The community patterns and assembly mechanism varied in eukaryotic microorganisms with different cell sizes.

Cell size is a fundamental determinant of eukaryotic microorganisms' ecology, influencing community dynamics, physiological processes, and ecological interactions. However, it is still unclear whether eukaryotic microorganisms of different size have similar structural and assembly mechanisms. In this study, we sorted eukaryotic microorganisms' cells into two sizes classes (large-sized eukaryotic microorganisms (LE) and small-sized eukaryotic microorganisms (SE)) and compared their structural and assembly mechanisms along the Han River. The results showed that LE fractions had higher diversity compared to SE fractions, the two size classes harbored distinct eukaryotic microorganisms' communities. And. Assembly mechanisms of the two classes were significantly different. We conclude that cell size is an important factor that determines eukaryotic microorganisms' structure and assembly mechanisms. These findings underscore the critical role of cell size in determining eukaryotic microorganisms' community and uncover a direct relationship between microbial morphology and ecological traits.