Toxic Effects Of Mixotrophic Dinoflagellate Akashiwo Sanguinea On Aquatic Animals

Species of Akashiwo sanguinea is a typical mixotrophic dinoflagellate.In recent years,harmful algal blooms of this species have frequently occurred throughout the world and often caused serious economic losses.Previously research has shown that A.sanguinea have obvious inhibitory effects on co-occuring phytoplankton and can cause death of many aquatic animals.However,the toxicity mechanism of this species is still unclear.This study investigated the lethal effects of 4 toxic A.sanguinea on the two model organisms(Oryzias melastigma and Brachionus plicatilis),the effects and mechanisms of algal toxins,and the regulatory mechanisms of nutrients on algal toxins.The main results were as follows:(1)Crude extract of A.sanguinea JX14 had serious lethal effects on the two model marine organisms,and the mortality will increase with the increase of toxin content,and also increase with the extension of the experimental time.The Crude extract of A.sanguinea JX14 produced significant toxicity to the larvae of the O.melastigma.When the dose of toxin reached a certain concentration,the larvae of O.melastigma would immediately have an escape response.The lethal effects on B.plicatilis were more pronounced,which suggested that different species may have different sensitivities to the same toxin.(2)Nutrient conditions had very important regulatory effects on the toxicity and growth of A.sanguinea.Four A.sanguinea strains(JX14,JX13,ASNP6,CCMA256)cultured in different nitrogen sources caused significantly different mortalities on target species O.melastigma and B.plicatilis.The urea treatments for all four strains displayed the highest mortalities,indicating that A.sanguinea may produced more toxins when cultured with urea as a nitrogen source.It is wroth noting that the highest mortality for O.melastigma of A.sanguinea at urea culture,and the specific growth rate of A.sanguinea is the highest,and is significantly higher than the control culture with uera.The limitation of nutrient also can promote the production of toxin and growth of A.sanguinea,and the highest mortalities of target animals were observed in N: P=10: 1,followed by N: P=30: 1,and nutrients-balanced treatments(20: 1),indicating that the toxicity of A.sanguinea was enhanced with nitrogen and phosphorus ratio imbalance,especially in the case of N-limitation.Besides,the highest mortality for O.melastigma of A.sanguinea under N-limited treatments(10: 1),the specific growth rate of A.sanguinea is the highest,and is significantly higher than the control culture within N-limitation.This shows that A.sanguinea in some specific nutrient environments could kills aquatic animals,at the same time,it may further use the nutrients in the environment to further increase its own specific growth rate and form algal blooms.(3)Four culture components of the four strains of A.sanguinea(JX14,JX13,ASNP6,CCMA256)had significantly different lethal effects on O.melastigma and B.plicatilis.The lethal rate of the two kinds of aquatic organisms showed that the sonicated was the strongest,followed by the filtrates of sonicated and the whole-cell culture(there was no significant difference between those two components),the filtrate of the whole-cell culture had no significant lethal effects.This result indicates that toxic substances of the A.sanguinea are mainly exist inside the cells,but also are secreted and released to ambient environments.When the algal cells break,they may release large amounts of algae toxins and thus have significantly toxic effects on aquatic organisms.The results of our study indicated that the mixotrophic dinoflagellates A.sanguinea could caused significant toxic effects on the two modle marine organisms.The toxins play a crucial role in this process,and nutrients composition and ratio in the environment have regulatory on the production of toxins.In certain nutritional environments A.sanguinea could enhance the toxicity to kill aquatic animals,meanwhile the growth of A.sanguinea increased.These results provide evidences to further understanding the mechanism of A.sanguinea blooms and its hazard to the marine ecosystems.