MadSci Network: Microbiology

Re: What is rhizobium leguminosarum?

Date: Fri Nov 14 18:57:11 2003
Posted By: Alex Barron, Graduate Student, Ecology(Biogeochemistry)
Area of science: Microbiology
ID: 1068752341.Mi

Ashley -

Rhizobia are a group of bacteria which can form symbioses with members of the legume family (which includes beans, alfalfa, acacia and many rainforest trees). The plants release chemical signals which attract the rhizobia, which in turn release chemicals which cause the plant to form small nodules on its roots. Rhizobia then live inside these nodules. The plant provides them with food (carbon) and the rhizobia produce nitrogen which the plant can use. It is one of the most fascinating examples of a symbiosis found in nature.

Rhizobia don't really "feed" on nitrogen. Like us, rhizobia are heterotrophs; they break down complex carbon compounds for energy. However, whereas most living creatures depend upon inorganic forms of nitrogen (nitrate (NO3-) and ammonium (NH4+)), rhizobia possess a very special enzyme called nitrogenase. This enzyme allows them to convert N2 gas (which makes up most of the atmosphere) into ammonium. They still need carbon for energy but they can "make" their own nitrogen from the air by converting nitrogen from the unavailable form (N2) to an inorganic one which all plants and microbes can use (NH4+).

Rhizobia are divided into a number of genera (Rhizobium, Bradyrhizobium, Sinorhizobium) and Rhizobium leguminosarum is a particular species in the genus Rhizobium. R. leguminosarum is capable of forming nodules on clovers, peas, lentils, and many beans.

Whether or not you need to be in a lab to work with Rhizobia depends upon what you want to do. Rhizobia can be a little difficult to grow in laboratory culture, and not all strains will fix nitrogen in lab conditions. It is, however, very easy to inoculate leguminous plants with rhizobia and study the resulting symbiosis. You could, for example, compare the growth of legumes that had been inoculated with rhizobia with those that had not.

If you are searching for inoculant or more information, check out this web page: Rhizobium Research Lab FAQ

There are some very good books (including one by J. Postgate) which summarize nitrogen fixation. You may be able to find them at a good library in Chicago.

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