Oligopeptide chemotypes of the toxic freshwater cyanobacterium Planktothrix can form subpopulations with dissimilar ecological traits

Nonribosomal oligopeptides were used as qualitative and quantitative markers to test whether populations of the toxic freshwater cyanobacterium Planktothrix comprise subpopulations with dissimilar ecological traits. A field program was conducted in Lake Steinsfjorden (Norway), where Planktothrix has dominated the phytoplankton community for decades, allowing the present study to disregard other potential producers of nonribosomal oligopeptides. Four chemotypes with distinct cellular oligopeptide patterns were found in the lake. The chemotypes occurred largely unaltered throughout a period of up to 33 yr and differed with respect to seasonal dynamics, depth distribution, and participation in loss processes. Changes in the relative abundance of chemotypes occurred almost constantly and could not be explained with fluctuations in light, temperature, or concentration of macronutrients but might have been due to differences among chemotypes in depth regulation or interaction with grazers or pathogens. Chemotypes correlated weakly with taxonomic groups and genotypes defined on the basis of phycocyanin operon deoxyribonucleic acid (DNA) sequences. Our findings suggest that first, oligopeptide chemotypes can have dissimilar ecological traits and therefore interact differently with their environment; second, populations of toxic freshwater cyanobacteria can comprise multiple ecologically distinct subpopulations; and, third, the relative abundance of these may vary, causing a high variability in whole-population properties. The latter was demonstrated for the microcystin-related toxicity of Planktothrix. The consequences of the present findings for the taxonomy of Planktothrix are discussed.