Average DNA base compositions, as determined from 50% thermal denaturation temperatures and from buoyant densities, were 38, 41, 43, and 40 to 41 mol% G+C for sp. strains P1-15, C5-34, AK1-6, and C3-40, respectively. These phenotypically similar organisms, which are representatives of the cluster within the group of chroococcacean cyanobacteria, have a smaller range of DNA base compositions (38 to 43 mol% G+C) than the group as a whole does (30 to 48 mol% G+C). Because the DNA base ratios of the cultures adjoin or overlap those of the low- and high-G+C-content clusters of the group, DNA base composition data do not differentiate the cluster from these two clusters. By bridging the gap between the low- and high-G+C-content clusters of the group, the data for create a continuum of DNA base ratios within the group and diminish the usefulness of G+C values for discerning clusters within the phenotypically heterogeneous group.


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