Spatial patterns of functional diversity and composition in marine benthic ciliates along the coast of China

Large-scale patterns of community composition and diversity along environmental gradients have been well studied for macroorganisms. However, the biogeography of microorganisms, especially ciliated protozoa, remains understudied. Here, we analyzed a comprehensive database of marine benthic ciliates found along the coast of China from 1991 to 2018 to examine the geographical patterns in species and trait composition and functional diversity. According to redundancy analysis conducted at large spatial scales, environmental variables, i.e. habitat type and salinity, explained more variance in species composition than latitude. In contrast, trait composition was better explained by spatial and climatic variables. At small spatial scales, both species and trait composition were probably influenced by mass effects due to the high dispersal ability of ciliates at such small scales, resulting in spatially homogenized communities. Several traits, including body size, feeding type and mobility, exhibited significant positive or negative latitudinal gradients. Functional diversity showed a significant positive correlation with latitude between 20 and 40° N, which may be caused by certain groups of ciliates possessing particular traits related to temperature. Our study is the first comprehensive evaluation of how trait composition and functional diversity of marine ciliated protozoa vary at large scales and can thus make a major contribution to the study of microbial biogeography.