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Role of Diazotrophic Bacteria in Biological Nitrogen Fixation and Plant Growth Improvement
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Role of Diazotrophic Bacteria in Biological Nitrogen Fixation and Plant Growth Improvement
Shin, Wansik; Islam, Rashedul; Benson, Abitha; Joe, Manoharan Melvin; Kim, Kiyoon; Gopal, Selvakumar; Samaddar, Sandipan; Banerjee, Somak; Sa, Tongmin;
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Though there is an abundant supply of nitrogen in the atmosphere, it cannot be used directly by the biological systems since it has to be combined with the element hydrogen before their incorporation. This process of nitrogen fixation (-fixation) may be accomplished either chemically or biologically. Between the two elements, biological nitrogen fixation (BNF) is a microbiological process that converts atmospheric di-nitrogen () into plant-usable form. In this review, the genetics and mechanism of nitrogen fixation including genes responsible for it, their types and role in BNF are discussed in detail. Nitrogen fixation in the different agricultural systems using different methods is discussed to understand the actual rather than the potential -fixation procedure. The mechanism by which the diazotrophic bacteria improve plant growth apart from nitrogen fixation such as inhibition of plant ethylene synthesis, improvement of nutrient uptake, stress tolerance enhancement, solubilization of inorganic phosphate and mineralization of organic phosphate is also discussed. Role of diazotrophic bacteria in the enhancement of nitrogen fixation is also dealt with suitable examples. This mini review attempts to address the importance of diazotrophic bacteria in nitrogen fixation and plant growth improvement.
Biological nitrogen fixation (BNF);Diazotrophic bacteria;Plant growth promotion; fixing prokaryotes;nif genes;
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Affourtit, J., J.P. Zehr, and H.W. Paerl. 2001. Distribution of nitrogen-fixing microorganisms along the Neuse River Estuary, North Carolina. Microb. Ecol. 41:114-123.

Araujo, A.E. da S., V.L.D. Baldani, P. de S. Galisa, J.A. Pereira, and J.I. Baldani. 2013. Response of traditional upland rice varieties to inoculation with selected diazotrophic bacteria isolated from rice cropped at the Northeast region of Brazil. Appl. Soil Ecol. 64:49-55. doi:10.1016/j.apsoil.2012.10.004. crossref(new window)

Aslantas, R., R. Cakmakci, and F. Sahin. 2007. Effect of plant growth promoting rhizobacteria on young apple tree growth and fruit yield under orchard conditions. Sci. Hort. 111:371-377. doi:10.1016/j.scienta.2006.12.016. crossref(new window)

Bahulikar, R.A., I. Torres-Jerez, E. Worley, K. Craven, and M.K. Udvardi. 2014. Diversity of nitrogen-fixing bacteria associated with switchgrass in the native tallgrass prairie of northern Oklahoma. Appl. Environ. Microbiol. 80:5636-43. doi:10.1128/AEM.02091-14. crossref(new window)

Banchio, E., P.C. Bogino, J. Zygadlo, and W. Giordano. 2008. Plant growth promoting rhizobacteria improve growth and essential oil yield in Origanum majorana L. Biochem. Syst. Ecol. 36:766-771. doi:10.1016/j.bse.2008.08.006. crossref(new window)

Barua, S., S. Tripathi, A. Chakraborty, S. Ghosh, and K. Chakrabarti. 2012. Characterization and crop production efficiency of diazotrophic bacterial isolates from coastal saline soils. Microbiol. Res. 167:95-102. doi:10.1016/j.micres.2011.04.001. crossref(new window)

Bashan, Y., G. Holguin, and L.E. de Bashan. 2004. Azospirillumplant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003) Can. J. Microbiol. 50:521-577. crossref(new window)

Beijerinck, M.W. 1901. Uber oligonitrophile Mikroben. Zentral Bakteriologische Parasitenk Abt. II 7:561-565.

Bishop, P.E., and R. Premakumar. 1992. Alternative nitrogen fixation systems, pp. 736-762. In G. Stacey, R. H. Burris, and H. J. Evans (ed.), Biological Nitrogen Fixation. Chapman and Hall, New York, NY.

Bishop, P.E., R. Premakumar, D.R. Dean, M.R. Jacobsen, J.R. Chisnell, T.M. Rizzo, and J. Kopczynski. 1986. Nitrogen fixation by Azotobacter vinelandii strains having deletions in structural genes for nitrogenase. Science 232:92-94. crossref(new window)

Buyer, J.S., and D.D. Kaufman. 1996. Microbial diversity in the rhizosphere of corn grown under conventional and lowinput systems. Appl. Soil Ecol. 5:21-27.

Carrillo-Garcia, A., Y. Bashan, E.D. Rivera, and G.J. Bethlenfalvay. 2000. Effects of Resource-Island Soils, Competition, and Inoculation with Azospirillum on Survival and Growth of Pachycereus pringlei, the Giant Cactus of the Sonoran Desert. Restor. Ecol. 8:65-73. doi:10.1046/j.1526-100x.2000.80009.x. crossref(new window)

Chien, Y.T. and S.H. Zinder. 1996. Cloning, functional organization, transcript studies, and phylogenetic analysis of the complete nitrogenase structural genes (nifHDK2) and associated genes in the archaeon Methanosarcina barkeri 227. J. Bacteriol. 178:143-148. crossref(new window)

Cleveland, C.C., A.R. Townsend, D.S. Schimel, H. Fisher, R.W. Howarth, L.O. Hedin, S.S. Perakis, E.F. Latty, J.C. Von Fischer, A. Elseroad, and M.F. Wasson. 1999. Global patterns of terrestrial biological nitrogen ($N_2$) fixation in natural ecosystems. Global Biogeochem. Cy. 13:623-645. crossref(new window)

Danso, S.K.A. 1995. Assessment of biological nitrogen fixation. Nutrient Cycling in Agroecosystems. 42:33-41. doi:10.1007/bf00750498. crossref(new window)

de Souza, R., A. Beneduzi, A. Ambrosini, P.B. da Costa, J. Meyer, L.K. Vargas, R. Schoenfeld, and L.M.P. Passaglia. 2012. The effect of plant growth-promoting rhizobacteria on the growth of rice (Oryza sativa L.) cropped in southern Brazilian fields. Plant Soil 366:585-603. doi:10.1007/s11104-012-1430-1. crossref(new window)

Dean, D.R. and M.R. Jacobson. 1992. Biochemical genetics of nitrogenase. In: Stacy, G., Burris, R.H., Evans, H.J. (Eds.), Biological Nitrogen Fixation. pp. 763-834. Chapman and Hall, New York.

Delwiche, C.C. 1981. The nitrogen cycle and nitrous oxide, pp. 1-15. In C. C. Delwiche (ed.), Denitrification, nitrification and atmospheric nitrous oxide. John Wiley and Sons, New York, NY.

Dey, R., K.K. Pal, D.M. Bhatt, and S.M. Chauhan. 2004. Growth promotion and yield enhancement of peanut (Arachis hypogaea L.) by application of plant growth-promoting rhizobacteria. Microbiol. Res. 159:371-394. doi:10.1016/j.micres.2004.08.004. crossref(new window)

Diallo D.M., A. Willems, N. Vloemans, S. Cousin, T.T. Vandekerckhove, P. de Lajudie, M. Neyra, W. Vyverman, et al. 2004. Polymerase chain reaction denaturing gradient gel electrophoresis analysis of the N2-fixing bacterial diversity in soil under Acacia tortilis ssp. raddiana and Balanites aegyptiaca in the dryland part of Senegal. Environ. Microbiol. 6:400-15. crossref(new window)

Dobbelaere, S., J. Vanderleyden, and Y. Okon. 2003. Plant growth-promoting effects of diazotrophs in the rhizosphere. Crit. Rev. Plant Sci. 22:107-149. crossref(new window)

Estrada, G.A., V.L.D. Baldani, D.M. de Oliveira, S. Urquiaga, and J.I. Baldani. 2012. Selection of phosphate-solubilizing diazotrophic Herbaspirillum and Burkholderia strains and their effect on rice crop yield and nutrient uptake. Plant Soil 369:115-129. doi:10.1007/s11104-012-1550-7. crossref(new window)

Fani, R., R. Gallo, and P. Lio. 2000. Molecular evolution of nitrogen fixation: The evolutionary history of the nifD, nifK, nifE and nifN genes. J. Mol. Evol. 51:1-11. crossref(new window)

Fischer, D., B. Pfitzner, M. Schmid, J. L. Simoes-Araujo, V. M. Reis, W. Pereira, E. Ormeño-Orrillo, B. Hai, et al. 2011. Molecular characterisation of the diazotrophic bacterial community in uninoculated and inoculated field-grown sugarcane (Saccharum sp.). Plant Soil 356:83-99. doi:10.1007/s11104-011-0812-0. crossref(new window)

Franche, C., K. Lindstrom, and C. Elmerich. 2009. Nitrogenfixing bacteria associated with leguminous and non-leguminous plants. Plant Soil. DOI 10.1007/s11104-008-9833-8. crossref(new window)

Giller, K.E., and J.M. Day. 1985. Nitrogen fixation in the rhizosphere: significance in natural and agricultural systems. In A. H. Fitter (ed.), Ecological interactions in soil. Plants microbes and animals. Blackwell Scientific Publications, Oxford.

Glick, B.R., B. Todorovic, J. Czarny, Z. Cheng, J. Duan, and B. McConkey. 2007. Promotion of plant growth by bacterial ACC deaminase. Crit. Rev. Plant Sci. 26:1-16. crossref(new window)

Halbleib, C.M., and P.W. Ludden. 2000. Regulation of biological nitrogen fixation. J. Nutr. 130:1081-1084. crossref(new window)

Halbleib, C.M., Y.P. Zhang, and P.W. Ludden. 2000. Regulation of dinitrogenase reductase ADP-ribosyltransferase and dinitrogenase reductase-activating glycohydrolase by a redox-dependent conformational change of nitrogenase Fe protein. J. Biol. Chem. 275:3493-3500. crossref(new window)

Hardy, R.W.F., R.C. Burns, and R.D. Holsten. 1973. Applications of the acetylene-ethylene assay for measurement of nitrogen fixation. Soil Biol. Biochem. 5:47-81. doi:10.1016/0038-0717(73)90093-X. crossref(new window)

Hassan, W., R. Bano, F. Bashir, and J. David. 2013. Comparative effectiveness of ACC-deaminase and/or nitrogen-fixing rhizobacteria in promotion of maize (Zea mays L.) growth under lead pollution. Environ. Sci. Poll. Res. 21:10983-10996. doi:10.1007/s11356-014-3083-5. crossref(new window)

Hassan, Y.I., D. Lepp, J. He, and T. Zhou. 2014. Draft Genome Sequences of Devosia sp. Strain 17-2-E-8 and Devosia riboflavina Strain IFO13584. Genome announcements 2. doi:10.1128/genomeA.00994-14. crossref(new window)

Hellriegel, H. and H. Wilfarth. 1886. Untersuchungen uber die stickstoff-nahrung der gramineen und leguminosen. Beilageheft zu der Zeitschrift des Vereins der Ruebenzucker-Industrie Deutschen Reiche. pp. 1-234.

Herridge, D.F., M.B. Peoples, and R.M. Boddey. 2008. Global inputs of biological nitrogen fixation in agricultural systems. Plant Soil 311:1-18. doi:10.1007/s11104-008-9668-3. crossref(new window)

Hill, S. 1992. Physiology of nitrogen fixation in free-living heterotrophs, pp. 87-134. In G. Stacey, R. H. Burris, and H. J. Evans (ed.), Biological Nitrogen Fixation. Chapman and Hall, New York, NY.

Hirsch, A.M., H.I. McKhann, A. Reddy, J.Y. Liao Y.W. Fang, and C.R. Marshall. 1995. Assessing horizontal transfer of nifHDK genes in eubacteria: nucleotide sequence of nifK from Frankia strain HFPCcI3. Mol. Biol. Evol. 12:16-27. crossref(new window)

Holguin, G., and B. R. Glick. 2001. Expression of the ACC deaminase gene from Enterobacter cloacae UW4 in Azospirillum brasilense. Microbial Ecol. 41:281-288. doi: 10.1007/s002480000040. crossref(new window)

Isawa, T., M. Yasuda, H. Awazaki, K. Minamisawa, S. Shinozaki, and H. Nakashita. 2010. Azospirillum sp. Strain B510 Enhances Rice Growth and Yield. Microb. Environ. 25:58-61. doi: 10.1264/jsme2.ME09174. crossref(new window)

Kennedy, I. R., and N. Islam. 2001. The current and potential contribution of asymbiotic nitrogen fixation to nitrogen requirements on farms: a review. Animal Prod. Sci. 41. CSIRO Publishing: 447-457. doi:10.1071/EA00081. crossref(new window)

Kuklinsky-Sobral, J., W.L. Araujo, R. Mendes, I.O. Geraldi, A.A. Pizzirani-Kleiner, and J.L Azevedo. 2004. Isolation and characterization of soybean-associated bacteria and their potential for plant growth promotion. Environ. Microbiol. 6:1244-1251. crossref(new window)

Madigan, M.T., J.M. Martinko, and J. Parker. 2000. Brock Biology of Microorganisms, Ninth ed. Prentice Hall, Upper Saddle River, NJ.

Mahieu, S., J. Escarre, B. Brunel, A. Mejamolle, S. Soussou, A. Galiana, and J.-C. Cleyet-Marel. 2013. Soil nitrogen balance resulting from N fixation and rhizodeposition by the symbiotic association Anthyllis vulneraria/Mesorhizobium metallidurans grown in highly polluted Zn, Pb and Cd mine tailings. Plant Soil 375:175-188. doi:10.1007/s11104-013-1941-4. crossref(new window)

Malik, K.A. and H.G. Schlegel. 1981. Chemolithoautotrophic growth of bacteria able to grow under N2-fixing conditions. FEMS Microbiol. Lett. 11:63-67. crossref(new window)

Marschner, H. 1995. Mineral nutrition of higher plants. 2nd edition. Academic Press, London.

Miche, L., M.L. Bouillant, R. Rohr, G. Salle, and R. Bally. 2000. Physiological and cytological studies on the inhibition of Striga seed germination by the plant growth-promoting bacterium Azospirillum brasilense. Eur. J. Plant Pathol. 106:347-351. crossref(new window)

Novoa, R., and R.S. Loomis. 1981. Nitrogen and plant production. Plant soil, 58(1-3), 177-204. crossref(new window)

Ohkuma, M., S. Noda, R. Usami, K. Horikoshi, and T. Kudo. 1996. Diversity of nitrogen fixation genes in the symbiotic intestinal microflora of the termite Reticulitermes speratus. Appl. Environ. Microbiol. 62:2747-2752.

Olson, N.D., T.D. Ainsworth, R.D. Gates, and M. Takabayashi. 2009. Diazotrophic bacteria associated with Hawaiian Montipora corals: Diversity and abundance in correlation with symbiotic dinoflagellates. J. Exp. Mar. Biol. Ecol. 371:140-146. doi:10.1016/j.jembe. 2009.01.012. crossref(new window)

Paul, E.A., and G.E. Clark. 1996. Soil Microbiology and Biochemistry, 2nd ed. Academic Press, San Diego, CA.

Piceno, Y.M., P.A. Noble, and C. R. Lovell. 1999. Spatial and temporal assessment of diazotroph assemblage composition in vegetated salt marsh sediments using denaturing gradient gel electrophoresis analysis. Microb. Ecol. 38:157-167. crossref(new window)

Poly, F., L. Ranjard, S. Nazaret, F. Gourbiere, and L.J. Monrozier. 2001. Comparison of nifH gene pools in soils and soil microenvironments with contrasting properties. Appl. Environ. Microbiol. 67:2255-2262. crossref(new window)

Prigent-Combaret, C., D. Blaha, F. Pothier, L. Vial, M.A. Poirier, F. Wisniewski-Dye, and Moenne-Loccoz, Y. 2008. Physical organization and phylogenetic analysis of acdR as leucine-responsive regulator of the 1-aminocyclopropane-1-carboxylate deaminase gene acdS in phytobeneficial Azospirillum lipoferum 4B and other Proteobacteria. FEMS Microbiol. Ecol. 65:202-219. crossref(new window)

Puri, A. 2016. Plant growth promotion nitrogen fixation by Paenibacillus polymxya in corn and canola. University of British, Columbia.

Ribbe, M., D. Gadkari, and O. Meyer. 1997. $N_2$ fixation by Streptomyces thermoautotrophicus involves a molybdenum-dinitrogenase and a manganesesuperoxide oxidoreductase that couple $N_2$ reduction to the oxidation of superoxide produced from $O_2$ by a molybdenum-CO dehydrogenase. J. Biol. Chem. 272:26627-26633. crossref(new window)

Richardson, A.E., J.M. Barea, A.M. McNeill and C. Prigent-Combaret. 2009. Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms. Plant Soil. DOI 10.1007/s11104-009-9895-2 crossref(new window)

Roesch, L.F.W., F.A.O. Camargo, F.M. Bento, and E.W. Triplett. 2007. Biodiversity of diazotrophic bacteria within the soil, root and stem of field-grown maize. Plant Soil 302: 91-104. doi:10.1007/s11104-007-9458-3. crossref(new window)

Rondon, M. A., J. Lehmann, J. Ramirez, and M. Hurtado. 2006. Biological nitrogen fixation by common beans (Phaseolus vulgaris L.) increases with bio-char additions. Biol. Fert. Soil. 43:699-708. doi:10.1007/s00374-006-0152-z. crossref(new window)

Rothballer, M., B. Eckert, M. Schmid, M., Fekete, A., Scholter, M., Lehner, A., Pollmann, S. and Hartmann, A. 2008. Endophytic root colonization of gramineous plants by Herbaspirillum frisingense. FEMS Microbiol. Ecol. 66:85-95. crossref(new window)

Sasaki, K., S. Ikeda, S. Eda, H. Mitsui, E. Hanzawa, C. Kisara, Y. Kazama, A. Kushida, et al. 2010. Impact of plant genotype and nitrogen level on rice growth response to inoculation with Azospirillum sp. strain B510 under paddy field conditions. Soil Sci. Plant Nut. 56:636-644. doi: 10.1111/j.1747-0765.2010.00499.x. crossref(new window)

Shu, W., G.P. Pablo, Y. Jun, and H. Danfeng. 2011. Abundance and diversity of nitrogen-fixing bacteria in rhizosphere and bulk paddy soil under different duration of organic management. World J. Microbiol. Biotechnol. 28:493-503. doi: 10.1007/s11274-011-0840-1. crossref(new window)

Steenhoudt, O. and Vanderleyden, J. 2000. Azospirillum, a freeliving nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects. FEMS Microbiol. Rev. 24:487-506. crossref(new window)

Stewart, W.D.P. 1969. Biological and ecological aspects of nitrogen fixation by free-living microorganisms. Proc. Roy. Soc. B. (London) 172:367-388. crossref(new window)

Suman, A., A. Gaur, A.K. Shrivastava, and R.L. Yadav. 2005a. Improving Sugarcane Growth and Nutrient Uptake by Inoculating Gluconacetobacter diazotrophicus. Plant Growth Reg. 47:155-162. doi:10.1007/s10725-005-2847-9. crossref(new window)

Swedrzynska, D., and A. Sawicka. 2000. Effect of Inoculation with Azospirillum brasilense on Development and Yielding of Maize (Zea mays ssp. Saccharata L.) under Different Cultivation Conditions. Pol. J. Environ. Stud. 6.

Taule, C., C. Mareque, C. Barlocco, F. Hackembruch, V. M. Reis, M. Sicardi, and F. Battistoni. 2011. The contribution of nitrogen fixation to sugarcane (Saccharum officinarum L.), and the identification and characterization of part of the associated diazotrophic bacterial community. Plant Soil 356:35-49. doi:10.1007/s11104-011-1023-4. crossref(new window)

Tchan, Y.T. 1988. Some aspects of non-rhizobial diazotrophs: their past and their future. In I. R. Kennedy (ed.), Microbiology in action. Research Studies Press Ltd., Letchworth.

Thiel, T., E.M. Lyons, J.C. Erker., and A. Ernst. 1995. A second nitrogenase in vegetative cells of a heterocyst forming cyanobacterium. Proc. Natl. Acad. Sci . USA 92: 9358-9362. crossref(new window)

Tilman, D., K.G. Cassman, P.A. Matson, R. Naylor, and S. Polasky. 2002. Agricultural sustainability and intensive production practices. Nature, 418(6898):671-677. crossref(new window)

Ueda, T., Y. Suga, N. Yahiro, and T. Matsuguchi. 1995. Remarkable $N_2$-fixing bacterial diversity detected in rice roots by molecular evolutionary analysis of nifH gene sequences. J. Bacteriol. 177:1414-1417. crossref(new window)

Unkovich, M., D. Herridge, and M. Peoples. 2008. Measuring plant-associated nitrogen fixation in agricultural systems. Australian Centre for Intern. Agri.l Res. 258.

Vaishampayan, A., R.P. Sinha, D.P. Hader, T. Dey, A.K. Gupta, U. Bhan, and A.L. Rao. 2001. Cyanobacterial biofertilizers in rice agriculture. Bot. Rev. 67:453-516. crossref(new window)

Vance, C.P. and P. H. Graham. 1995. Nitrogen fixation in agriculture: application and perspectives., pp. 77-86. In I. A. Tikhonovich, N. A. Provorov, V. I. Romanov and W. E. Newton (ed.), Nitrogen Fixation: Fundamentals and Applications. Kluwer Academic Publishers, Dordrecht.

Vessey, J.K. 2003. Plant growth promoting rhizobacteria as biofertilizers. Plant Soil 255:571-586. crossref(new window)

Vitousek, P.M., and P.A. Matson. 2009. Nutrient cycling and biogeochemistry. Princeton University Press, New Jersey.

Wang, L., Z. Yu, J. Yang, and J. Zhou. 2015. Diazotrophic bacterial community variability in a subtropical deep reservoir is correlated with seasonal changes in nitrogen. Environ. Sci. Poll. Res. Int. 22:19695-705. doi:10.1007/s11356-015-5144-9. crossref(new window)

Wang, S., J. Chen, and J.L. Johnson.1988. The presence of five nifH-like sequences in Clostridium pasteurianum: sequence divergence and transcription properties. Nucleic Acids Res. 16:439-454. crossref(new window)

Ward, D.M., R. Weller, and M.M. Baeson. 1990. 16S RNA sequences reveal numerous uncultured microorganisms in a natural community. Nature 345:63-65. crossref(new window)

Wartiainen, I., T. Eriksson, W. Zheng, and U. Rasmussen. 2008. Variation in the active diazotrophic community in rice paddy-nifH PCR-DGGE analysis of rhizosphere and bulk soil. Appl. Soil Ecol. 39:65-75. crossref(new window)

Weber, O.B., R.N. Lima, L.A. Crisóstomo, J.A.D. Freitas, A.C.P.P. Carvalho, and A.H.N. Maia. 2009. Effect of diazotrophic bacterium inoculation and organic fertilization on yield of Champaka pineapple intercropped with irrigated sapota. Plant Soil 327:355-364. doi:10.1007/s11104-009-0059-1. crossref(new window)

Yan, Y., J. Yang, Y. Dou, M. Chen, S. Ping, J. Peng, and W. Lu. 2008. Nitrogen fixation island and rhizosphere competence traits in the genome of root-associated Pseudomonas stutzeri A1501. Proc. Natl. Acad. Sci. USA. 105: 7564-7569. crossref(new window)

Young, J.P.W. 1992. Phylogenetic classification of nitrogen-fixing organisms, pp. 43-86. In G. Stacey, R. H. Burris, and H. J. Evans (ed.), Biological Nitrogen Fixation. Chapman and Hall, New York, NY.

Zani, S., M.T. Mellon, J.L. Collier, and J.P. Zehr. 2000. Expression of nifH genes in natural microbial assemblages in Lake George, New York, detected by reverse transcriptase PCR. Appl. Environ. Microbiol. 66:3119-3124. crossref(new window)

Zehr, J.P. and L.A. McReynolds. 1989. Use of degenerate oligonucleotides for amplification of the nifH gene from the marine cyanobacterium Trichodesmium thiebautii. Appl. Environ. Microbiol. 55:2522-2526.

Zehr, J.P., S. Braun, Y.B. Chen. and M. Mellon. 1996. Nitrogen fixation in the marine environment: relating genetic potential to nitrogenase activity. J. Exp. Mar. Biol. Ecol. 203:61-73. crossref(new window)

Zehr, J.P., B.D. Jenkins, S.M. Short, and G.F. Steward. 2003. Nitrogenase gene diversity and microbial community structure: a cross-system comparison. Environ. Microbiol. 5:539-554. crossref(new window)