Characteristics of Nitrate Respiration of Isolated Soybean Bacteroids

Abstract

Soybean bacteroids were isolated from nodules by sucrose density gradient centrifugation, and some characteristics of the bacteroid respiration associated with nitrate reduction were investigated. Under anaerobic conditions, CO2 evolution from the bacteroid suspension was very low. However, when 1 mM NO2 − was added, the CO2 evolution increased by ca. 18 times. The rate of CO2 release by anaerobic nitrate respiration was ca. 20% of that of the aerobic CO2 evolution at a pO2 of 0.2 atm. The CO2 release associated with NO2 − reduction was inhibited by the addition of 1 mM DNP or HgCl2, whereas the addition of 1 mM KCN did not exert any effect. On the other hand, when the bacteroids respired O2 under aerobic conditions, both the O2 uptake and CO2 evolution were severely inhibited by KCN and HgCl2. In addition, neither NO2 − nor NH2OH could replace the role of NO3 − as an electron acceptor for anaerobic respiration. 15NO3 − was added to the bacteroid suspension under aerobic, microaerobic and anaerobic conditions, and the fate of N was investigated. The rates of NO3 − decrease and NO2 − increase in the medium, were both high under anaerobic conditions, and they were quite low under aerobic conditions. Based on the 15N analysis, NO2 − could be further reduced to ammonia, and the ammonia produced was incorporated into some unidentified cationic compound(s) other than amino acids or amides. N2O production was not observed during the 40 min incubation after addition of nitrate.