Photosynthetic and hemolytic activity in Amphidinium carterae in response to light irradiance and light quality

The benthic dinoflagellate Amphidinium carterae is notable for its hemolytic toxins and blooms in many coastal regions, threatening local aquaculture and marine ecosystems. Light is the trigger for the photosynthetic and hemolytic activity of A. carterae; however, the response of photosystem II activity and hemolytic activity to light irradiance and quality is still unclear. Therefore, a series of experiments were conducted on A. carterae to investigate how light irradiance (10, 25, 50, 100, and 200 μmol·m^-2·s^-1 of LED white light) and wavelength (white, blue, green, and red LED light at 50 μmol·m^-2·s^-1) interact with the photosynthetic and hemolytic activities of A. carterae. The results revealed that half saturation irradiance (50 μmol·m^-2·s^-1) maintained high photosynthetic efficiency (F_v/F_m) and light-regulated nonphotochemical energy dissipation (Y_NPQ) but reduced the effective photochemical quantum yield (Y_II) and cell growth. High light, >50 μmol·m^-2·s^-1, increased the Y_II, relative electron transport rate (rETR) and cell growth but reduced F_v/F_m and Y_NPQ, similar to blue light. However, red light increased F_v/F_m and Y_NPQ but reduced cell growth. Interestingly, the hemolytic activity of A. carterae was strongly affected by light irradiance but not by light quality. Low light, < 50 μmol·m^-2·s^-1, limited the growth and toxin quota of A. carterae, as well as Y_II. High levels of hemolytic activity were produced by A. carterae, but relatively fast cell growth redirected cellular resources toward growth rather than toward toxin synthesis, resulting in a negative correlation between the hemolytic toxin production rate and the growth rate when light irradiance exceeded 50 μmol·m^-2·s^-1 and across all light spectra. Whereas higher photoprotection (Y_NPQ) of A. carterae induced hemolytic activity under high light, which indicates that hemolytic components associated with photoprotection may be induced during light stress. Taken together, the ability of A. carterae to produce high levels of hemolytic toxins at irradiances greater than 50 μmol·m^-2·s^-1 makes it possible for A. carterae to become a toxic vector in the corresponding environment, whereas the migration of cells from high light (surface) to low light (bottom) may reduce the toxic effect of A. carterae during blooms.