The objective of this project was to use ribosomal RNA genes, cloned from natural populations of Sargasso Sea bacterioplankton, as markers for picoplankton diversity. It is widely recognised that a majority of microorganisms have yet to be cultivated, and therefore much of extant microbial diversity remains unknown (50). The method described here for analyzing natural bacterial communities circumvents this problem by utilizing ribosomal RNA, found in all life forms, for defining and enumerating the components of natural populations. Two different clone libraries of eubacterial 16S rRNA genes amplified from a natural population of Sargasso Sea picoplankton by the polymerase chain reaction (11) have been phylogenetically analysed. The analyses indicate the presence of a wide variety of novel microorganisms, representing members of the α and γ proteobacteria and the oxyphototroph (13, 47) eubacterial phyla. One group of novel clones, represented by SAR 83, were found to be most closely related to Erythrobacter, a genus of aerobic bacteriochlorophyll~ containing organisms. The results imply that many closely related 16S rRNA lineages, or clusters of lineages, coexist within bacterioplankton communities. The significance of these clusters is uncertain. One interpretation, that they represent clonal structure within bacterial species, suggests that populations of marine bacteria are very ancient. / Graduation date: 1991