2.1 The culture of algae and copepods
Two species of PSNs , P. cuspidata (toxic) and P. brasiliana (non-toxic), were isolated from the coastal waters of Guangdong, China, and used as prey for copepods. The algae were cultured in 1-liter flasks containing 0.2 μm-filtered sterilized seawater supplemented with f/2 medium (Guillard and Ryther 1962). Cultures were maintained in a temperature-controlled walk-in chamber at 22°C with a 14:10 hour light:dark cycle. To ensure the freshness of the algae and adequate levels of DA, the PSNs were used for experiments during the early stationary phase. Intracellular DA content was measured the day before the experiment using the method described by Niu et al. (2024).
Copepods (A. erythraea ) were collected from Port Shelter, Hong Kong, using a WP2 net with a mesh size of 200 μm. The collected copepods were filtered through 1000 μm and 450 μm meshes to remove other species and immature individuals. We selected copepods that passed through the 1000 μm net but were retained by the 450 μm net. Individuals were identified under a dissection microscope (Olympus IX51), and the dominant species, A. erythraea were isolated. The selected copepods were cultured in multiple 50 L tanks filled with 0.2 μm-filtered seawater under a 12:12 hour light:dark cycle. Saturating amounts of PSNs were also provided to the selected copepods, and continuous aeration maintained oxygen levels and prevented PSNs from settling. Dead copepods were promptly removed, and the seawater was changed daily.