Cortical structure of macroalgae influences epiphytic eukaryotic assemblages

Macroalgae host diverse epiphytic eukaryotes that contribute to coastal biodiversity and ecosystem function. Yet how fine-scale host traits shape these communities remains unclear. Using small organelle-enriched metagenomics (SoEM), we profiled epiphytes on eight intertidal macroalgal species from Korea and tested whether host cortical cell layer thickness predicts community structure. Epiphytic composition separated strongly by cortical thickness (thick > 8 cells vs. thin ≤ 5 cells), independent of host phylum. Thick-layered hosts predominantly harbored macrofauna (e.g., Malacostraca, Hexapoda), whereas thin-layered hosts were dominated by microalgae (diatoms). Phylogenetic diversity and the net relatedness index (NRI) indicated phylogenetic clustering in most thick-layered hosts (suggesting habitat filtering), while thin-layered hosts tended toward near-random assembly. Our findings identify cortical cell layer thickness as a key morphological trait structuring epiphytic eukaryotic communities. This trait-based perspective clarifies host–epiphyte interactions and motivates replicated, multi-scale studies integrating morphological and functional traits with genomic data for coastal biodiversity management.