Seasonal dynamics of eukaryotic microalgae in Osaka Bay revealed from time series data by 18S rRNA amplicon sequencing analysis

Eukaryotic microalgae fundamentally contribute to marine primary production throughout the year. However, a comprehensive understanding of their year-round fluctuations in coastal ecosystems remains limited. In this study, we identified the major microalgae and their fluctuation patterns in Osaka Bay by analyzing the 18S rRNA gene amplicon sequencing of seawater samples collected monthly for 18 months. The eukaryotic community structure, comprising over 3000 amplicon sequence variants (ASVs), exhibited clear seasonal dynamics. In particular, Picochlorum geojense ( Trebouxiophyceae ) was the major microalga in Osaka Bay, reaching a relative abundance of over 60% in summer. The highest relative abundance of P. geojense was even higher than that of other bloom-forming species in Osaka Bay: the raphidophycean Heterosigma akashiwo and the diatom Skeletonema spp. The canonical correspondence analysis suggested that temperature and phosphate significantly influenced the community dynamics. Furthermore, co-occurrence network analysis identified a positive correlation between the heterotrophic bacterium Cryomorphaceae and P. geojense , suggesting a potential biotic association. Similarly, ASVs of major diatoms and dinoflagellates were estimated to function as highly connected ecological hubs, given their co-occurrence with multiple ASVs. These results showed that the seasonal dynamics of dominant microalgae in Osaka Bay were associated with environmental factors and showed co-occurrence patterns with specific prokaryotes.