Morphological, Chemical, and Genetic Diversity of Tropical Marine Cyanobacteria Lyngbya spp. and Symploca spp. (Oscillatoriales)

Although diverse natural products have been isolated from the benthic, filamentous cyanobacterium Lyngbya majuscula, it is unclear whether this chemical variation can be used to establish taxonomic relationships among disparate collections. We compared morphological characteristics, secondary-metabolite compositions, and partial 16S ribosomal DNA (rDNA) sequences among several collections of L. majuscula Gomont, Lyngbya spp., and Symploca spp. from Guam and the Republic of Palau. The morphological characteristics examined were cell length, cell width, and the presence or absence of a calyptra. Secondary metabolites were analyzed by two-dimensional thin-layer chromatography. Each collection possessed a distinct cellular morphology that readily distinguished Lyngbya spp. from Symploca spp. Each collection yielded a unique chemotype, but common chemical characteristics were shared among four collections of L. majuscula. A phylogeny based on secondary-metabolite composition supported the reciprocal monophyly of Lyngbya and Symploca but yielded a basal polytomy for Lyngbya. Pairwise sequence divergence among species ranged from 10 to 14% across 605 bp of 16S rDNA, while collections of L. majuscula showed 0 to 1.3% divergence. Although the phylogeny of 16S rDNA sequences strongly supported the reciprocal monophyly of Lyngbya and Symploca as well as the monophyly of Lyngbya bouillonii and L. majuscula, genetic divergence was not correlated with chemical and morphological differences. These data suggest that 16S rDNA sequence analyses do not predict chemical variability among Lyngbya species. Other mechanisms, including higher rates of evolution for biosynthetic genes, horizontal gene transfer, and interactions between different genotypes and environmental conditions, may play important roles in generating qualitative and quantitative chemical variation within and among Lyngbya species.