Regulation Of Light And Nutrients On The Phagotrophic Behavior Of Harmful Dinoflagellates

Laboratory experiments were conduct to study phagotrophic behavior of 6 typical Harmful Algal Bloom Species:Karenia mikimotoi, Alexandrium catenella, Prorocentrum donghaiense, Prorocentrum micans, Scrippsiella trochoidea, Prorocentrum minimum and their responses to light and nutrients. Traditional microscope and fluorescence hybridization method were applied to our experiments. Field survey, ship-based mesocosm experiments were also conducted to investigate phagotrophy of mixotrophic dinoflagellates in the high frequency Harmful Algal Bloom (HAB) areas (28°N-34°N,123°～126°E) in East China Sea in spring 2010. Combined with other physicochemical parameters, we studied the effects of environmental factors on the phagotrophic red-tide dinoflagellates, explored phagotrophy mechanism of these mixotrophic dinoflagellates and their importance in microbial food web, bloom dynamics and population ecology. The results would enrich the studies on nutritional mechanism and physiological ecology of microalgae, and provided us a comprehensive basis for HAB prediction and precaution. The main results were as follows:1. In situ study showed that the dominant red-tide algae P. donghaiense were observed to ingest Fluorescent LabeledⅠ. galbana (FLIg) and Fluorescent Labeled Skeletonema sp (FLS) in the high frequency HAB areas in East China Sea in spring 2010. Among 27 sample stations, feeding occurrences were found at 16 stations in south dc section (nearby Shanghai and Zhejiang), but no feeding occurrences were found at 11 stations in the north dc section. The ingestion frequencies of P. donghaiense on these two Fluorescent labeled algae(FLA) were very low, only 0.2～1.4%. In the feeding experiment by adding FLIg, ingestion by P. donghaiense was found at 12 stations. Salinity,DOP,DIN:DIP all had significant effects on its ingestion(P=0.025, P=0.024, P=0.035). While adding FLS, ingestion by P. donghaiense was found at 11 stations, DOP had significant effects on its ingestion (P=0.011). Igestion frequencies on FLIg and FLS had some correlation (r=0.609, n=30, P=0). Their maximum feeding frequencies were both in the high P. donghaiense abundance section, station zb7. Field observations suggested that P deficiency, or N:P ratios that were substantially higher than the optimum 10:1 ration for dinoflagellates may stimulate feeding behavior in these species. In our study, no feeding occurrences of Prorocentrum balticum and S. trochoidea were found, but the ingestions of 2 suspected Prorocentrum sp. on FLIg were found at za6b and 2 suspected Protoperidinium sp. on FLS and P. donghaiense were found at rb17.2. In the prophase of the mesocosm experiment, P. donghaiense could ingest both FLIg and FLS, the ingestion frequencies were0.2～1.4%. Phagotropgic behavior disappeared along with time. In metaphase and anaphase, no feeding occurrency was found. The cause of this phenomenon was unknown and needed further study.3. In the lab, we observed the nutritional strategy of 6 HAS growing under nutrient deficiency/sufficiency and darkness/normal light by feeding several prey species with different sizes. Results showed that K. mikimotoi could ingest live Isochrysis galbana under nutrient rich, normal irradiated condition. A. catenella could ingest fluorescent labeled Thalassiosira curviseriata (FLTc) in nutrient rich condition both in dark and light. P. donghaiense could ingest FLIg in nutrient-poor condition after mixcultured in darkness for 24hr. No matter at what conditions, only little amount of dinoflagellates were found to ingest cells. These results indicated that autotrophy rather than phagotrophy might play the main role in nutrients acquisition in these mixotrophic dinoflagellates, nutrient deficiency and darkness had little effects on their ingestions. In our research, no feeding occurrencies by P. micans,S. trochoidea,P. minimum were found. Detection and enumeration of cells containing food vacuoles with prey (i.e. phagotrophy) was based on the detection of fluorescence of acidotropic probe (LysoSensorTM), but no food vacuole was found in these 6 dinoflagellates. It meant that even these dinoflagellates were mixotroph, their feeding frequency were very low.Overall K. mikimotoi, A. catenella and P. donghaiense all could ingest cells, belonging to mixotrophic organisms, but feeding frequencies were very low. Light and nutrient had no impact on their feeding, autotrophy rather than phagotrophy played the main role in their growth. In East China Sea, P. donghaiense and other mixotrophic dinflagellates might make contributions to the outbreak,maintenance and succession of HABs.