Bacterial microbiota influence algal growth and physiology, yet experimental validation in toxic benthic dinoflagellates remains limited. Here, we investigated the microbiota of six geographically distinct strains of the harmful benthic dinoflagellate Prorocentrum lima (P. lima) and identified core bacterial taxa shared across strains. Microbiota transplantation into an axenic P. lima strain revealed that donor consortia significantly enhanced early phase growth, increasing the specific growth rate by 61.1% compared to axenic controls. Coculture experiments with Marinobacter adhaerens (M. adhaerens), a core taxon isolated from P. lima, resulted in a significantly increased specific growth rate─up to 164.1% higher than the axenic control─while no such enhancement was observed in other tested microalgal species, indicating a host-specific interaction. Among several nutrient-limited conditions tested, this enhancement was uniquely observed under trace metal restriction. Genomic analysis and CAS assays revealed that M. adhaerens lacks canonical siderophore pathways but employs a noncanonical iron-acquisition strategy, which may contribute to enhanced iron availability under trace metal-limited conditions. These results provide experimental evidence that specific bacterial lineages closely associated with toxic dinoflagellates can promote their proliferation. Our findings highlight the ecological significance of dinoflagellate-associated core microbiota and offer new directions for microbiome-informed strategies in HAB monitoring and management.