Endophytic Micromonospora from Medicago sativa are apparently not able to fix atmospheric nitrogen

It has been reported that Micromonospora strains isolated from root nodules of Casuarina and Lupinus could be capable of fixing atmospheric nitrogen (N2) both inside root nodules and as free-living bacteria. Given the potential significance of such findings, we performed similar studies using isolates from Medicago root nodules in order to confirm if this phenomenon was a universal characteristic in nodule-derived Micromonospora isolates. Our results showed that of the Micromonospora strains studied: (i) they did not grow in N-free culture media, (ii) nitrogenase activity or incorporation of N2 to the biomass was not detected, neither in trials with free-living bacteria, nor in symbiosis with plants and (iii) the presence of genes for nitrogen fixation in these strains was impossible to confirm. It can be concluded that, under the conditions tested and with the variety of techniques used, Micromonospora strains employed do not necessarily fix N2 as free-living diazotrophs nor in symbiosis with alfalfa plants.