Toxin production can act as a chemical defense mechanism against predation. It is well established that toxic diatom Pseudo-nitzschia can increase domoic acid (DA) production in response to grazing pressure from copepods, either through direct interaction or chemical cues like copepodamides. However, our recent studies reveal that this response is not limited to copepods. Protozoa, such as mixotrophic dinoflagellate Lepidodinium sp. and heterotrophic dinoflagellates Noctiluca scintillans, can also stimulate increased toxicity in Pseudo-nitzschia. This suggests that the succession of toxic diatom-dinoflagellate blooms, and their impact on marine ecosystems, may be more complex than previously thought. We also found that protozoan feeding on toxic diatoms can significantly influence the ecological fate of algal toxins. For example, after one day of incubation with P. fukuyoi, N. scintillans accumulated 67.94±2.72 pg DA per cell, with an accumulation efficiency of 273.53±21.20%. During the first 6 hours of starvation, 82.73±9.91% of the DA is degraded into other molecules, with a DA depuration rate of 0.38 h^-¹, which is higher than in some copepod species. Furthermore, N. scintillans can release dissolved DA back into the environment, accounting for 11% of the accumulated DA. These findings improve our understanding of the trophic interactions between toxic diatoms and protozoa, providing new insights into the dynamics of harmful algal blooms (HABs), the migration and transformation of toxins at the bottom of the marine food web, and their broader ecological implications for ecosystem health.
 

Pseudo-nitzschia, portozoa, chemical defense, domoic acid, toxin migration and transformation