A Study On The Changes And Omics Of Phytoplankton Community In The Qinhuangdao Coastal Waters

Qinhuangdao,located along the western boundary of the Bohai Sea and opposite to the Bohai Strait,is a tourist city that is well known both within China and abroad.In recent years,due to the deterioration of the ecological environment in the Bohai Sea,harmful algal blooms have become more and more serious.Since 2009,the newly recorded species Aureococcus anophagefferens has successively induced brown tides in the Qinhuangdao coastal waters,causing significant damage to the local shellfish industry and coastal tourism.However,studies about the outbreak mechanism of brown tides and environmental characteristics of the Qinhuangdao coastal waters have rarely been reported.Here,in order to explore the internal and external factors driving community succession and to provide a scientific basis for the further explanation of the outbreak mechanism of harmful algal blooms,we conducted a series of studies about the characteristics of the phytoplankton community and its responses to environmental changes from March 2013 to January 2014,using microscopy techniques,chlorophyll a(Chl-a)analyses,flow cytometry,18 S rDNA sequencing and metatranscriptome sequencing.In total,123 species of phytoplankton belonging to 56 genera were identified via microscopy,including 87 species of diatom belonging to 39 genera,33 species of dinoflagellate belonging to 16 genera,and 1 species each of chlorophyte,euglenophyte and chrysophyte.The phytoplankton community underwent succession from nanocelled chained diatoms to dinoflagellates in the spring and summer,followed by succession to micro-celled diatoms and nano-celled chained diatoms as the most prominent groups in the subsequent autumn and winter.Canonical correspondence analysis revealed that changes in the abundance of diatoms were related to nutrient availability,whereas water temperature was the crucial factor influencing the abundance of dinoflagellates.To better study the size-fractionated composition of the phytoplankton community,Chl-a analyses and flow cytometry were applied.Analyses of the size-fractionated Chla concentration showed that micro-and nanophytoplankton represented the majority of total Chl-a.During the study period,phytoplankton < 10 ?m had high cell abundance(>108 cells/L),which was more than 10,000 times the result observed using microscopy.As picophytoplankton are difficult to distinguish and count by microscopy,the results above indicate the predominance of picophytoplankton in the group of phytoplankton < 10 ?m.A.anophagefferens had high cell abundances in August and September and contributed small proportions(3.83% to 22.90%)of the phytoplankton < 10 ?m during the study period.To evaluate the species composition and relative abundance of the eukaryotic phytoplankton community(including the picophytoplankton)from a molecular biological perspective,we sequenced 18 S rRNA genes extracted from field samples.A total of 71 algal species were identified,among which 34 were new records.Further analysis revealed that half of the identified species were known bloom-forming or toxic algae,including 11 harmful algal species that were previously undetected or unreported in the Bohai Sea.Dinoflagellates stood out from the phytoplankton community in many respects,including sequence abundance,OTU richness,new records,harmful algal species and nutritional modes.The more abundant genera among different phytoplankton lineages were almost pico-or nano-celled algae,as were the most abundant identified algal species.A metatranscriptomics technique was employed to reveal the functional responses during the phytoplankton succession in the Bohai Sea.Results showed that phytoplankton < 10 ?m had high contributions to the gene expression in the phytoplankton community.Expression levels of the pathways Photosynthesis,Photosynthesis-antenna proteins,Phagosome and Lysosome varied among algal species and samples.Pre-and post-bloom gene expression levels of most basal metabolic pathways were higher than those during the bloom.In summary,we used a variety of techniques to study the characteristics of the phytoplankton community in the Qinhuangdao coastal waters and found that picophytoplankton formed a dominant group.Many algal species were newly recorded in the Bohai Sea,broadening our understanding about the composition and biomass of the eukaryotic phytoplankton community.Moreover,we described the succession characteristics of the eukaryotic phytoplankton community in the Qinhuangdao coastal waters,providing a scientific basis for revealing the processes underlying the evolution of the ecosystem and revealing the mechanism of algal bloom formations.