Effect of haemoglobin and other nitrogenous compounds on the respiration of the Rhizobia

Abstract

Certain amino acids and inorganic N salts increased the respiration of washed suspensions of root-nodule bacteria furnished with glucose. This stimulation was immediate and did not increase appreciably with time during the period of measurement (up to 4 hrs.). The initial increase in rate was constant over a wide range of concns., but the total increase of O2 uptake was dependent upon the concn. of nitrogenous compound supplied. Young cells were stimulated somewhat more than old cells. In the presence of glycine, the uptake of O2 by Rhizobium trifolii furnished with glucose was more rapid than in its absence; much though not all, of this stimulation apparently arises from O2 used in the oxidation of the glycine. The respiration rate of washed cells of R. leguminosarum supplied with glucose was enhanced by the addition of hog hemoglobin. The percentage increase was greater at a PO2 of 0.01 atm. than at 0.20 atm. A Pco of 0.05 atm. did not reduce this stimulation at a PO2 atm. but did so at a PO2 atm. Heat-denatured hemoglobin had little influence on the rate of uptake. Even though the respiration of the rhizobia is enhanced by added nitrogenous compounds, the assumption that hemoglobin increases respiration by the same mechanism does not necessarily follow. Certain exptl. findings are inconsistent with this explanation. Although direct participation of the nodular hemoglobin in the N-fixation reaction appears to be an attractive and plausible function, examination of the evidence to date suggests that other alternatives have not been eliminated.