Algicidal infochemicals produced by marine bacteria have the unique ability to influence the structure of prokaryotic and eukaryotic microbial communities simultaneously. 2-heptyl-4-quinolone (HHQ), a quorum-sensing molecule produced by the marine Gammaproteobacterium Pseudoalteromonas piscicida, was recently shown to arrest the growth of certain eukaryotic phytoplankton. To investigate effects of HHQ in-situ, we exposed natural microbial communities from the Norwegian coast to nanomolar concentrations of HHQ over the course of a phytoplankton bloom. Amplicon sequencing (16S and 18S rRNA genes) revealed distinct patterns in both free-living and particle-associated bacteria and eukaryotic phytoplankton community structure.