eukaryotic phytoplankton has been fairly well characterized by 

 classical morphology, and major classes of protozoa have similarly 

 been identified. Epifluorescence microscopy has provided details 

 about the abundance of phototrophic and heterotrophic prokaryotes. 

 Within the oxygenic photosynthetic prokaryotes, two major groups 

 occur in the marine environment (cyanobacteria and prochlorophytes) 

 and several groups are found in freshwater. The identification of 

 heterotrophs, however, is difficult, largely because many cannot be 

 cultured. 



Several interactions within the microbial loop are 

 particularly interesting in understanding the stability and 

 structure of microbial communities. Competition is an extremely 

 important process which affects their overall structure. How do 

 variations in resources, such as different nutrients, affect the 

 species composition in aquatic microbial loops? Production is 

 extremely important in the dynamics of microbial loops. How do 

 changes in specific protozoan populations affect the diversity of 

 their prokaryotic prey? Does grazing increase species diversity by 

 removing the dominant prey species? What are the grazing 

 specifications of particular protozoans? 



Microbes do not exist as individuals in the environment, but 

 interact as closely linked, interdependent communities. Stable pH 

 and oxygen gradients have been measured in some marine 

 particulates, indicating that anaerobic processes may take place 

 within aggregate microhabitats, in an otherwise aerobic 

 environment. These transient communities may be extremely 

 sensitive to perturbation, which could have a dramatic effect on 

 carbon flow through the system. What is the community structure of 

 the microhabitats, and their sensitivity to physical perturbation? 

 What successional events take place during community change? How 

 do these affect community function? 



The function of viruses in microbial communities needs to be 

 assessed. Proctor and Fuhrman (1991) report that viruses in 

 microbial food webs are potentially important in both organic 

 matter release and particle formation. Host defense mechanisms 

 against viruses can include the release of proteases and nucleases. 

 Molecular methods are likely to be necessary to understand these 

 processes. 



Assessment of Function within Closely Linked Opposite Pathways 



III-9 



