Anaerobic Growth and Denitrification among Different Serogroups of Soybean Rhizobia

We screened soybean rhizobia originating from three germplasm collections for the ability to grow anaerobically in the presence of NO(3) and for differences in final product formation from anaerobic NO(3) metabolism. Denitrification abilities of selected strains as free-living bacteria and as bacteroids were compared. Anaerobic growth in the presence of NO(3) 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 NO(3). An investigation with several strains indicated that bacteria not growing anaerobically in the presence of NO(3) also did not utilize NO(3) as the sole N source aerobically. An exception was strain USDA 33, which grew on NO(3) but failed to denitrify. Dissimilation of NO(3) by the free-living cultures proceeded without the significant release of intermediate products. Nitrous oxide reductase was inhibited by C(2)H(2), but preceding steps of denitrification were not affected. Final products of denitrification were NO(2), N(2)O, or N(2); serogroups 31, 46, 76, and 94 predominantly liberated NO(2), whereas evolution of N(2) 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 NO(3) by bacteroid preparations of Bradyrhizobium japonicum proceeded without delay and was evident by NO(2) accumulation irrespective of which final product was formed by the strain as free-living bacteria. Anaerobic C(2)H(2) reduction in the presence of NO(3) was observed in bacteroid preparations capable of NO(3) respiration but was absent in bacteria that were determined to be deficient in dissimilatory nitrate reductase.