Differential effect of exudates of Zostera marina and Gracilaria vermiculophylla on the growth rate and toxin production of the dinoflagellate Alexandrium minutum

Harmful algal blooms (HABs) caused by the toxic dinoflagellate Alexandrium minutum pose problems for coastal communities worldwide. It has been shown that the growth of HAB species is negatively affected by biomolecules released by Zostera marina . Seagrass meadows dominated by Z . marina have been reported to be occupied by the invasive red alga Gracilaria vermiculophylla . The aim of this study was to assess the effect of exudates from Z. marina and G. vermiculophylla on the growth and toxicity of A. minutum . Our results confirm the significant decrease in A. minutum growth rate when exposed to Z. marina exudates. In contrast, no inhibitory effect was detected when cultures were amended with exudates from G. vermiculophylla ; instead, a positive effect was occasionally observed. A significant increase in toxin content per cell in A. minutum was found when exposed to G. vermiculophylla either alone or in a mixture with exudates of Z. marina . Overall, our results suggest that the invasion of seagrass meadows by G. vermiculophylla affects their potential capacity to control the growth of A. minutum . • Exudates of Zostera marina negatively affected Alexandrium minutum growth rate. • The negative effect of Z. marina remained in the presence of G. vermicullophylla exudates • G. vermiculophylla exudates did not inhibit the growth of A. minutum. • Toxin per cell was higher in A. minutum cells exposed to G. vermiculophylla exudates.