Hi Muhammed,

Sometimes ago, one of my students told me that one of the rhizobial strains he isolated as able to grow on a liquid medium containing no nitrogen source. However, after a rigourous verification, we found that one of the medium components contained very low amounts of nitrogen, which permitted rhizobial growth. When we changed this component, he obtained no growth.

As far as I know, rhizobia fix nitrogen in symbiosis with some legumes, and Parasponia sp. (Trema sp.) for some, inside roots because of some biochemical constraints: The nitrogenase complex is inhibited irreversibly by oxygen. So, inside the nodule, there is a limited oxygen and the bacteroids are protected by a peribacteroidal membrane in a symbiosome. However, for the oxydation processes, the oxygen is transported to the different compartments through the leghemoglobin... I think you know all the diferent details of these processes.

Greetings

Hi Dr. Mustapha Missbah El Idrissi, Thank you very much for providing nice information on Rhizobium and nitrogen fixation.

It´ll depend a lot on what you are calling liquid media. If it is in fact a semi-liquid one, one of the main phenotypical differential markers of Azorhizobium in relation to the remaining traditional rhizobia genera is exactly the in vitro nitrogen fixation. I am not sure of the details, though. Besides that, if you are talking about rhizobia, and not Rhizobium, some of the Burkholderia species are also endophytic nitrogen fixers in (I believe) sugar cane and sweet potatos, so they may be able to fix nitrogen in vitro.

This is an interesting issue. Papermill process waters are nitrogen deficient and recent microbiome testing is showing many rhizobia genera in the waters. It could be that they are within the biofilms, protected from oxygen (which is what leghemoglobin is supposed to do). I haven't worked in N-fixation for many years and my experience was with Frankia.