of Elemental Cycling 



1. Nitrogen Cycle 



Microbes play an important role in the global cycling of 

 nitrogen. Denitrifying bacteria reduce nitrate to NO^"/ nitrous 

 oxide, and eventually, to dinitrogen. Nitrogen lost by 

 denitrif ication must be resupplied to the biosphere, either by 

 atmospheric electrical discharge, or nitrogen fixation by microbes. 

 Nitrogen fixation is extremely important in certain terrestrial 

 (e.g., nitrogen-fixing nodules of legumes) and aquatic (e.g., 

 heterocyst-forming cyanobacteria) habitats. 



Although N seems to limit production in the sea, only a few 

 organisms have been identified as nitrogen fixers. Several 

 organisms (both free-living and symbiotic) potentially can fix N~, 

 but such activity may not have been detected because of 

 methodological difficulties. Furthermore, nitrogen fixation in 

 several marine environments has been debated, due to possible 

 artifacts generated by the methods of detection (e.g., in organic- 

 rich sediments) . The potential for N-fixation in these organisms 

 and environments can be determined directly by detecting the genes 

 responsible for N-fixation (nif genes) . Oligonucleotide primers 

 have been successfully used to amplify (using PCR) , clone, and 

 sequence nif genes from DNA from a variety of microorganisms and 

 natural environments, yielding information on the taxonomic 

 identity of the nitrogen-fixing microorganism. 



The use of an immunological probe in addition to a DNA probe 

 in samples from a natural community to detect the presence of 

 nitrogenase provides information on the expression of these genes. 

 Such an approach could yield answers to the following questions: 

 What is the distribution and diversity of nitrogen-fixing 

 microorganisms? What environmental factors regulate the expression 

 of nitrogenase? Are the numbers of N-fixing microorganisms 

 consistent with our estimates of nitrogen fixation in the sea? 

 More specifically, probes are available to show whether there is a 

 potential for nitrogen-fixation in either the picoplankton, in the 

 bacteria, or in the cyanobacteria component of the community. 



Because nitrifiers and denitrifiers utilize the products of 

 one reaction series as substrates for the opposite series, they 



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