Spatial Distribution Pattern Of Planktonic Ciliates In The Western Pacific Ocean And The Chukchi Sea

Ciliates are a large group of single-cell microbial eukaryotes that inhabit a wide range of marine,freshwater,and soil environments.With body size from 5 to 200 μm,they are an essential component of protozoa.As a linkage between the Microbial loop and the classic food web,ciliates act as the primary consumers of the pico/nanoplankton and major food sources of mesozooplankton,such as copepods and larval fish.Ciliates play a vital role in the material cycle and energy flow of the marine ecosystem.Due to limited sequencing length,ecology research of ciliates based on highthroughput sequencing mostly uses small subunit ribosomal gene variable regions to reveal the community composition.The community structure and diversity of ciliate reflected by different primers may be different due to the bias of different primers,and the differences have not been systematically evaluated,which limits the comparison between related works based on different primers to a certain extent.The latitudinal diversity gradient is one of the most significant patterns in macroecology and biogeography.At present,the research on the latitudinal gradient diversity of ciliates mainly focuses on tintinnids,but few studies exist on the entire ciliate community.Previous studies have shown that body size is an important feature to distinguish microplankton,that is,microplankton with different particle sizes have significantly different community composition.However,the study of the spatial distribution of ciliates at different particle sizes is very scarce at large spatial scales.The majority of microbial communities seem to be composed of a few abundant taxa with high abundance and a large number of rare species with low abundance.Thus,it is necessary to study the diversity and spatial pattern of abundant and rare subcommunity.Recently,a growing number of researches is focused on bacteria and phytoplankton,but research about abundant and rare of ciliate remain scarce,and further exploration is urgently needed.In this study,we carried out the following researches:(1)Based on high-throughput sequencing,we systematically evaluated the differences of two primer sets(Cil-F/Cil-R and 384F/1147R)on ciliate diversity,community structure,and spatial distribution patterns in different habitats(oceanic area,transitional area,and melt ponds)of the Chukchi Sea.The results showed that,ⅰ)ciliate community composition and diversity revealed by different primers are significantly different.In terms of composition,both pairs of primer sets indicated that Spirotrichea,Litostomatea,and Phyllopharyngea are the dominant groups in the Western Arctic Ocean region.Compared with Cil-F/Cil-R,the proportion of Spirotrichea and Phyllopharyngea in the 384F/1147R community decreased,while the percentage of Litostomatea increased.In terms of a diversity,the diversity of Cil-F/Cil-R is significantly higher than that of 384F/1147R;ⅱ)The ciliate community revealed based on different primers has a consistent spatial distribution pattern.All show that environmental and spatial factors are important factors affecting the spatial distribution of ciliates in the Western Arctic Ocean;ⅲ)Combined with the analysis of deep chlorophyll maximum layer(DCM)samples in the oceanic area and the transitional area,it is shown that under a sharp environmental gradient(i.e.,from oceanic area,transitional area to melting ponds),environmental factors are the most influential factors.In contrast,under gradual environmental gradient(i.e.,from oceanic area to transitional area),depth is the main factor controlling ciliate community structure and diversity.(2)Besides,we chose Cil-F/Cil-R,which can better detect the diversity of ciliates,to evaluated the latitudinal diversity gradient and spatial distribution pattern of ciliates with different body sizes(micro-,nano-)and different abundance sub-communities along a transect across the Western Pacific Ocean from the Japan Sea to the Bering Sea.We also analyzed the contribution of spatial and environmental factors to explain the geographical pattern of abundant and rare ciliate sub-communities.The results show that,ⅰ)The composition of ciliates with different body sizes was significantly different,and the Spirotrichea,Phyllopharyngea,and Colpodea were the main groups causing community differences;ⅱ)Compared with the nano group,the a diversity of the micro group exhibits latitudinal gradient distribution;ⅲ)The spatial distribution pattern of micro group is more evident than nano communities;ⅳ)The conditionally rare or abundant taxa(CRAT)and the conditionally abundant taxa(CAT),which are the main contributors to differences of community composition among different regions,and play an essential role in the geographical distribution of ciliates;ⅴ)Both environmental and spatial factors coordinately contributed to the spatial distribution of ciliate communities.Variation partitioning showed that micro community exhibited a stronger response to environmental and spatial factors than the nano-community.