Investigating the role of allelochemicals in the interaction between Alexandrium monilatum and other phytoplankton species.

Species of the dinoflagellate genus Alexandrium can release bioactive extracellular compounds with allelopathic effects (e.g., immobilization, inhibition of growth, photosynthesis or lysis) towards other phytoplanktonic organisms. In the lower Chesapeake Bay, Virginia, US, succession or co-occurrence of blooms of Akashiwo sanguinea, Margalefidinium polykrikoides and the goniodomin-producing A. monilatum tend to be common during summer months, however the allelopathic potential of A. monilatum, and how it may affect bloom dynamics, has not be studied. We used a rapid fluorescence-based bioassay as well as flow cytometry and an assessment of immobilization to determine the potential effects of A. monilatum culture supernatants on M. polykrikoides, A. sanguinea and the diatom Chaetoceros muelleri (included as a reference strain). In addition, we also investigated the effects of standards of known A. monilatum toxins goniodomin A (GDA) and GDA seco-acid (GDA-sa). Exposure of C. muelleri to culture supernatants from two different strains of A. monilatum resulted in an inhibition of the maximum quantum yield of the photosystem II (F/F) and cell lysis of the diatom, with strain-specific variation observed, highlighted by a 30-fold difference in the effective concentration resulting in 10 % inhibition (EC) and 2-fold difference in the lethal concentration resulting in 50 % mortality (LC) between the two A. monilatum strains tested. Exposure of M. polykrikoides to A. monilatum culture supernatants resulted in a decrease in motility (at ≥ 500 eq. cells mL) along with a decrease in F/F (at ≥ 1,500 cells mL) and altered cellular morphology (at ≥ 3,500 eq. cells mL). No effect was observed when A. sanguinea was exposed to A. monilatum culture supernatants (at the concentrations tested in these studies). When M. polykrikoides was exposed to the GDA standard, its F/F (at ≥ 1,951 nM) and motility (at ≥ 390 nM) decreased, and its morphology (at ≥ 975 nM) was modified. The study of supernatant time- and temperature-stability, and the absence of a relationship between observed effects and goniodomin concentrations suggested the presence of additional unknown allelochemicals distinct from goniodomins, as has been observed for other Alexandrium species. Immobilization of M. polykrikoides by A. monilatum culture supernatants and GDA standard in laboratory-based exposure experiments may indicate that A. monilatum has a competitive advantage when both species co-occur in the Chesapeake Bay and this warrants further testing.