Dynamic Shifts Of Phytoplankton Assemblage And Metatranscriptomic Signatures In The Developmental Course Of A Harmful Algal Bloom

Marine phytoplanktons are the major players of marine carbon cycle,they are also important contributors of global primary productivity,their seasonal succession often lead to harmful algal blooms(HABs).Diatoms and dinoflagellates are two major groups of phytoplanktons,which are distributed widely and abundantly.While diatoms normally dominate,dinoflagellates can out grow them seasonally or on occasions of environmental perturbations,often resulting in harmful algal blooms(HABs)that exact devastating impacts on the marine ecosystem,economy,and public health.Currently,most research on marine phytoplankton are focusing on the distribution,ecological survey and physiological response to environmental factors.How these two keystone phytoplankton groups are so successful and how their"seesaw" dynamics tips towards a bloom are fundamental yet poorly understood marine ecology questions.In this study,we took the 18S rRNA-rDNA profiling approach to investigate the how the community composition,diversity and the physiological activity changed in the time course from pre-bloom to the dinoflagellates bloom;we employed time-sequential(month long)metatranscriptomics to characterize the molecular evolution of a phytoplankton community from diatom dominance(dominantly Skeletonema)to dinoflagellate(Prorocentrum donghaiense)bloom,an annual event observed in the study area in East China Sea for more than a decade;three types of MEI2-like proteins are identified from P.donghaiense based on the transcriptome data,and their expression pattern were explored and functions were predicted.The major findings are summarized as below:1.A dinoflagellate bloom often succeeds a diatom-dominant phytoplankton community.Information on how the phytoplankton community shifts in the course of the bloom is crucial for understanding the mechanism of bloom formation,but is limited.Here we report the temporal changes in the general community and metabolically active sub-community over a three-week period in East China Sea in the course of P.donghaiense bloom development.High-throughput sequencing of 18S rRNA and 18S rDNA(18S rRNA gene)was carried out for plankton samples(3.0-200 ?m)collected in the pre-bloom(April 30)and bloom(May 13,15 and 20)periods.From a total of 383.716 high-quality contigs,452 operational taxonomic units(OTUs,defined by 97%sequence identity)were retrieved from all the samples.Alpha diversity index of both 18S rDNA and rRNA in the pre-bloom community was greater than that in the bloom community.The dramatic increase in P.donghaiense rDNA abundance and rRNA:rDNA ratios from pre-bloom to bloom stages indicated that the non-bloom to bloom evolution of the phytoplankton community was driven by high metabolic activities of the bloom species.These findings demonstrate that simultaneous analysis of 18S rDNA and 18S rRNA is more insightful than rDNA analysis alone for understanding how each phytoplankton species contributes in the community and whether bottom-up or top-down control regulates the dynamics of a harmful algal bloom.2.Here we conducted time-sequential metatranscriptomic profiling on a natural assemblage that evolved from diatom dominance to a dinoflagellate bloom to interrogate the underlying major metabolic and ecological drivers.Data reveals similarity between diatoms and dinoflagellates in exhibiting high capacities of energy production,nutrient acquisition,and stress protection in their respective dominance stages.The diatom-to-dinoflagellate succession coincided with an increase in turbidity and sharp declines in silicate and phosphate availability,concomitant with the transcriptomic shift from expression of silicate uptake and urea utilization genes in diatoms to that of genes for light harvesting,diversified phosphorus acquisition and autophagy-based internal nutrient recycling in dinoflagellates.Furthermore,the diatom-dominant community featured strong potential to carbohydrate metabolism and a strikingly high expression of trypsin potentially promoting frustule building.In contrast,the dinoflagellate bloom featured elevated expression of xanthorhodopsin,and antimicrobial defensin genes,indicating potential importance of energy harnessing and microbial defense in bloom development.This study sheds light on mechanisms potentially governing diatom-and dinoflagellate-dominance and regulating bloom development in the natural environment and raises new questions to be addressed in future studies.3.Two types of MEI2-like genes were identified from P.donghaiense based on the transcriptome and their functions were explored.Their expressions were very high during the P.donghaiense bloom and were down-regulated under UV radiation.We conferred MEI2-like might participate in cell proliferation.In addition,there was no significant relationship between cell cycle and these genes expression,which indicated their function might not involve in mitosis.As previous studies showed that MEI2-like genes are involved in initiation of DNA synthesis of pre-meiosis and meiosis I,we conferred that the expression of MEI2-like genes might be the molecular marker of meiosis in P.donghaiense.However,so far,we don't have any direct evidences that showed the functions of these genes,which need to be further investigated.