conspire to constrain the total amount of nitrogen available in the 

 environment and its chemical composition. Thus, the degree of 

 coupling between nitrification and denitrification may control the 

 distribution of trace gases such as NO and ^0, and, in balance 

 with nitrogen fixation, may limit total global productivity by 

 limiting the availability of fixed nitrogen. Autotrophic, rather 

 than heterotrophic, nitrifiers make the major impact on nitrogen 

 cycling in the environment, and by virtue of their highly 

 specialized metabolism (obligately chemolithotrophic) , they are a 

 small, but phylogenetically diverse, group. Even though they 

 comprise a very minor percentage of the total community, species- 

 specific immunological probes and nucleic acid probes for 

 individual strains are likely to be useful tools in quantifying and 

 identifying nitrifying bacteria in complex assemblages. In 

 contrast, denitrifiers are a much more diverse group, largely 

 because the ability to denitrify is widespread and is insufficient 

 to identify a metabolically coherent group. In this case, probes 

 for functional entities, such as particular denitrifying enzymes 

 and cof actors, will be more useful. Since denitrification, unlike 

 nitrification, is an inducible metabolism, probes for gene 

 expression and enzyme activity will be important in distinguishing 

 the presence of the organism from the actual activity of the 

 induced metabolism. Nitrifiers and denitrifiers occur as membranes 

 of complex communities, on both large (oceanic oxygen-minimum 

 zones) and small (sediment surface communities in shallow coastal 

 environments) scales. Probes for specific individual strains or 

 gene expression on the single cell basis will be necessary to 

 dissect the interactions of these bacteria (see microbial consortia 

 below) . In terms of limiting factors, the 

 nitrification/denitrification couple is important because we need 

 to know not only what environmental factors control these two 

 processes themselves, but what controls the effect of their 

 interaction on the larger ecosystem: What determines the degree of 

 linkage between nitrification and denitrification, and therefore, 

 the net flux and loss of fixed nitrogen from the system? 



Organism Function in the Methane Cycle 



Methane is a radiatively active atmospheric trace gas that is 

 currently increasing at a rate of approximately 1% per year (Blake 

 and Rowland, 1988) . If this increase continues unabated, 

 predictions have been made that the effect of methane on global 



III-ll 



