I am currently a graduate student at BYU, in the Microbiology and Molecular Biology Department. I work in Dr. Griffitts' lab studying the symbiosis between rhizobia and legumes. Read on to find out what that means.
All living things are made up of atoms. The most abundant atoms in living things are represented by the acronym CHNOPS: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur. Nitrogen makes up 78% of our atmosphere [1], but it exists in a form that we can't use, called dinitrogen. Dinitrogen consists of two nitrogen atoms that have a triple bond (this means that they share three of their electrons). Breaking that triple bond so that individual nitrogen atoms are available requires a lot of energy. There are four basic processes which break the triple bond of dinitrogen: fire, lightning, the Haber-Bosch Process (the industrial process used to produce nitrogen fertilizer), and biological nitrogen fixation. The amounts of nitrogen produced by the first two are negligible, so we'll only discuss the last two.
The Haber-Bosch process was developed in 1909 by two German chemists. It converts dinitrogen from the atmosphere into ammonia (which is one nitrogen atom bonded to three hydrogen atoms). For best results, the Haber-Bosch process requires temperatures between 300 and 550 °C (that's 570 to 1000 °F) and pressures that are 150 to 250 times that of sea level.[2] To generate those kinds of temperatures, and to provide the hydrogen atoms needed to produce the ammonia, fossil fuels are burned. Fertilizer produced by the Haber-Bosch process is responsible for about 40% of the world's food supply.[3] Any fertilizer that isn't taken up by the plants ends up washing down into rivers and streams, polluting them.
The rest of the nitrogen found in plants (and thus in our food) comes from biological nitrogen fixation. Biological nitrogen fixation is the process that living organisms use to convert dinitrogen to ammonia. To date the only organisms known to do this are bacteria. Some of these bacteria will actually form a symbiosis with plants. These symbiotic bacteria are collectively called rhizobia. The rhizobia provide nitrogen to the plant in exchange for sugar (a source of carbon). The plants then convert the ammonia into proteins.
The plants which form this symbiosis with rhizobia are usually legumes. Legumes are high in protein content because of the nitrogen fixation performed by their rhizobia. There are important legumes which are forage crops (alfalfa, clover, vetch, etc) and food crops (beans, peas, soybeans, peanuts, lentils, etc.). Not only do legumes provide protein to humans and animals, they also enrich the soil with nitrogen. This is why they are used for crop rotations, for intercropping, and as green manures.
In contrast to applied fertilizers, nitrogen from rhizobia does not pollute the water supply, it doesn't require fossil fuels, and it takes place at normal temperatures and pressures. So there are many things that make it preferable to the industrial Haber-Bosch process. This is why we want to better understand biological nitrogen fixation.
When the plant and the bacteria form the symbiosis, the plant produces a special organ for the bacteria to live in, called a root nodule which look like little pink warts on the roots (see the picture to the right).
Now, there are between 100 million and 3 billion bacteria per gram of soil, depending on the location.[4] The vast majority of these are not rhizobia, and many will cause disease if they manage to infect the plant. So how does the plant let in the right bacteria and exclude all the rest? That is the question I am trying to answer.
In particular, I am studying the interaction between a single species of rhizobia called Sinorhizobium meliloti and its plant hosts. There are only three types of plants which will form the symbiosis with it: medics (which include alfalfa), sweetclovers, and fenugreeks. All other legumes reject Sinorhizobium meliloti as though it were just another potentially harmful soil bacteria. I want to know why and how.
Notes:
[3] Smil, V. (2002) "Nitrogen and Food Production: Proteins for Human Diets." (.pdf) Ambio 31 (2), p. 126.
Image attributions:
Alfalfa is by Ben L, available at http://picasaweb.google.com/lh/photo/-gDTzpwJdLV-487Xa2-pmg.
Root nodules are from http://commons.wikimedia.org/wiki/File:Medicago italica root nodules 2.JPG. It was actually me who donated this image to Wikipedia!
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