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.