For identifying organisms and populations, the concept of 

 "species" may or may not be useful in predicting succession in the 

 plankton or in the sedimentary environment. The concept of 

 "functional groups" may be more useful. (Often, but not always, 

 functional groups will be taxonoraically based. The "species," 

 however, may not be the important functional unit.) A species 

 appears in the plankton and becomes "abundant" in part because it 

 has the necessary pathways or molecular apparatus for survival. In 

 part, this is a stochastic function of whether or not the species 

 is present in sufficient numbers to serve as an inoculum. The 

 classification of "functional" groups is a formidable problem and 

 is primarily a conceptual one, in the sense of defining the set of 

 environmental conditions in which the species must function to 

 persist. 



Functional groups based on metabolic pathways and other 

 parameters are discussed below, to emphasize some of the possible 

 routes that might be taken in the solution of this obviously very 

 complex issue. 



Determining Organism Function in Consortia 



Many geochemical processes co-occur in microbial assemblages 

 such as mats, planktonic and soil consortia. Consortia means a 

 suite of organisms interacting as a functional group within a 

 larger community of organisms. Degradation of a chlorinated 

 hydrocarbon pesticide by a group of microorganisms, each performing 

 part of the overall reaction that destroys the pesticide, is an 

 example of a consortium which can be studied in the laboratory. 

 Many processes in carbon, nitrogen, and sulfur cycles are likely to 

 be mediated by similar consortia. Processing of nitrogen in 

 microbial mats is such an example, in which multiple opposing 

 transformations (e.g., oxidation and reduction of nitrogen oxides) 

 are mediated by diverse but physically and biochemically 

 coordinated organisms. Net chemical fluxes through consortia are 

 determined by the integrated activity of the consortia members and 

 their response to external cues. 



Microbial Loop of Marine Food Webs 



Some general details are known about the community structure 

 of marine webs; i.e., a large fraction of their carbon is cycled 

 through the "microbial loop." The species composition of large 



III-8 



