Phagotrophic Characteristics Of Causative Species In Large-scale HABs In East China Sea And Its Role In HABs Formation

Large-scale dinoflagelllate blooms have outbroken in the East Sea sea every year. Karenia mikimotoi, Prorocentrum donghaiense and Alexandrium catenella are three major causative species. The East Sea areas is looked as eutrophication waters with higher nitrogen concentration and lower phosphorus, therefore, it is thought that some special adaptation strategies of dinoflagellate would be the main reason to cause the formation of large-scale HABs in East China Sea. Now many special characteristics of dinoflagelllate including the ability to use the dissolved organic materials, luxury absorption of nutrient and moving ability are researched in details, but we still know nothing about the phagotrophic ability of these dinoflagelllates. In this study, using the marine microalgae such as Isochrysis galbana and Hemiselmis virescens, and bacteria such as Escherichia coli and the bacteria isolated from dinoflagellate as phagotrophic objects, phagotrophic ability of seven dinoflagellates are studied, under different light density and different nutrimental conditions.From our study, it is found that K. mikimotoi have a strong phagotrphic ability, and can engulf bacteria, fluorescent microsphere (FM), I. galbana and H. virescens. Furthermore, when adding bacteria to the medium of K. mikimotoi under the conditions of N-depleted, P-depleted, N, P-depleted and N and P-replete, the max biomass of K. mikimotoi increased 12%, 20%, 14% and 55%, respectively. Under the same nutrimental conditions, it increased 36%, 35%, 16% and 40% when adding I. galbana. Similarly, under nutrition-depleted and nutrition-replete conditions, it increase 137% and 25%,respectively when adding H. virescens. It is indicated that K. mikimotoi has a strong phagotrophic ability to promote its growth obviously, even that phagotrophy help it grow faster than just rely on photosynthesis.P. donghaiense also has phagotrophic ability, and can engulf living E. coli and H. virescens, but not ingest actionless fluorescence-labeled I. galbana (FLA), fluorescence-labeled bacteria (FLB) and fluorescent microsphere (FM). Under nutrition-depleted conditions, the max biomass increase 39%, when adding E. coli. At the same time, they increase 10% and 17% respectively when adding H. virescens under nutrition-depleted and nutrition-replete conditions. It is found that Prorocentrum micans can engulf FLA, FLB and FM0.5 with diameter 0.5μm and Prorocentrum minimum can only engulf FM0.5. Under the conditions of N-depleted, P-depleted, N, P-depleted and N and P-replete, the max biomass of P. micans increased 6%, 14%, 26% and 3%, respectively. Under the same nutrimental conditions, it increased 34%,18%, 11% and 4% when adding I. galbana. It is indicated that phagotrophy have a stimulative effect on growth of Prorocentrum sp. under nutrition-depleted conditions, but it has not effect on growth of Prorocentrum sp. in eutrophic environment.A. catenella can phagocytose bacteria, I. galbana and FM0.5. The max biomass of A. catenella increase 63% and 77% respectively when adding 0.2OD and 0.4OD E .coli. But it mainly kill them by allelopathy, not by phagotrophy when adding the other microalgae such as I. galbana and H. virescens. Though other studies, it is confirmed that Alexandrium tamatense can ingest FLB and FM0.5, and Alexandrium minutum can ingest FLA and FM0.5. It is declared that A. catenella can acquire its nutrition by ingesting bacteria and kill them by allelopathy, not by phagotrophy when it is symbiosis with other microalgae.Through the comprehensive contrast, it is clear that the phagotrophic ability of K. mikimotoi is strongest, and Prorocentrum sp. and Alexandrium sp. are inferior. Similarly, the stimulative effect on growth of K. mikimotoi is most obvious, Prorocentrum sp. is second, and Alexandrium sp. is not obvious, when adding same particulate organic matter. By comparison, K. mikimitoi is belong to"ideal"type mixotrophy and Prorocentrum sp. is belong to"phagotrophic algae"type mixotrophy. But Alexandrium sp. is very special one. It acquires its nutrition by ingesting bacteria and kill them by allelopathy when it is symbiosis with other microalgae.Combined with the characteristics of HABs in East China Sea and other ther physiological and ecological characteristics of these three dinoflagellates, it is indicated that phagotrophy of dinoflagellate would impact HABs formation in East China Sea. The phagotrophy play an important role in the formation large-scale K. mikimitoi bloom in East China Sea. The photosynthesis ability of Prorocentrum sp. is stronger than its phagotrophy. So, the phagotrophy effect on the formation of large-scale Prorocentrum bloom in East China Sea, may be limited. But growth of Alexandrium sp. was promoted and maintained thought the combination with engulfing bacteria and killing potential nutrition competitors/predators to getting more niche. Through this study, it is provided a new scientific perspective for formation mechanism of large-scale HABs in East China Sea.