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.