Control of ichthyotoxic Cochlodinium polykrikoides using the mixotrophic dinoflagellate Alexandrium pohangense: A potential effective sustainable method

Red tides dominated by Cochlodinium polykrikoides often lead to great economic losses and some methods of controlling these red tides have been developed. However, due to possible adverse effects and the short persistence of their control actions, safer and more effective sustainable methods should be developed. The non-toxic dinoflagellate Alexandrium pohangense is known to grow well mixotrophically feeding on C. polykrikoides, and populations are also maintained by photosynthesis. Thus, compared with other methods, the use of mass-cultured A. pohangense is safer and the effects can be maintained in the long term. To develop an effective method, the concentrations of A. pohangense cells and culture filtrate resulting in the death of C. polykrikoides cells were determined by adding the cells or filtrates to cultured and natural populations of C. polykrikoides. Cultures containing 800 A. pohangense cells ml^?1 eliminated almost all cultured C. polykrikoides cells at a concentration of 1000 cells ml^?1 within 24 h. Furthermore, the addition of A. pohangense cultures at a concentration of 800 cells ml^?1 to C. polykrikoides populations from a red-tide patch resulted in the death of most C. polykrikoides cells (99.8%) within 24 h. This addition of A. pohangense cells also lowered the abundances of total phototrophic dinoflagellates excluding C. polykrikoides, but did not lower the abundance of total diatoms. Filtrate from 800 cells ml^?1 A. pohangense cultures reduced the population of cultured C. polykrikoides by 80% within 48 h. This suggests that A. pohangense cells eliminate C. polykrikoides by feeding and releasing extracellular compounds. Over time, A. pohangense concentrations gradually increased when incubated with C. polykrikoides. Thus, an increase in the concentration of A. pohangense by feeding may lead to A. pohangense cells eliminating more C. polykrikoides cells in larger volumes. Based on the results of this study, a 1 m³ stock culture of A. pohangense at 4000 cells ml^?1 is calculated to remove all C. polykrikoides cells in ca. 200 m³ within 6 days. Furthermore, maintenance of A. pohangense populations through photosynthesis prepared A. pohangense to eliminate C. polykrikoides cells in future red-tide patches. Moreover, incubation of A. pohangense at 2000 cells ml^?1 with juvenile olive flounder Paralichthys olivaceus for 3 days did not result in the death of fish. Therefore, the method developed in this study is a safe and effective way of controlling C. polykrikoides populations and can be easily applied to aqua-tanks on land.