Growth energetics of Rhizobium leguminosarum in chemostat culture

Shell Research Ltd, Sittingbourne Research Centre, Sittingbourne, Kent ME9 8 AG, U.K. Department of Applied Biology, University of Wales Institute of Science and Technology, King Edward VII Avenue, Cardiff CF1 3NU, U.K. ABSTRACT SUMMARY: The efficiency of oxidative phosphorylation in symbiotic nitrogen-fixing bacteria of the genus Rhizobium in the root nodules of leguminous plants is of fundamental importance as it influences the efficiency of ATP generation from the oxidation of plant photosynthate for the ATP-dependent process of nitrogen fixation. A poor efficiency of energy coupling was found for Rhizobium leguminosarum grown glucose-limited in chemostat culture as indicated by low growth yields on glucose [ Y max glucose = 80.3 g dry wt bacteria (mol glucose) -1 and O 2 [ Y max oxygen = 25.6 g dry wt bacteria (mol O 2 ) -1 . (These values are low when compared with those for aerobic heterotrophic bacteria which have two or three sites of oxidative phosphorylation.) Although H + /O measurements suggested a potential P/O ratio of 3 there was a very rapid rate of proton decay across the cell membrane; this coupled with the low growth yields suggested that the actual P/O ratio was nearer 1. Under conditions of excess dissolved oxygen cytochromes of the c, b, o and aa 3 types were detected. Under oxygen limitation a d type terminal cytochrome oxidase was also produced and was associated with a further decrease in the efficiency of energy conservation. It remains to be seen whether such a low efficiency of energy conservation is expressed by R. leguminosarum within the root nodules of a host legume. Present address: Biological Laboratory, University of Kent at Canterbury, Kent CT2 7NJ, U.K. Present address: Biotechnology Department, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, Kuwait.