Can we increase the nitrogen fixation efficiency of Sinorhizobium meliloti?

Abstract

Rhizobia are soil bacteria that can grow and reproduce in nodular swellings on legume plant roots. In return for carbon resources, rhizobia provide their host plant with fixed nitrogen (N2 → NH3) for proteins, etc. In alfalfa nodules, rhizobia that fix atmospheric nitrogen lose their ability to reproduce. These non-reproductive rhizobia may divert resources from nitrogen fixation to rhizopine, which can be used as an additional carbon source for still- eproductive rhizobia in the same nodule. Consequentially, rhizopine producing rhizobia would benefit at the host’s expense and could harm agriculture. From June 2010 to March 2011, I have been measuring nitrogen-fixing efficiency to determine if rhizopine production actually reduces the quantity of nitrogen fixation in several genetically different strains of rhizobia. I measured how much carbon dioxide (from respiration) and atmospheric hydrogen (a byproduct of nitrogen fixation) were given off by nodules as a function of oxygen concentration. Of the four strains of rhizobia tested by this method, two strains synthesized rhizopines and two strains did not. The respiration and fixation data for each strain fit a linear regression with the inverse slope = nitrogen-fixation efficiency and the y-intercept = carbon cost in the absence of nitrogen fixation. My results show that one rhizopine producing strain had lower nitrogen fixation efficiency. However, this did not occur in a commonly used rhizopine producing laboratory strain. Therefore, I was able to conclude that rhizopine production may not be the most important factor controlling nitrogen-fixation efficiency.