Marine Bacterium Kordia algicida Reshapes Plankton Microbiome and Induces Metabolomic Rewiring, Independent of Heatwave or Worst-Case Climate Scenarios

Marine bacteria are integral components of planktonic communities, where they regulate algal growth, induce cell death, and contribute to bloom termination and species succession. They also play a key role in marine biogeochemical cycling by recycling algal-derived organic matter and releasing bioactive metabolites. Despite their ecological importance, bacterial–plankton interactions and their consequences for community structure and chemistry remain poorly understood. We investigated the impact of the algicidal marine bacterium Kordia algicida OT-1 on a natural plankton microbiome collected from a mesocosm experiment simulating present and future climate conditions. Plankton communities were exposed to ambient conditions or to a worst-case climate scenario, with a subset further subjected to a one-week heatwave. After 24 h of incubation, K. algicida significantly altered phytoplankton abundance and phylum-level community composition, independent of the applied abiotic conditions. Chemical changes induced by bacterial interactions were assessed by extracting filtrates from cocultures and analyzing them using ultra-high-performance liquid chromatography–high-resolution mass spectrometry (UHPLC-HRMS). Four natural products, i.e., adenosylhomocysteine, two indole alkaloid derivatives, and 5-bromotryptophan, were identified among metabolites released in response to bacterial exposure. Overall, shifts in the planktonic chemical landscape were primarily driven by bacterial activity, rather than abiotic conditions.