Toxic Effects Of Mixotrophic Dinoflagellate Akashiwo Sanguinea On Co-Occurring Phytoplankton

The mixotrophic dinoflagellate Akashiwo sanguinea is a cosmopolitan harmful species and has been observed forming blooms around the world, causing mortality of multiple aquatic organisms. Previous studies showed A. sanguine had lethal effects on finfish, shellfish, and shrimp, but the toxicity of this species to phytoplankton has not been well investigated. This study investigated the toxic effects of A. sanguinea on multiple phytoplankton species, an how nutritional conditions regulated the toxicity. The main results are briefly highlighted as follows:(1) Clonal cultures of A. sanguinea from different geographic origins were able to inhibit the growth of co-occuring phytoplankton, including Rhodomonas salina, Scrippsiella trochoidea and Phaeocystis globosa. The inhibiting effects were positively correlated with the cell density of A. sanguinea. We found cells of phytoplankton co-cultivated with A. sanguinea were lysed within 6 h at elevated cell densities of A. sanguinea. In comparison, the strain from Xiamen, Fujian, Chian(A. sanguinea CCMA256) exhibited the highest 94% inhibition rate on R. salina under 2000 cells/mL, while the strains from Daya Bay, Guangdong, China(JX13, JX14) also had high inhibitory effects on R. salina, but the strain from Long Island, New York, USA(AS2) showed virtually no negative effect on R. salina. O n the other hand, the growth rates of the strains JX13 and JX14 could be significantly promoted by R. salina in a dose-response manner while the strain AS2 was almost not effectted by R. salina.(2) Crude extract of A. sanguinea JX14 could lyse R. salina, and the inhibition on R. salina was increased with the concentration of crude extract. Four components of A. sanguinea JX14 culture all inhibited the growth of R. salina. Specifically, the strongest effect was observed in the sonicated culture, followed by the filtrate of sonicated culture, the whole-cell culture, and the cell- free filtrate of whole-cell culture. These results showed the toxic substances from A. sanguinea not only exist inside the cell, but are released into the ambient medium also.(3) The toxic effects of A. sanguinea on co-occuring phytoplankton observed to be regulated by nutritional conditions, Since the toxicity of A. sanguinea was enhanced significantly via increasing nutrient concentrations, indicating high level of nutrients probably benefited A. sanguinea producing more toxins. The toxicity of A. sanguinea at lower(10: 1) and higher N/P ratio(30: 1) was significantly stronger than those at normal N/P ratio(20: 1), indicating N-deficient and P-deficient conditions could enhance the toxicity of A. sanguinea.The toxins of A. sanguinea exhibited to lyse cells of phytoplankton, and the lysed cell debris of phytoplankton could then be functioning as important nutrient sources to promote the growth of A. sanguinea population. Heterotrophy could be an important competing strategy for A. sanguinea and thus, plays an important role in the formation and maintenance of A. sanguinea blooms.