Contrasting growth phase-dependent microbiota dynamics between dinoflagellates and diatoms.

Bacteria associated with phytoplankton can profoundly influence host physiology and bloom dynamics. Yet it remains poorly understood whether the microbiota of dinoflagellates is restructured across host growth phases, and how such dynamics differ from those of other phytoplankton. Here, we tracked the microbiota of three harmful algal bloom (HAB)-forming dinoflagellates (Alexandrium catenella, A. pacificum, Gymnodinium catenatum) and three diatoms (Cylindrotheca closterium, Pseudo-nitzschia pungens, Coscinodiscus granii), each represented by two strains of distinct spatiotemporal origin and sampled at three growth phases (P1, P2, and P3). Microbiota composition was clearly differentiated by host species (PERMANOVA, R² = 0.799, n = 72, P < 0.001). However, the temporal dynamics of microbiota differed markedly between taxonomic groups. In dinoflagellates, microbiota was consistently restructured with each growth phase transition; in G. catenatum, growth phase accounted for the majority of microbiota variation (PERMANOVA, R² = 0.849, n = 12, P < 0.001) while intraspecific strain identity had negligible effect (PERMANOVA, R² = 0.014, n = 12, P > 0.05). In diatoms, by contrast, microbiota composition was strongly tied to intraspecific strain identity, with limited phase-dependent shifts. Dinoflagellates also harbored larger core microbiota than diatoms (15-37 vs. 2-9 core microbial lineages per host), exhibited higher centrality of core taxa within co-occurrence networks, and showed markedly lower normalized stochasticity ratios (NST; mean 15.7% vs. 35.5%). Microbiota dissimilarity was most strongly correlated with host phylogenetic distance (Mantel r = 0.617, P < 0.001), and the Bray-Curtis-based microbiota dendrogram was in complete topological congruence with the host phylogeny (nRF = 0.00, P < 0.01). Together, these findings demonstrate that dinoflagellates maintain a more deterministic and growth phase-linked microbiota than diatoms, indicative of strong host filtering.