Dr. Sherri Morris
The Importance of Nitrogen-fixing Symbiotic Extremophiles
Extremophile: a word combination of the Latin extremus (extreme) and Greek philiā (love). Contrary to the average human, extremophiles love the extreme, preferring seemingly uninhabitable environments to resource rich habitats. These archaea are the toughest out there, and are found thriving in deep-sea thermal vents and sub-glacial lakes. A few of the outlandish environmental niches extremophiles have adapted to are high/low temperatures, pH extremes, the absence and excess of oxygen, and high salinity. While all these environments seem very extreme to humans, several extremophiles have invaded other organisms themselves, giving a whole new meaning to the word “extreme”.
The species of interest, Rhizobium leguminosarum, falls under the broad group of rhizobia, which are classified as nitrogen-fixing nodules that form on the roots of leguminous plants (Young and Haukka 1996). Rhizobium leguminosarum was the first discovered species of rhizobia, and is classified under the Rhizobiaceae family of the Rhizobium genus. This classification is all part of the alpha subdivision of Proteobacteria. Since rhizobia have such a broad range of organisms in it, it is split up into a total of three genera: Rhizobium, Bradyrhizobium and Azorhizobium. Rhizobia are similar in the fact that they fix nitrogen symbiotically through root nodules, but diversely distinctive by the specific way they accomplish that means, hence all the different classifications.
It was in the year of 1888 that the studies of two German scientists, Hellriegel and Wilfarth, proved that it was the root nodule bacteria that provided nitrogen to their host plants. The next year, Frank (1889) published the nodular symbiont under the name Rhizobium leguminosarum, and it has remained to this day. These studies brought to light the importance of rhizobia and leguminous nitrogen fixation to the scientific community. Since its discovery, Rhizobium leguminosarum and rhizobia in general have been extensively studied and several rhizobia organisms have had their entire genomes completely mapped. This scientific progress has allowed us to further understand rhizobia’s structure and function. Biosphere nitrogen is eventually lost into the atmosphere, and requires constant maintenance and replenishment (Mylona, Pawlowski, and Bisseling 1995). Only some prokaryotes can perform this highly oxygen-sensitive reduction process of nitrogen to ammonia. Efficient nitrogen fixers establish a symbiosis with higher plants where the plant partner provides the energy for nitrogen fixation. Rhizobium leguminosarum is one of these highly efficient nitrogen fixers, and it has some very unique growth factors. Initially, the rhizobia bacteria lives freely in the soil, typically around the roots of various plants such as clover, alfalfa, beans, and soy (Kimball 2011). But the bacteria do not stop here, as they cannot fix any nitrogen until the invasion of the roots of their appropriate legume. Once this invasion is complete, the rhizobia are engulfed into membrane-enclosed symbiosomes in the cytoplasm of the legume’s root cells. Another set of growth conditions worthy of the title “extremophile” is the symbiotic fixation of nitrogen in the guts of termites (Ohkuma, Noda, and Kudo 1999). Rhizobia and its related symbiotic nitrogen fixers are an important part of the agricultural world, as nitrogen is the most common nutrient deficiency. Due to the importance of nitrogen fixing bacteria, there is much literature on these extremophiles and their methods.
To comprehensively grasp how rhizobia are the specific extremophiles that they are, one must look first to the physiology of these root-nodule bacteria (Dilworth, James, Sprent, and Newton 2007). The rhizobia must survive for extended periods of time submerged in soils, which is often under stress from varying factors, such as pH...
Cited: Jones, K.M., H. Kobayashi, B.W. Davies, M.E. Taga, and G.C. Walker. 2007. How rhizobial symbionts invade plants: the Sinorhizobium– Medicago model. Nature Reviews Microbiology 5:619-633.
Kimball, J.W. (2011). Symbiotic Nitrogen Fixation. In Kimball’s Biology Pages.
Retrieved October 13, 2012, from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenFixation.html.
Mylona, P., K. Pawlowski, and T. Bisseling. 1995. Symbiotic nitrogen fixation. The Plant Cell 7:869-885.
Ohkuma, M., S. Noda, and T. Kudo. 1999. Phylogenetic diversity of nitrogen fixation genes in the symbiotic microbial community in the gut of diverse termites. Applied and Environmental Microbiology 65:4926-4934.
Poole, P.S., Hynes, M.F., Johnston, A.W.B., Tiwari, R.P., Reeve, W.G., Downie, J.A. 2007. Physiology of root-nodule bacteria. In: Nitrogen-fixing Leguminous Symbioses (Dilworth, M.J., James, E.K., Sprent, J.I., Newton, W.E., eds.). Springer, Dordrecht, The Netherlands, pp. 241-245.
Vance, C.P. 2001. Symbiotic nitrogen fixation and phosphorus acquisition. plant nutrition in a world of declining renewable resources. Plant Physiology 127:390-397.
Young, J.P.W., and K.E. Haukka. 1996. Diversity and phylogeny of rhizobia. New Phytologist 133:87-94.
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