The Effect Of Environmental Factors On Phaeocystis Ingestion By Rotifers And The Analysis Of The Differences At The Multi-omics Level

Due to the effect of human activities,global warming and sea water eutrophication are becoming more and more serious,the red tide of Phaeocystis outbreak frequently in the ocean,which leads to water hypoxia and water quality deterioration,and cause a serious threat to marine ecosystem.Biological method is considered as an environment-friendly method to control harmful algal blooms.Our previous studies have shown that rotifer can eliminate the population of Phaeocystis well and transmit energy to higher trophic levels by ingesting Phaeocystis.Due to rotifer has the following advantages of no food selectivity,short life span,easy for culture and large reproduction,it has become one of the most potential zooplankton to control Phaeocystis.However,in the complex natural environment,what kind of interference does the external environmental stress have on rotifer eliminate Phaeocystis?And how is the effect of rotifer long-term ingestion of Phaeocystis on transcriptome and metabonomic level regulated?This paper will focus on the above scientific questions,simulate environmental factor effect of rotifer ingesting Phaeocystis,combined with analysis on the differences of individual level,population level and molecular level of rotifer,to clarify the response of population dynamics of rotifer and Phaeocystis to a variety of environmental factors and its internal mechanism.The research contents and results of this thesis mainly include the following five parts:1.Temperature-dependent elimination efficiency on Phaeocystis globosa by different initial population sizes of rotifer Brachionus plicatilisWe cultured Phaeocystis with different initial population densities of rotifer(1,3,5 inds mL-1)at 19,22,25,28,and 31? for 9-16 d until all the Phaeocystis were eliminated.Results showed that the population of rotifer feeding on Phaeocystis increased rapidly and higher temperatures favored the growth of Phaeocystis and rotifer.With increased initial rotifer density and temperature,both the clearance rate of rotifer and the reduction rate of Phaeocystis increased,and thus Phaeocystis were eliminated earlier.Both temperature and initial rotifer density had significant effects on clearance rate of rotifer and the time to Phaeocystis extinction,and there was a significant interaction between the two factors on the two parameters,i.e.,the effect of initial rotifer density on eliminating Phaeocystis decreased with increasing temperature.In conclusion,higher temperature and bigger initial rotifer density promote rotifer to eliminate the harmful Phaeocystis,and the optimal temperature for rotifer to clear Phaeocystis is 28?.2.The effects of cadmium on life history parameters of Brachionus plicatilis and rotifer-Phaeocystis population dynamicsPhytoplankton abundance,heavy metal and temperature may have interactive way on zooplankton.Therefore,in this study we investigated the combined effects of different cadmium(Cd)levels and Phaeocystis concentrations on some key life-history traits of the rotifer and the combined effects of non-lethal Cd concentration and temperature on the population dynamics of rotifer and Phaeocystis.Results showed that the Cd level and Phaeocystis concentration had a significant interaction on the key life-history parameters of the rotifer.Mid-level algal concentrations(5-36×104 cells mL-1)had an apparent effect on brood production and the number of rotifers producing offspring at high Cd level.The time to first reproduction,the survival time exponentially decreased with increasing Phaeocystis concentrations under any Cd levels and then subsequently reached a constant value.With increasing Phaeocystis concentration,the total number of offspring exponentially increased and then reached the asymptotic value;High concentration of Phaeocystis can reduce the sensitivity of rotifer to heavy metal.However,the negative effects could not be eliminated completely by the increasing Phaeocystis concentration.These results of life history combined with population dynamics experiment show that under the non-lethal environmental concentration of Cd,which rotifer can complete the whole life history and reproduce a considerable number of offspring,it will also lead to the failure of rotifer to control Phaeocystis.This may be due to the maternal effect of rotifer,the new individuals and their offspring continue to be under Cd stress,which eventually leads to the decline of overall performance of rotifer individuals and rotifer population declined,so it could not inhibit the rapid growth of Phaeocystis population.At the same time,the negative effect of Cd on rotifer was aggravated by temperature increased.3.The interference effect of microplastics on the ingestion of Phaeocystis by rotifer:based on the changes of life history traits of rotifers and rotifer-Phaeocystis population dynamicsTo evaluate the effects of microplastics on rotifers,we designed a series of experiments concerning rotifer life-history traits and rotifer-Phaeocystis(predator-prey)population dynamics under different concentrations and sizes of microplastics.The results showed that small-sized microplastics(0.07 ?m)at high levels(?5 ?g mL-1)decreased rotifer survival and reproduction,prolonged the time to maturation,and reduced the body size at maturation,whereas large-sized microplastics(0.7 and 7 ?m)had no effect on rotifer life-history traits.For rotifer-Phaeocystis population levels,small-sized microplastics(0.07 ?m)significantly delayed the elimination of Phaeocystis by rotifers;The results of rotifer-Phaeocystis population dynamics are consistent with the changes in the life-history traits of rotifers and further confirm the negative effects of small-sized microplastics(0.07 ?m)on rotifers.These findings help to reveal the effect of pollutants on predator-prey population dynamics.4.Trade-off between reproduction and lifespan of the rotifer Brachionus plicatilis and the difference analysis of transcriptome level under different food conditions(Phaeocystis or Chlorella)To explore whether there is a trade-off between rotifer reproduction and life history under different food conditions and the internal mechanism of life-history parameter changes,we exposed the rotifer to cultures with 100%Chlorella,a mixture of 50%Phaeocystis and 50%Chlorella,or 100%Phaeocystis,and the life history parameters were recorded.At the same time,we selected rotifers that cultures with 100%Chlorella and 100%Phaeocystis for transcriptome sequencing.Results showed that the difference of food quality had a significant impact on the life-history parameters of rotifers.The overall performances(combining reproduction and lifespan together)of rotifers cultured in 100%Chlorella or mixed diets were significantly higher than those cultured in 100%Phaeocystis.In general,Chlorella favors rotifers reproduction at the cost of shorter lifespan,whereas Phaeocystis tends to extend the lifespan of rotifers with lower fecundity,indicating that trade-off exists between reproduction and lifespan under different food conditions.In addition,the expression trends of rotifer feeding on Phaeocystis and Chlorella in carbohydrate digestion and absorption,glycolysis,gluconeogenesis,biosynthesis of unsaturated fatty acid and apoptosis pathways were consistent with the results of rotifer life history,which confirmed the internal mechanism of different food conditions on the external overall performance of rotifer.5.Adaptive tolerance of rotifers for long-term ingestion Phaeocystis and the analysis mechanism of multiomicsIn order to research the effect on the rotifer life history and the internal mechanism changes on rotifer ingesting Phaeocystis for a long time,the life history experiments were carried out by using the rotifer cultured with Phaeocystis for a short time and in a long time,then the rotifer samples were collected for transcriptome and metabolome sequencing.The results showed that the rotifer culture with Phaeocystis for a long time,the total offspring per rotifer was 15%higher than that of the rotifers fed on Phaeocystis for a short time.The experimental data of transcriptome and metabolome showed that the differentially expressed genes and metabolites were mainly related to energy metabolism,carbohydrate metabolism and multiple nutrient metabolism pathways.The changes of life history parameters were consistent with the changes of multiomics regulatory pathways,which indicated that rotifer had a certain adaptive tolerance to Phaeocystis due to its long-term nutrient deficiency.In conclusion,the life history of rotifers and rotifer-Phaeocystis population dynamics were significantly mediated by external environmental factors after ingesting Phaeocystis:higher temperature and higher initial rotifer density promoted rotifer to eliminate harmful Phaeocystis;heavy metals and microplastics affected the life history of rotifers feeding on Phaeocystis,and interfered with or completely hindered rotifer from eliminating Phaeocystis;trade-off existed between reproduction and lifespan of rotifer under different food conditions(Phaeocystis or Chlorella).These changes were related to the expression trends of carbohydrate digestion and absorption,glycolysis,gluconeogenesis,unsaturated fatty acid biosynthesis and apoptosis;rotifers had adaptive tolerance after ingestion Phaeocystis for a long time.The differentially expressed genes and metabolites were mainly related to energy metabolism,carbohydrate metabolism and various nutrient metabolism pathways.These results have an important theoretical significance for clarifying the response of the interspecific relationship between rotifer and Phaeocystis to a variety of environmental factors and for further revealing the adaptive tolerance mechanism of rotifer to Phaeocystis.They are also have a great practical significance to evaluate the actual effect of algae control and turn harmful algae into benefit.