Study On Diversity Of Microbial Eukaryotes In Sediments From The China Seas And Western Pacific Ocean

Microbial eukaryotes(=microeukaryotes)comprise of the commonly known protists and lower fungi.The estimation of the diversity and distribution of microbial eukaryotes has long been an ongoing debate and attracted considerable attention.Compared with pelagic seawater,marine sediments inhabit a much higher diversity of microbial eukaryotes.However,our understanding on their diversity and distribution in marine sediments is hampered by the methodological limitations.Hitherto,the questions about how environmental conditions,and latitudinal and depth gradients shap the diversity and distribution of microbial eukaryotes in marine sediments,are still unclear.The aim of this study is to answer the questions mentioned above.In this study,we firstly evaluated the influence of the environmental DNA(eDNA)in waters on assessing the microbial eukaryotes diversity in sediments.Based on the sediment and overlying water samples collected from two stations in the Yellow Sea Cold Water Mass(YSCWM),we evaluated the ciliate molecular diversity in sediments in relation to that in the upper water layers by using DNA and cDNA(complementary DNA)high-throughput sequencing.TheDNA in sediments was extracted by the direct extraction and elution methods,respectively.The results showed that the planktonic eDNA which affeced the evaluation of the molecular diversity of ciliates in sediments was mainly derived from the upper water layers.In contrast with DNA sequencing by the direct extraction of DNA from the sediments,DNA sequencing by the eluted DNA as well as cDNA sequencing could reduce the influences of planktonic eDNA on the evaluation of molecular diversity of ciliates in sediments.Furthermore,we compared the diversity and community compositions of microbial eukaryotes revealed by DNA and cDNA sequencing.The results showed that cDNA sequencing could reduce the influence of the metazoan DNA on the assessment of microbial eukaryote diversity in comparison with DNA sequencing.Moreover,the communities of microbial eukaryotes revealed by the two methods from the same station were significantly different.cDNA sequencing revealed Diatomea was the most dominant group,which was similar with the results based on morphological methods,whileDNA sequencing showed Dinoflagellata was the most dominant group.The results indicate that the cDNA sequencing method is more practicable in detecting the diversity and the distribution pattern of microbial eukaryotes in marine sediments.Based on the results of methodological evaluation,we utilized the cDNA highthroughput sequencing method to investigate the influence of the Yellow Sea Cold Water Mass(YSCWM)on the diversity and distribution of microbial eukaryotes in sediments in the Yellow Sea and adjacent sea area.The results showed that a similar pattern with that of meio-and macrobenthos was detected for the microbial eukaryotes,whose communities were significantly different inside and outside the YSCWM,and were significantly different in the north and south Yellow Sea.The mean richness of microbial eukaryotes inside the YSCWM was lower than that outside the YSCWM.About 31% of the total OTUs were exclusively obtained inside or outside the YSCWM,where they were generally in low abundance and were easy to be neglected by traditional morphological methods.Moreover,these OTUs had low identities to known species in the databases,and thus belong to the novel taxa.The dispersal limitation(spatial factors)had stronger effects in shaping the total and abundant communities than environmental factors,while environmental factors(depth and bottom water temperature)were more important in shaping the rare communities.We utilized the cDNA high-throughput sequencing method to investigate the microeukaryotes diversity and their distribution in sediments along a latitudinal gradient in the Yellow Sea(YS),East China Sea(ECS)and South China Sea(SCS).A total of 8,785 microeukaryotes OTUs were obtained in the three sea areas,and the number of OTUs obtained at the SCS was the highest,followed by the YS and ECS.By contrast,the mean OTU richness at each station was the highest in the YS,followed by the SCS and ECS.This pattern is different from the latitudinal distribution pattern of macrobenthos in the sea areas of China.The Spearman analysis showed that the number of OTUs was positively correlated with the latitude,and negatively correlated with the bottom water temperature.In term of community composition,the most dominant group varied in the three sea areas.The Diatomea,Cercozoa,and Ciliophora predominated in the YS,ECS and SCS,respectively.Cercozoa had the highest OTUs richness in all the three seas.The communities of microbial eukaryotes in the three seas were significantly different,and the similarity between the communities in the YS and ECS was higher,compared to those with the SCS.The dispersal limitation had stronger effects in shaping the total,abundant and rare communities than environmental factors did.We further utilized the cDNA high-throughput sequencing method to investigate the influence of depth gradient on the diversity and distribution of microbial eukaryotes from intertidal through continental shelf to deep-sea sediments(depth up to 6,000 m)in the same latitude(36°N).A total of 3,855 microeukaryote OTUs were obtained from all the samples.The total and mean OTU richness of microbial eukaryotes in the continental shelf(YS)was the highest,followed by the habitats of the deep sea(Northwestern Pacific Ocean)and intertidal zone(Qingdao Bay).The number of OTUs was negatively correlated with the depth and positively correlated with chlorophyll a and pheophytin a content in the coastal and deep-sea sediments.The communities of microbial eukaryotes from the three habitats were significantly different,and the community from the continental shelf was more similar with that from the intertidal zone than that from the deep-sea sediments.The most dominant group in the intertidal zone was Diatomea,and Cercozoa predominated in both the continental shelf and deep-sea sediments.The environmental factors had stronger effects in shaping the total(depth,chlorophyll a content and total organic matter content),abundant(depth,chlorophyll a content and total nitrogen content)and rare(depth and chlorophyll a content)communities than spatial factors did.In sum,we improved and optimized the research protocal on the diversity of microbial eukaryotes in marine sediments,and illustrated,for the first time,the influences of YSCWM on the mesoscale distribution pattern of benthic microbial eukaryotes.We also revealed the distribution pattern of benthic microbial eukaryotes along the latitudinal gradients on a large scale(China seas),showing a different latitudinal pattern from macrobenthos.Meanwhile,we revealed the distribution pattern of benthic microbial eukaryotes along a large spatial and water-depth gradient.Overall,this study indicates that the distribution of microbial eukaryotes in marine sediments,particularly that of rare taxa,was geographically restricted and was shaped by both the environmental factors and dispersal limitation.The relative importance of the environmental factors and dispersal limitation in shaping the distribution of microbial eukaryotes varied with different sampling scales as well as multiple habitats.