Toxic oligopeptides in the cyanobacterium Planktothrix agardhii-dominated blooms and their effects on duckweed (Lemnaceae) development

Abstract

Cyanobacterial toxins are a global threat to aquatic organisms; however, they represent only one group of bioactive cyanobacterial metabolites. Very little is known about the effects of other cyanobacterial products (e.g., non-ribosomal oligopeptides) on freshwater macrophytes. Our experimental study revealed that the development of young duckweed Spirodela polyrhiza was inhibited by two aquatic extracts of cyanobacterial bloom samples predominated by Planktothrix agardhii and pure microcystin-LR (MC-LR). The extracts differed considerably in the content of MCs and other oligopeptides; they contained three or four MC variants and several other oligopeptides such as anabaenopeptins, aeruginosins, and planktocyclin. Their toxic effects on young plants (first frond area, root number, fresh biomass, and chlorophyll a content) were different. The more phytotoxic extract obtained from a higher cyanobacterial biomass contained a lower total MC concentration and different anabaenopeptin variants (e.g., anabaenopeptin H, G, HU892, and E/F) as well as planktocyclin, which were not present in another extract with a higher MC concentration. The obtained results suggested that P. agardhii oligopeptides other than MCs are harmful to young duckweeds and may exert even stronger toxic effects than MC-LR. The production of various variants of MCs together with other oligopeptides, and their toxicity to aquatic plants varied over seasons depending on the taxonomic composition of the cyanobacterial bloom. Germinated turions of S. polyrhiza can be used as a sensitive bioindicator of the cyanobacterial threat in aquatic ecosystems.