence of NO3-. An investigation with several strains indicated that bacteria not growing anaerobically in the presence of NO3- also did not utilize NO3- as the sole N source aerobically. An exception was strain USDA 33, which grew on NO3- but failed to denitrify. Dissimilation of NO3- by the free-living cultures proceeded without the significant release of intermediate products. Nitrous oxide reductase was inhibited by C2H2, but preceding steps of denitrification were not affected. Final products of denitrification were NO2-, N2O, or N2; serogroups 31, 46, 76, and 94 predominantly liberated NO2-, whereas evolution of N2 was prevalent in serogroups 110 and 122, and all three were formed as final products by strains belonging to serogroups 6 and 123. Anaerobic metabolism of NO3- by bacteroid preparations of Bradyrhizobium japonicum proceeded without delay and was evident by NO2- accumulation irrespective of which final product was formed by the strain as free-living bacteria. Anaerobic C2H2 reduction in the presence of NO3- was observed in bacteroid preparations capable of NO3- respiration but was absent in bacteria that were determined to be deficient in dissimilatory nitrate reductase.