Ecological Characteristics Of Protist In The Western Pacific Ocean

Protists are vital components of microbial food web in the ocean,including pigmented nanoeukaryotes(PNE),heterotrophic nanoflagellate(HNF)and ciliates.HNF are major predators of heterotrophic bacteria and picophytoplankton,while ciliate are important consumers of HNF and nanophytoplankton.PNE are great contribution to the marine primary production.Therefore,they play significant roles in the marine carbon and energy cycle.Horizontal and vertical distribution variation in the abundance and biomass of protists and the reponse of ciliate community structure to different water masses were investigated during three cruises between November 2014 and October 2016 in the Western Pacific Ocean.The main results as follows:(1)In the November 2014,ciliates were more abundant in the deep chlorophyll a maximum(DCM)layer and above in the Western Pacific Ocean.A total of 56 species(34 naked ciliates and 22 loricate ciliates)were observed,and the number of dominant species was 10.Strombidium epidemum was most dominant.Our results showed that the ciliates were significant negative correlation with depth.Light level and ocean current may influence the surface ciliates pattern.The food availability and nutrient concentration are high in DCM leading to the most ciliate abundance.Ciliates in deep sea implied that they had habits of high pressure and lower temperature.(2)In September2015,ciliates and PNE were more abundant in the deep chlorophyll a maximum(DCM)layer and above in the Western Pacific Ocean.HNF was mainly 2-5 ?m size HNF population,and the 2-5 ?m size HNF population decreased with depth.The number of dominant species were 12,and the most dominant specie was Strombidium epidemum in the surface layer.Strobilidium spirale was an indictor in the subsurface water.Leegaagdiella sol was an indictor in the intermediate water.We observed that the distribution of PNE and ciliate are influenced by depth and concentration of nutrient and food in the Western Pacific Ocean.In the euphotic layer,HNF abundance was controlled by bacteria and picophytoplankton(bottom-up),as well as ciliates grazing(top-down).In the deep sea,HNF abundance was controlled by bacteria(bottom-up).(3)In October 2016,average HNF abundance was approximately 3-4 times lower than those in 2015.While the average ciliate abundance was about 1-2 times higher than those in 2015.The results showed there was the significant positive correlation between HNF abundance and bacteria abundance implying a tighter HNF-bacteria coupling in the Western Pacific Ocean.There were different mechanisms of control abundance of HNF and bacteria or HNF and ciliate in the euphotic layer and deep sea.In the euphotic zone,HNF abundance was controlled by bacteria while ciliate controlled by the food availability.In deep sea,HNF was controlled by bacteria and ciliate was controlled by predator.Grazing rates of HNF on bacteria were higher than those in deep sea indicting temperature and the numbers of predator are key factors to grazing rates.In addition,HNF release particle organic matter and dissolved organic matter through preying on bacteria,which contribute to nutrient remineralization.