Growth enhancement of harmful dinoflagellate Prorocentrum cordatum by bacterial community shifts driven by dissolved organic matter derived from raphidophyte Heterosigma akashiwo

Phytoplankton blooms release dissolved organic matter (DOM), altering the composition of marine environments, which impacts microbial communities. Variation in DOM utilization among bacterial taxa can shift bacterial community structure, potentially affecting phytoplankton growth and species composition. Given the observed bloom successions of Heterosigma akashiwo and Prorocentrum cordatum, we hypothesized that H. akashiwo-derived DOM could influence P. cordatum growth via alterations in the bacterial community. This study investigated how DOM-modified bacterial communities affect P. cordatum growth. Our results showed that P. cordatum growth was enhanced by 76–220 % when exposed to DOM-altered bacterial communities, while Skeletonema costatum growth was suppressed by 24–28 %. This study demonstrates that the ratio of humic-like carbon to fulvic-like carbon in H. akashiwo-derived DOM, particularly during the stationary and decline phases, promotes bacterial taxa capable of degrading high-molecular-weight compounds, thereby facilitating P. cordatum growth. Functional predictions identified specific bacterial taxa with potential growth-promoting effects. Among them, Dokdonia, a genus within the family Flavobacteriaceae, was enriched in DOM-exposed bacterial communities and associated with growth enhancement, whereas algicidal and methanotrophic bacteria were more abundant in the absence of DOM. These findings highlight the role of bloom-phase-specific DOM in shaping bacterial communities and driving phytoplankton bloom succession.