Potent allelopathy and non-PSTs, non-spirolides toxicity of the dinoflagellate Alexandrium leei to phytoplankton, finfish and zooplankton observed from laboratory bioassays.

The dinoflagellate genus Alexandrium has been well known for causing paralytic shellfish poisoning (PSP) worldwide. Several non-PSP toxin-producing species, however, have shown to exhibit fish-killing toxicity. Here, we report the allelopathic activity of Alexandrium leei from Malaysia to other algal species, and its toxicity to finfish and zooplankton, via laboratory bioassays. Thirteen microalgal species that co-cultured with Al. leei revealed large variability in the allelopathic effects of Al. leei on the test algae, with the growth inhibition rates ranging from 0 to 100%. The negative allelopathic effects of Al. leei on microalgae included loss of flagella and thus the motility, damages of chain structure, deformation in cell morphology, and eventually cell lysis. The finfish experienced 100% mortality within 24 h exposed to the live culture (2000-6710 cells·mL-1), while the rotifer and brine shrimp exhibited 96-100% and 90-100% mortalities within 48 h when exposed to 500-6000 cells·mL-1 of Al. leei. The mortality of the test animals depended on the Al. leei cell density exposed, leading to a linear relationship between mortality and cell density for the finfish, and a logarithmic relationship for the two zooplankters. When exposed to the treatments using Al. leei whole live culture, cell-free culture medium, extract of algal cells in the f/2-Si medium, extract of methanol, and the re-suspended freeze-and-thaw algal cells, the test organisms (Ak. sanguinea and rotifers) all died at the cell density of 8100 cells·mL-1 within 24 h. Toxin analyses by HILIC-ESI-TOF/MS and LC-ESI-MS/MS demonstrated that Al. leei did not produce PSP-toxins and 13-desmethyl spirolide C. Overall, our findings demonstrated potent allelopathy and toxicity of Al. leei, which do not only pose threats to the aquaculture industry, fisheries, and marine ecosystems but may also play a part role in the population dynamics and bloom formation of this species.