Studies On The Diversity And Activity Of Protist In The Western Pacific Ocean And Northern South China Sea

Protists play an important role in the biogenic element cycle and energy flow in oligotrophic ecosystems.The development of molecular biology technology has enabled us to link the protistan diversity with its ecological functions,which has greatly improved our understanding of marine ecosystem function.Natural microbial assemblages inferred from the 18S rRNA gene sequencing can not distinguish the active groups from the inactive ones.Howerer,the rRNA-based sequencing approach was more inclined to characterize the metabolically active groups.Thus,combined the rDNA and rRNA sequencing approaches provide complementary information for understanding protistan diversity in a natural euvironment.The protistan biodiversity and activity are poorly studied in the western Pacific Ocean.Combining the rDNA/rRNA highthrough-put sequencing,high performance liquid chromatography,flow cytometry and scanning electron microscopy,we investigated the diversity and activity of protist(with different sizes)in the western Pacific Ocean and in the northern South China Sea in the present study.The following are the main findings:(1)Picoeukaryotes existing high and variable diversity in the northwestern Pacific Ocean with different hydrologic conditions,and their diversity was mainly affected by temperature and salinity.Picoeukaryotic community composition inferred from the DNA sequnencing approach was dominated by Mamiellophyceae,MAST,MALV-?,Spirotrichea,Prymnesiophyceae,and MALV-groups(69.33%of the total DNA sequences).While,Picoeukaryotic community composition inferred from the RNA sequnencing approach was mainly represented by Spirotrichea,Mamiellophyceae,MAST,Pelagophyceae and MALV-? groups(67.46%of the total RNA sequences).Principal component analysis revealed that picoeukaryotic community composition inferred from the rDNA and rRNA approaches was significantly different,and these difference were mainly attributed to the Mamiellophyceae,Spirotrichea,and Pelagophyceae groups.The RNA:DNA ratio was uesd as a proxy for the relative metabolic activity of the individual picoeukaryotic OTUs.Our results showed that the the nutrient concentration,(i.e.,the NO3+NO2 and SiO3 concentration)was the most important environmental factor affecting the relative metabolic activities of Mamiellophyceae,Spirotrichea,and Pelagophyceae OTUs in the northwestern Pacific Ocean.(2)With the increase of depth,the biodiversity of protist in the twilight zone was generally decreased.Rhizaria was the dominated group based on the the DNA sequencing approach,however,the relative abundance of this group was decreased significantly in the RNA sequencing survry.We speculate that part of the sequence obtained by DNA sequencing may obtain from dormant cells or fragments of dead materials setting from the euphotic layer.The relative abundance of Ciliophora in the RNA sequencing survey was significantly higher than that in the DNA sequencing results,indicating that Ciliophora were still highly active in the twilight zone of the northwestern Pacific Ocean.(3)Kuroshio intrusion had little influence on the diversity and community composition of nano-and pico-eukaryotes in the northern South China Sea.Nano-and pico-eukaryotic community composition in the northern South China Sea was mainly shaped by depth.Dinophyceae was the dominated group in nano-eukaryotes both in the DNA and RNA surveys;picoeukaryotic communities were dominant by Dinophyceae and Radiolaria in the DNA survey and by Dinophyceae,MAST and MALV-? in the RNA survey.Significant differences were observed in the picoeukaryotic community composition between the DNA-based and RNA-based molecular approaches,and these difference were manily attributed to the Radiolaria,MALV-?,Pelagophyceae,Mamiellophyceae,and MAST groups.The relative metabolic activity of picoeukaryotes was more susceptible to the lateral Kuroshio intrusion.The relative metabolic activities of picoeukaryotes were highly affected by temperature and nutrient concentration.(4)Picoeukaryotes showed high diversity in the western Pacific Ocean of lower latitude.Although the geographic locations of sampling stations were very different,the picoeukaryotic diversity and community composition was not influenced by the geographical distance.Picoeukaryotic community composition inferred from the DNA and RNA sequnencing approaches was mainly represented by MALV-?,MALV-?,MAST,and Radiolaria groups.The MALV-? group was metabolically inactive during the sampling period,while the MAST and pelagophyceae groups had high metabolic activities.