Abstract: Symbiotic and parasitic eukaryotes play essential roles in maintaining marine ecosystem stability and regulating plankton community structure. During the summer of 2021 and the winter of 2022, surface water samples were collected from seven stations in the Pearl River Estuary. High-throughput sequencing of the 18S rDNA V4 region was used to investigate the community structure and seasonal dynamics of symbiotic and parasitic eukaryotes. A total of 623 ASVs, obtained from symbiotic and parasitic organisms, belonged to five phyla and eleven families. The predominant groups of parasites were Syndiniales Group I and II and the Pirsoniaceae whereas Symbiodiniaceae and Phaeocystis were one of the main symbiotic groups. Seasonal variations in their community structures were observed between the summer and winter. In particular, Syndiniales Group II was the predominant group in both seasons (relative abundance: 36.00~80.79%) while Symbiodiniaceae was more abundant in offshore waters. Our results suggest that the dynamics of these groups of eukaryotes is influenced by environmental gradients and hydrodynamic processes and plays an important role in the regulation of planktonic community structure in the estuary. Thus, this work offers significant information about the ecological functions of eukaryotic symbionts and parasites in estuarine environments.

Key words: Eukaryotic symbionts and parasites, Plankton community, Community composition, High-Throughput Sequencing, 18S rDNA V4 region, Pearl River Estuary