Scientists can use Nitrogen-Fixing Bacteria to extract certain kinds of pollutants out of soils to bring them back to their natural states. Nitrogen-Fixing Bacteria is available in many different types such as the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and Azorhiobium (MacLean,2007). They may vary in size which can determine where they would be best utilized and determine the re-embodiment of soils (MacLean,2007). Also their location in the soil is a critical factor in the development of ecosystems. Legume-rhizobium symbiosis is what we can use to re-embodiment of the land that is contaminated (MacLean,2007). We are going to concentrate on how this process is used in the extraction of Arsenic and Copper contaminated lands. Legume-rhizobium symbiosis is also being used to fix the toxic spill that occurred at the Aznalcollar pyrite mine in southern Spain (Carrasco et all,2005). Free-Living Heterotrophic Bacteria are extremely important for the natural reduction of contaminated Antarctic soils for re-growth. Finding out what happens in Antarctica is important for better understanding what diazotrophs are doing in the soils of Antarctica (Eckford et all,2002). 

      Bacteria forming, nitrogen-fixing symbiosis is provided by legumes that replace the nitrogen in soils. The rhizobium has larger genomes than most bacteria and B. japonicum has the largest chromosome sizes because of its extra chromosomal DNA. Bradyrhzobia have the smaller genomes because they lack extra chromosomal DNA (MacLean et all 2007).

      The use of legume-rhizobium symbiosis is essential for the N-cycle for both ecosystems and legumes we understand that they provide nitrogen in the form of ammonia (Sylvia, 2005). Rhizobial bacteria are structured to tolerate high Arsenic only by internally maintaining a low Arsenic concentration. Interestingly enough, rhizobiums can fix inert nitrogen gas from the atmosphere and supply it to the plant as NH⁴⁺ which can be used by plants for growth. In a recent study it was shown that injecting (DW) biomass increases the tolerance of Arsenic to plants to due to a high solution of P (Phosphorus) concentrates. On the contrary when it comes to the increasing Arsenic in alfalfa, it reduces its nodules by 50 percent. Rhizobium was found to increase the sensitivity of the legume host to Arsenic in the growing medium. It was also found that Arsenic concentrations did not have a significant effect on the root nodules with potential N-fixing capabilities (Reichman, 2007). The study concluded, rhizobium has a potential advantage when it comes to the remediation of heavy metal contaminated sites because it is produced cheaply and in large quantities.

      At the Aznalcollar mine in southern Spain occurred a drastic mining spill during April 1998 where the dam collapsed and contaminated the croplands along the Agrio and Guadiamar river valleys (Carrasco et all,2004). Eventhough, the clean up had taken out most of the sludge there was still patches of high concentrations of metals that remained in the soil. Some investigators decided to us Phytoremediation which is the technology that uses nitrogen fixing bacteria in plants to clean up pollutants from the soil (Carrasco et all,2004). Investigators isolated Rhizobium to resist Arsenic and heavy metal from the contaminated soil and use them with metal accumulating legumes for bioremediation. They isolated about 100 Rhizobium strains with the native legumes that had functional nodules to find the strain that would work most efficiently. Bioaccumulation results indicated that the strain S. meliloti Alf 12 was the best fit for the clean up (Carrasco et all,2004). Though this strain was very successful in the clean up it lacked the resistance to Copper (CU). So they used the strain Med4D which can handle the high concentrates of Cu in the contaminated areas (Carrasco et all,2004). The studies at the Aznalcollar mine resulted in over fifty successful strains of Rhizobium on highly concentrated metals along the Agrio and Guadiamar river valleys soils to be used in the remediation of affected areas around the world.

      Free-living phototrophic nitrogen fixing cynanobacteria have been found to exist in surface soils, snow, sea ice and lakes in Antarctica (Eckford et all,2002). Spills of fossil fuels, most commonly diesel and jet fuel on soils results in high soil/carbon nitrogen ratios (Eckford et all 2002). This can limit the microbial activity greatly. The Antarctic Treaty bans the use of microbes for bioremediation for the remediation of soils on the continent. So it is important for us to understand what the potential biodegradative abilities of the indigenous microflora can do for the environment. Interestingly enough, the Antarctic soils have partial organic carbon whereas fuel-contaminated soils have enough carbon but limited nitrogen (Eckford et all, 2002). The organic carbon can be used for heterotrophic diazotrophs. Hydrocarbon degradation under nitrogen-limited conditions can still have the potential for growth without nitrogen. The results concur that it is possible that diazotrophs fix nitrogen and degrade hydrocarbons (Eckford et all,2002). So it is possible that they alternate between hydrocarbon utilization and diazotrophy.

      In the results above you can understand some situations where Nitrogen-Fixing Bacteria can be used to extract certain kinds of pollutants from out of soils. The most common bacteria used in the situations were Rhizobium which is used to remove heavy metal contaminates like Arsenic and Copper. Legume-rhizobium symbiosis is one of the greatest environmental finding when it comes to cleaning up our chemical/metal spills to bring our ecosystems to their natural states. When the Aznalcollar pyrite mine had its spill they used different strains of rhizobium to relieve the land from heavy metals. Lastly, we examined the hypothesis that scientists brought around that diazotrophs alternate between hydrocarbon utilization and diazotrophy. This was important to understand how the environment is fixing itself in the Antarctic.

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Works Cited:

Carrasco, J.A., Armanio, P, Pajuelo, E, Burgos, A, Caviedes, M.A., & Lopez, R , Chamber, M.A., Palomeras, A.J.,(2005). Isolation and characterization of symbiotically effective rhizobium resistant to arsenic and heavy metals after the toxic spill at the aznalcollar pyrite mine. 37, 1131-1140.

Eckford, R, Cook, F.D., Saul, D, Aislabie, J, & Foght, J (2002). Free-living heterotrophic nitrogen-fixing bacteria isolated from fuel contaminated antarctic soils. Applied and Environmental Microbiology. 68, 5181-5185.

MacLean, A.M., Finan, T.M., & Sadowsky, M.J. (2007). Genomes of the symbiotic nitrogen-fixing bacteria of legumes. Plant Physiology. 144, 615-622.

Reichman, S.M. (2007).The potential use of the legume- rhizobium symbiosis for the remediation of arsenic contaminated sites. Soil Biology and Biochemistry. 39, 2587-2593.

Sylvia, D.M., Fuhrmann, J.J., Hartel, P.G., Zuberer, D.A., 2005. Biological dinitrogen fixation: Symbiotic. Graham, P.H. Principles and Applications of Soil Microbiology, 2^nd ed, pp. 406-408. Pearson Education Inc., New Jersey