Program     Marine Carbon Sequestration (MARCO): Multiscale Regulation and Response to Global Change       Poster An integrative approach reveals an unexpected high diversity and distinct community structure of ciliates in mesopelagic water of the northeastern South China Sea P-SPS5-07 Ping Sun* , Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystem, College of the Environment and Ecology, Xiamen University Liying Huang, Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystem, College of the Environment and Ecology, Xiamen University Presenter Email: psun@xmu.edu.cn Ciliates play key roles in the microbial loop of marine ecosystems. However, the partitioning of spatial, temporal, and environmental effects on ciliate distribution in transitions between environments remains largely unknown. An integrative approach was used to analyze ciliate communities by combining high-throughput cDNA sequencing and the quantitative protargol stain on seasonal samples collected along a transect from coastal to oceanic regions at depths ranging from the surface to 1000 m. Surprisingly, the alpha-diversity in the surface (3 m) and mesopalegic (200-1000 m) waters were comparable. Integrating size observations from morphological examination, we found this unexpected deep ocean diversity mainly came from small-sized (<60 μm, mostly 20-40 µm) ciliates. Distinct community composition above and below the photic zone was revealed and commensal/parasitic OTUs affiliated with Astomatida and Apostomatida (Oligohymenophorea) were more abundant in mesopelagic waters than above, hence implying they are an important component of food webs in the mesopelagic zone. Depth and its related environmental factors, not either geographic distance or time, were the best predictors of ciliate diversity and community structure. Collectively, these findings expand the knowledge of the diversity, community structure and forcing factors of ciliates in the environment, particularly in the mesopelagic waters