Biotic and Abiotic Factors Shaping the Metacommunity Structure of Free-Living Bacterioplankton and Nanoflagellates in the Kuroshio Region

Substantial community variation in free-living bacterioplankton and their main predators—heterotrophic and mixotrophic nanoflagellates (HNFs)—often remains unexplained due to the frequent oversight of their mutual dependency through reciprocal influences. Here, we sampled bacterioplankton and HNFs from both the surface layer and the deep chlorophyll maximum (DCM) across a 13-degree latitudinal transect in the Kuroshio, spanning from Taiwan to Japan. Using three-way variation partitioning complemented by a null model approach, we disentangled the reciprocal influences between these microbial groups alongside the effects of dispersal and the abiotic environment. Our analyses reveal that bacterioplankton and nanoflagellates reciprocally explain over 10% of compositional variation when both layers are analysed together, highlighting the significance of their reciprocal influences. Dispersal contributes a comparable 10% of the explained variation, whereas environmental dissimilarity plays a minor role in shaping both trophic levels. When examining each layer separately, bacterioplankton composition is primarily driven by dispersal, whereas the composition of heterotrophic and mixotrophic nanoflagellates cannot be uniquely attributed to bacterioplankton, dispersal, or environmental factors. These findings suggest that biotic factors become more pronounced when considering vertical variation, whereas horizontal dispersal predominantly drives community composition within a given water layer. By incorporating two trophic levels, this study provides new insights into the metacommunity dynamics of free-living bacterioplankton and HNFs, emphasising the role of their mutual influences across depth gradients.