Marine Bacterium <i>Kordia algicida</i> 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 <i>Kordia algicida</i> 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, <i>K. algicida</i> 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.