Allelopathic activity of the toxic dinoflagellate Alexandrium ostenfeldii: Intra-population variability and response of co-occurring dinoflagellates

The paralytic shellfish toxin (PST) producing dinoflagellate Alexandrium ostenfeldii forms dense, recurrent blooms during summer in shallow coastal areas of the Baltic Sea. We studied the intra-population variability of its allelochemical potency and the responses of co-occurring and potentially competing dinoflagellates to the allelochemicals. The lytic activity of 10 northern Baltic A. ostenfeldii strains was evaluated by their EC50 values (i.e. the cell concentration yielding a 50% decline in cryptophyte density), which were found to vary between 236 and 1726 cells ml−1. When co-occurring dinoflagellates (Kryptoperidinium foliaceum, Levanderina fissa and Heterocapsa triquetra) were exposed to filtrate of A. ostenfeldii, short-term (<1 h) responses of the target species after an initial immobilization were species-specific. Almost all of the K. foliaceum cells formed cysts, L. fissa cells lost their cell shape and lysed, whereas H. triquetra cells shed their thecae. After 24 h, K. foliaceum had returned into vegetative cells and the number of immotile L. fissa and H. triquetra cells had significantly decreased. The results indicate that A. ostenfeldii can disturb the growth of competing dinoflagellates by excreting allelochemicals at bloom concentrations and that co-occurring species may develop efficient means to escape and recover from the allelochemicals, allowing them to coexist with A. ostenfeldii.