Distribution,Community Composition,and Metabolic Potential Of Microeukaryotes In The Tropical Western Pacific Ocean

Marine microeukaryotes,referring to eukaryotes with sizes less than 200 μm,have multiple nutritional modes(e.g.,photoautotroph,heterotroph,and mixotroph).They play significant roles as primary producers,predators,symbiosis,or parasites participating and influencing biogeochemical cycles on a global scale directly or indirectly.However,It is still unclear on the role and ecological functions of marine microeukaryotes,especially those with sizes less than 20 μm in the epi-(water depths<200 m),meso-(water depths 200-1000 m),and bathypelagic(water depths>1000 m)zones.This research mainly focuses on the abundance,community composition,and metabolic potential of photoautotrophic,heterotrophic,and mixotrophic taxa with different sizes based on multiple methods,likely fluorescence microscopy,TSA-FISH,high-throughput sequencing of 18S rRNA and rDNA,and transcriptomics.In addition,we aim to provide some novel data on the ecological functions of marine microeukaryotes at different water depths of the tropical western Pacific Ocean.The main results of this thesis are as follows:1.Results in Chapter 2 show that the abundance distribution and potential drivers of heterotrophic eukaryotes(HEs)with sizes less than 20 μm.The average abundance of HEs was 340 ± 24(s.e.),70 ± 6,and 9 ± 1 cells mL-1 in the epi-,meso-,and bathypelagic zones,respectively.In addition,the abundance of heterotrophic pico-eukaryotes(HPEs,HEs with sizes less than 3 μm)dominated in the HEs abundance and the percentages increased from the epi-downward to meso-and bathypelagic zones.The results also indicated that HEs might have distinct food source at different water layers.The abundance of HEs was significantly correlated with the abundances of Prochlorococcus,Synechococcus,heterotrophic prokaryote and viruse at the epipelagic zone.And it was significantly correlated with the abundances of heterotrophic prokaryote and sinking particulate organic matter in the dark ocean.2.Results in Chapter 3 show that the average abundance of pigmented eukaryotes(PEs)with sizes less than 20 μm was 726±88,6.4±1.0,and 2.6 ± 0.3 cells mL-1 in the epi-,meso-and bathypelagic zones,respectively.Moreover,the abundance of pigmented pico-eukaryotes(PPEs,PEs with sizes less than 3 μm)dominated the abundance of PEs,and Haptophyta was the major taxon of PEs.The proportion of Haptophyta in the PEs abundance increased from the epi-downdward to the meso-and bathypelagic zones.PEs and Haptophyta had different survival strategies in the epipelagic zone.PEs was extremely abundant in the deep chlorophyll maximum layer,twice the average abundance in the epipelagic layer.Whereas the abundance of Haptophyta remained stable at the upper layer over 150 m,was extremely abundant at 25-50 m but not in the deep chlorophyll maximum layer.3.Results in Chapter 4 show that HEs are ubiquitous in the dark ocean in spite of the food scarcity,probably owing to their highly diverse modes of nutrition(phagotrophy,osmotrophy,and paratrophy).Ciliate and Discoba were the major taxon of phagocytic HEs at 200 m and 2000 m,respectively.They were highly active and had high ingestion related transcription levels,indicating that they might survive in the dark ocean through predation.Labyrinthulomycotes and Fungi,representative species of osmoheterotrophic eukaryotes,had more diverse metabolic pathways and higher transcription levels in the deep sea than upper layers,suggesting that the deep ocean maybe favorable for their survival.In addition,parasitic eukaryotes,represented by Syndiniales,were more active at 2000 m than at 5 m and 200 m,suggesting that parasitism may also be an essential nutritional mode in the deep sea.4.PEs are ubiquitous in the dark ocean,this thesis studied the community composition and potential metabolism of PEs,and explored the significance of PEs existence in the dark sea.Results in Chapter 4 show that PEs consisted of photoautotrophic(represented by Chlorophyta and Bacillariophyceae)and mixotrophic taxa(represented by Dinophyceae and Haptophyta).At 5 m,PEs might survive through photosynthesis,owing to the high photosynthesis and carbon-related transcription level.At 200 m and 2000 m,photoautotrophic and mixotrophic taxa had different metabolic potential,and there may exist different survival strategies:photoautotrophic taxa may enter a dormant state and eventually fast sink to the deep ocean contributed to the carbon burial through the biological pump,whereas the mixotrophic taxa might survive in the deep sea through predation.5.Complex physical processes in the tropical western Pacific affected the distribution pattern of microeukaryotes:New Guinea Coastal Undercurrent carried high levels of nutrients could promote the growth of microeukaryotes.In addition,the existence of downflow in the Halmahera Eddy may enhance the efficiency of the biological pump,causing more PEs fast sink from the epipelagic zone downward to the meso-,and bathypelagic zones.