ribosomal RNA sequences. Giovannoni et al. (1988) used universal 

 rRNA primers and the polymerase chain reaction (PCR) (Saiki et al., 

 1988) to amplify rRNA genes from picoplankton collected in the 

 Sargasso Sea. A preliminary study in the Pacific Ocean using rRNA 

 genes isolated from a picoplankton clone has also been completed 

 (Schmidt et al., 1991). One noteworthy conclusion is that a 

 cluster of closely related bacteria was found in both the Atlantic 

 and Pacific Oceans. Early estimates suggest that this group of 

 bacteria may comprise 10-15% of the marine picoplankton from these 

 areas, and yet the group is not closely related to any cultured 

 organisms for which rRNA sequences are available. If these 

 estimates are correct, understanding their distribution and 

 metabolic capacity will be crucial to an analysis of the community. 



Molecular Assay of Intraspecies Variability 



Elucidation of the extent and role of intraspecies variability 

 is essential to understand how environmental stress or perturbation 

 may influence community structure and function. Intraspecies 

 variation may be an important buffer, maintaining community 

 stability in the face of environmental challenge. Molecular data 

 from bacterioplankton in the Sargasso Sea (Giovannoni et al., 1988) 

 and Central North Pacific imply that there is a large degree of 

 genetic diversity between closely related, coexisting marine 

 cyanobacterial populations. However, the overall extent of 

 intraspecific variability, its function and maintenance, are little 

 understood in these and other environments. For instance, it was 

 recently shown that viral populations are extremely abundant in the 

 oceans, present in about the same order of magnitude as 

 bacterioplankton populations (» 10V mL '* Suttle et al., 1990). These 

 populations are thought to be important in controlling the size of 

 phototrophic and heterotrophic microbial populations, and hence, 

 they may greatly influence both primary and secondary productivity. 

 Yet what prevents these presumably lytic viruses from eliminating 

 their host populations? Is intraspecific variation an important 

 factor in determining host-viral interactions? Can environmental 

 perturbance influence the interactions between viral and host 

 populations, or skew the intraspecific variability which may buffer 

 these populations? The answers to these and similar questions will 

 provide important information on the influence of chronic stress on 

 intraspecific variation, and its consequences. 



III-4 



