Allelopathy of Baltic Sea cyanobacteria: no evidence for the role of nodularin

Extracts of Aphanizomenon flos-aquae and Nodularia spumigena, the two most common cyanobacteria forming recurrent blooms in the Baltic Sea, decrease the abundance of some phytoplankton species via the release of allelopathic substances. We investigated how cell-free filtrates of the two cyanobacteria, as well as purified hepatotoxin nodularin, produced by N. spumigena affected cell numbers, chlorophyll a content and 14CO2 uptake of the cryptophyte Rhodomonas sp. Both cyanobacterial filtrates significantly retarded the growth of Rhodomonas sp., A. flos-aquae filtrate up to 46%, whereas purified nodularin showed no significant effect on any of the growth parameters of the cryptophyte. These results suggest that the allelopathic effect of N. spumigena is most probably due to metabolite(s) other than nodularin, possibly acting via the damage of the target cells.