Utilization of two types of dissolved organic phosphate by axenic and nonaxenic harmful Amphidinium carterae: Response of photosynthetic and hemolytic activity

Phosphorus (P) is a major limiting nutrient of phytoplankton in the current coastal ocean due to the overload of nitrogen; however, the dominance of the harmful dinoflagellate Amphidinium carterae has increased in the coastal ocean, leading to the hypothesis that A. carterae potentially utilizes dissolved organic phosphate (DOP). Therefore, a series of experiments were conducted on axenic and nonaxenic A. carterae to investigate how dissolved inorganic phosphate (DIP) and two types of DOP (β-glycerol phosphate disodium salt hydrate (BGP) and 2-aminoethylphosphonic acid (2-AEP)) interact with the photosynthetic and hemolytic activities of A. carterae. The results showed that A. carterae could directly utilize BGP (C-O-P bonds of DOP) but had a limited ability to use 2-AEP (C-P bonds of DOP). A low amount of DIP (<0.36 µM) reduced the growth, biomass, and photosynthetic activity of A. carterae; in contrast, it induced the hemolytic activity of A. carterae. However, the reduction in the hemolytic activity of A. carterae mostly relied on cell division rather than the utilization of DIP or DOP, as indicated by the significant negative relationship between the growth rate and hemolytic activity of A. carterae. The healthier the cells (high value of photosynthetic efficiency), the lower the hemolytic activity of A. carterae. Taken together, the ability to utilize the BGP makes it possible for A. carterae to become dominant in a BGP-sufficient environment, whereas the stress of cells, i.e., in P-limited conditions, may drive toxic events during blooms of A. carterae.