Response And Underlying Mechanism Of Different Strains Of Prorocentrum Donghaiense To The Allelopathy Of Karenia Mikimotoi

Prorocentrum donghaiense and Karenia mikimotoi are two main harmful algal blooms(HABs)-forming dinoflagellates along the coast of China,which often formed the bilateral HABs in some sea areas,recently.It is found that K.mikimotoi have allelopathic effects on the growth of P.donghaiense,which might play imortant roles in the development of the HABs.Up to now,the studies about allelopathy are mainly focused on the physiology changes of target algal species,the extract and identification of allelochemicals,and the possible mechanism of allelopathy.However,no studies has been conducted on the difference in response of different strains of P.donghaiensis to the K.mikimotoi allelopathy,and underlying mechanism remains unclear.In order to further explore the competitive effect between P.donghaiense and K.mikimotoi and reveal the allelopthic mechanism of K.mikimotoi,the response of different strains of P.donghaiensis from the East China Sea to the K.mikimotoi allelopathy were compared and possible menanism were analyzed.In this study,we observed the growth of different strains of P.donghaiense under co-culture with K.mikimotoi or filtrate culture from K.mikimotoi.Meanwhile,the physiological and biochemical changes and molecular responses of different strains of P.donghaiense to cell free filtrate of K.mikimotoi were analyzed.The results showed that the growth of different strains of P.donghaiense varied in response to K.mikimotoi.Among the three strains,strain F3-1 is the most sensitive to the allelopathy of K.mikimotoi.Compared with the co-culture with K.mikimotoi,the cell-free filtrate of K.mikimotoi displayed some weaker inhibition on the three strains of P.donghaiense.In the later stage of the experiment,the growth rate of the three P.donghaiense strains in the filtrate increased significantly,suggesting that the allelochemicals in the K.mikimotoi filtrate are unstable and may be degraded.Further analyses on physiological and biochemical indicators showed that the K.mikimotoi filtrate reduced Fv/Fm value and total carbohydrates of P.donghaiense F3-1 cells,increased the levels of total protein and neutral lipid,decreased superoxide dismutase activity,and increased malondialdehyde content.These outcomes suggest that the allelochemicals from K.mikimotoi may affect the carbon flow in the strain P.donghaiense F3-1,causing more carbohydrates to flow in the direction of protein and neutral lipids.In contrast,the strain P.donghaiense Ya only displayed a weak response to the cell free filtrate of K.mikimotoi.Compared with control counerpart,the content of active oxygen species,neutral lipids and malondialdehyde were significantly increased in the strain P.donghaiense Ya when cultured with K.mikimotoi filtrate,suggesting that the strain P.donghaiense Ya may have a strong resistance on K.mikimotoi allelopathy.Transcriptome data demonstrated that the two strains of P.donghaiensis underwent great changes in the expression of key genes concerning photosynthesis,oxidative phosphorylation,antioxidant system and absorption of nitrogen and phosphorus nutrients when cultured with K.mikimotoi filtrate.The expression of genes related to photosynthesis was decreased,while most genes concerning oxidative phosphorylation,and some antioxidant genes were up-regulated.There were significant differences in the expression of genes related to endocytosis,energy metabolism and ABC transporter between the two strains of P.donghaiensis.In F3-1strain,the genes associated with endocytosis were singnificantly down-regulated,while those in YA strain were up-regulated,suggesting that endocytosis in P.donghaiensis may also be an important mechanism for P.donghaiensis to respond to the K.mikimotoi allelopathy.The expression of genes related to glycolysis,pyruvate metabolism and tricarboxylic acid cycle in F3-1 and YA strains were significantly different,suggesting that difference in energy metabolism might contribute the growth difference between the two strains.The difference in ABC transporter gene expression suggests that ABC transporter may play an important role in P.donghaiense against K.mikimotoi allelopathy,which can effectively pump allelochemicals out of the cell and reduce the harmful effects of K.mikimotoi allelopathy.In conclusion,K.mikimotoi allelopathy caused a series of physiological and biochemical changes in P.donghaiensis,concerning photosynthesis,oxidative phosphorylation,antioxidant system,absorption of nutrients,carbon metabolism,endocytosis and ABC transporter,etc.Different strains of P.donghaiense varied in response to the K.mikimotoi allelopathy,and the difference in energy metabolism,ABC transporter,and endocytosis may contribute to the difference in response to K.mikimotoi allelopathy in the two strains.The ABC transporter can effectively pump allelochemicals out of the cell,thereby reduce the harmful effects of allelochemicals.Endocytosis may be an important pathway for P.donghaiensis to obtain nutrients from the environment under stress condition.Our finding might advance the understanding of the coexistence and inter-species competition of the two algal species.It is of great significance to explain the environmental adaptation mechanism of P.donghaiensis.