Haptophytes are single-celled eukaryotic plankton that are widespread in the open oceans. Revealing the differences between size-fractionated haptophyte communities is crucial in understanding their diversity and the role in the marine ecosystem. In this study, the diversity and community structure of nano- and pico-sized haptophytes in the Western Pacific Ocean (WPO) were investigated using 18S rRNA genes with haptophyta-specific amplification. Notably, the 28 samples were distinctly divided into two groups based on particle size (nano and pico), with 33 haptophyte genera detected in both subcommunities. Interestingly, the pico-sized community displayed higher alpha diversity compared to the nano subcommunity. Environmental and biotic factors were the main drivers of these differences. Furthermore, community assembly analysis using null model (NM) and neutral community model (NCM) showed that the pico-sized community appeared more influenced by stochastic processes, whereas nano-sized communities exhibited greater sensitivity to environmental factors, suggesting a stronger influence of determinated processes. Co-occurrence network analysis revealed tighter interconnections within the nano-sized community, which also harbored more keystone species. Additionally, intermediate taxa (IT) with relative abundance of 0.01–0.1% not only encompassed the highest zero-radius OTU (ZOTU) richness among keystone species but also accounted for the largest proportion of abundance in both networks, indicating their pivotal role in shaping the co-occurrence networks. This study highlights significant differences in diversity and community structure between nano- and pico-sized haptophytes, enhancing our understanding of the ecological characteristics of size-fractionated haptophytes in the Western Pacific Ocean.
 

haptophytes, 18S rRNA gene, community structure, community assembly ,co-occurrence network, intermediate taxa