中国东海微型鞭毛虫群落的分布特征及其环境影响因子

海洋微型鞭毛虫是海洋原生生物中一类高度异质化的类群，物种多样性高，具有多种营养方式，在全球海洋生态系统中占据广阔的生态位，在生物地球化学循环中发挥着关键作用。然而关于其生物多样性和群落结构的认识十分有限，特别是有关环境因子与其生物地理分布关系的研究更为罕见。为了探究微型鞭毛虫群落多样性、群落结构以及影响其生物地理分布格局的环境因素，将高通量测序技术与传统的显微镜观测方法相结合，全面调查了中国东海春季和秋季微型鞭毛虫的群落特征，并深入探讨了与环境因子之间的关系。结果表明：东海微型鞭毛虫的丰度平均为2.27×10~3个/mL,表现为近岸处较高、随离岸距离的增加而下降的趋势；Shannon多样性指数呈现表层低于底层、近岸区低于陆架区的特征，反映了生物群落的稳定程度以及对环境条件的适应程度；不同类群的鞭毛虫具有各自独特的营养模式和相对固定的粒级，表现出对温度、盐度、溶解氧等环境因素的不同响应，从而使群落的物种组成和分布模式呈现明显的季节变化和生境差异。研究结果可为深入认识东海海洋微型鞭毛虫的群落结构、分布格局以及环境影响因素提供理论依据。

Distribution of the microbial flagellates' community and the key affecting environmental factors in the East China Sea

Marine microbial flagellates are a highly heterogeneous group of protists with high biodiversity and multiple trophic modes. They occupy broad niches in global marine ecosystem and play essential roles in the biogeochemical cycles. However, little is known about the distribution of the diversity and community structure of the microbial flagellates in a regional marine area, especially the relationship between their biogeographic distribution and the environmental factors. The East China Sea (ECS) is a marginal sea with sloping topography, leading to a wide coastal area and its adjacent open continental shelf area. The hydrological conditions display gradient changing patterns from the coastal area to the shelf area in the ECS, which could affect the microbial flagellates'' communities differently, but is so far understudied. Thus, the ECS region provides a suitable system for the study of the relationship between environmental conditions and microbial flagellates'' communities. In order to adequately describe the diversity and community structure of the microbial flagellates, as well as the environmental factors that shape and drive their biogeographic distribution patterns, a comprehensive investigation on the microbial flagellates'' communities using combined methods of high-throughput sequencing of eukaryotic 18S rRNA gene V4 region and traditional microscopic observation was conducted in the ECS in spring and autumn, 2019. Intensive exploration also attempted using multiple statistical analyses on interpretation of the relationships between environmental factors and structure of microbial flagellates'' communities. The main findings are as follows:Firstly, the average abundance of the nano-sized flagellates in the ECS was 2.27×10³ cells/mL, which decreased with increasing distance from the shore. The heterotrophic nano-sized flagellates were dominant at the bottom layer of seawater while the surface seawater was dominated by the pigmented ones. Secondly, the community alpha diversity reflected the community stability and adaptability to environmental conditions, with higher Shannon diversity index both at the bottom seawater and in the shelf area than those at the surface seawater and in the coastal area, respectively. This indicated that the relatively stable environmental characteristics and environmental heterogeneity among different patches in the bottom layer and shelf area might cause sufficient competition and niche differentiation of the community, and thus led to higher biodiversity. Thirdly, different taxa groups of microbial flagellates owned unique nutritional modes and relatively fixed size fractions. The relative richnessnumbers of the Operational taxonomic units (OTUs)] and relative abundance (numbers of the sequencing reads) of various taxa were different. Alveolata were the most abundant and diverse taxa in the studied area. Archaeplastida, Hacrobia, and Stramenopiles displayed high relative abundance and richness in the pico-sized flagellates'' community, while Rhizaria showed high relative richness mostly in the nano-sized community. Lastly, the potential correlation between the microbial flagellates'' communities and environmental factors depended on the degree to which the biological community matched with or adapted to the changing environmental conditions. Different taxa showed different responses to the environmental factors such as temperature, salinity, and dissolved oxygen, etc. This led to the obviously seasonal dynamics and biogeographic distribution patterns of the community composition. Moreover, the geographic distances had an impact on the dispersal limitation of the microbial flagellates'' community, which might also result in spatial difference of the community. Specifically, the similarity of the community composition was greatly reduced with the increasing of the environmental heterogeneity and geographical distance. Overall, the findings of this study contribute to improve our understanding of the diversity, community structure, and affecting factors of marine microbial flagellates in the ECS, and to promote the research of marine flagellates from the phenomenon to the process and the mechanism.