Effects Of Harmful Dinoflagellates On Population And Community Levels

Harmful algal blooms(HABs)can cause adverse effects at levels from the organism to population,community and ecosystem.Therefore,studies on the effects of HABs on the population and community levels provide the vital basis or are parts of the most important components of the ecology of HABs and dinoflagellates in particular.Among all HABs-causing algae in the ocean,dinoflagellates are the main contributors(?75%of all HABs).Most harmful dinoflagellates can release one or several kinds of secondary metabolites(i.e.allelochemicals)to the environment and consequently affect the survival and population dynamics of other coexisting plants(mainly microalgae),while other HABs-causing dinoflagellates can produce phycotoxins causing toxic or lethal effects on coexisting animals.The toxic and allelopathic effects may make HABs-forming species more competitive than the co-occurring phytoplankton in aquatic systems.Moreover,HABs may also cause deleterious effects at community level,which has been generally an aspect lacking investigation in the ecology of HABs,and thus is worthy of more intensive studies in order to gain a comprehensive understanding about the ecological consequences of HABs.Regarding the effects of HABs-forming dinoflagellates on the population level,it is of great significance to study the allelopathic effects of HABs on co-occurring phytoplankton.The present dissertation thus selected and investigated their allelopathic effects of four HABs-forming dinofllagellates using another well-known HABs-forming but allelopathy-sensitive dinoflagellate,Akashiwo sanguinea as a model target species.Among them,the most toxic species,Margalefidinium polykrikoides,was selected to investigate the toxic effects of HABs dinoflagellates at the organism and population level,since M.polykrikoides,including its four different ribotypes from all geographic regions,has been shown to be able to produce ichthyotoxic compounds,while the toxicity of Chinese population,however,has never been carefully examined.This thesis investigated the toxcity potency of the Chinese population of M.polykrikoides via comparing the toxicity of multiple strains and ribotypes of the species from different geographical origins.Regarding the effects of HABs at community level,previous researches have usually been focusing on animals,those of comercial values in particular,and other environmental factors such as dissolved oxygen and nutrients.This thesis therefore investigated the effects of Prorocentrum donghaiense blooms on the species diversity and community structure of the dinoflagellate sub-community in the East China Sea.The major findings of the thesis are summarized as follows:1.Study on allelopathic effects of harmful dinoflagellates at the population level.This study investigated the allelopathic effects of four common HAB-forming dinoflagellates,Margalefidinium polykrikoides(American/Malaysian ribotype),Karlodinium veneficum,Karenia mikimotoi,and Biecheleria brevisulcata(isolated from China),on another common HAB-forming dinoflagellate,Akashiwo sanguinea.The choice of A.sanguinea as a test model organism in this current study was due to its reported sensitivity to allelopathy.Under our experimental conditions(72 h co-culturing),it was observed that the allelopathic effects varied from positive(growth-stimulating)to negative(growth-inhibiting)depending on the species and in particular the initial cell density ratio.When co-cultured with A.sanguinea(initial cell density 500 cells/m L),C.polykrikoides promoted the growth of A.sanguinea at an initial cell density of 250 cells/m L while K.mikimotoi(strain Su‘Ao)and B.brevisulcata promoted A.sanguinea at initial cell densities below 1,000 cells/m L.The growth of K.veneficum(with initial cell densities between 5,000 to 50,000 cells/m L)was promoted by A.sanguinea.In contrast,with the same initial cell density for A.sanguinea(500 cells/m L),the strain of K.mikimotoi from Shenzhen Bay inhibited the growth of A.sanguinea at the initial densities>500 cells/m L,and C.polykrikoides inhibited the growth of A.sanguinea at initial cell densities>250 cells/m L in a dose-response manner that reached 100%inhibition when the initial density was above 2,500 cells/m L.It is noteworthy that the two strains of K.mikimotoi(Su‘Ao and Shenzhen Bay)exhibited significantly different allelopathic potency.The results demonstrated that the allelopathic interactions among HAB-forming species change from positive to negative,depending on the species and the ratio of initial cell density.We believe that these results will contribute to further understanding of the mechanisms and dynamics of HABs.2.Study on animal toxicity effects of harmful dinoflagellates at the population level.We examined the toxicity of four strains of M.polykrikoides including MPJZB-C3 and MPJZB-D6(East Asian ribotype),isolated from Jiaozhou Bay,China,CP1(American/Malaysian ribotype),isolated from Paconic Bay,USA,and MPCo KK23(American/Malaysian ribotype),isolated from Kota Kinabalu,Malaysia,to three aquatic animals including the marine medaka(Oryzias melastigma),rotifer(Brachionus plicatilis,L-type),and brine shrimp(Artemia salina).With equivalent initial cell densities ranged from 1,000 to 7,000 cells/m L,CP1 and MPCo KK23displayed acute toxicity in a dose-response manner to the three target animals(100%mortality at higher cell densities)within 24 h,MPJZB-C3 also displayed acute toxicity to the fish and rotifer in 24 h(up to 50%mortality)when the cell densities were above 3,000 cells/m L,while MPJZB-D6 did not exhibit significant toxicity to all three animals in 24 h,but,in 120 h,caused 50-100%mortalities in the fish when the cell density was>3,000 cells/m L and 17%mortality in brine shrimp when the cell density was>5,000 cells/m L.Statistical analysis of the data for all strains of M.polykrikoides,test animals,cell densities,and time points indicated that the strains CP1 and MPCo KK23 exhibited significantly higher toxicity(50-100%mortalities)to all three animals than the strains MPJZB-C3 and MPJZB-D6(0-100%mortality)did within 24 h exposure,while,however,the difference in toxicity between the American and Malaysian strains and that between the two Chinese strains were both not significant in terms of the mortality of test animals(with MPJZB-D6 showing the least toxicity,but not statistically different from MPJZB-C3).While the cell-free culture media(by filtration)and sonicated cultures were less toxic to test animals than the live cultures for all M.polykrikoides strains,sonicated cultures were even less toxic than the cell-free culture media,indicating that the toxin is unstable under mechanical force.Among the three target animals,the marine medaka exhibited to be the most sensitive to the toxicity,followed by rotifer and then brine shrimp.Collectively,our results suggest the differences in toxicity to three test animals among the four clonal cultures of M.polykrikoides reflect the inherent difference in toxicity between East Asian and American/Malaysian ribotypes,which led us to a hypothesis that the different ribotypes may be also different toxicity ecotypes,which needs further comparisons among more strains of all four ribotypes.3.Effects on community level–harmful algal blooms affect species diversity and community structure.Here we reported an investigation on the effects of a HAB caused by the dinoflagellate Prorocentrum donghaiense on the species diversity and community structure of the dinoflagellate sub-community via a pyrosequencing approach for the samples taken before,during,and after the bloom season of P.donghaiense in the East China Sea.We sequenced partial 28S r RNA gene of dinoflagellates for the field samples and evaluated the species richness and diversity indices of the dinoflagellate community,as a sub-community of the total phytoplankton.We obtained 800,185 valid sequences,categorized into 560 operational taxonomic units OTUs(Operational taxonomic unit)of dinoflagellates from 50 samples and found that the biodiversity of dinoflagellate community was significantly reduced during the blooming period in comparison to that in pre-and after-blooming periods,as reflected in the four diversity indices:the species richness expressed as the number of OTUs,Chao1 Index,Shannon Index(evenness),and Gini-Simpson Index.These four indices were all found to be negatively correlated to the cell density of the bloom species P.donghaiense.Correlation analyses also revealed that the P.donghaiense cell abundance was correlated negatively with NO₃^--N,and NO₂^--N,but positively with total nitrogen(TN)and total phosphorus(TP).Principal coordinates analysis(PCoA)showed that the community structure of dinoflagellates was markedly different among the different sampling periods,while the redundancy analysis(RDA)revealed P.donghaiense abundance,salinity,NO₃^--N,and SiO₃^2-were the most four significant factors shaping the dinoflagellate community structure.Our results together demonstrated that HABs caused by the dinoflagellate P.donghaiense could strongly impact the aquatic ecosystem on the sub-community level which the blooming species belongs to.