The marine alpha-proteobacterium Phaeobacter inhibens engages in intermittent symbioses with microalgae. The symbiosis is biphasic and concludes in a parasitic phase, during which the bacteria release algaecidal metabolites in response to algal p-coumaric acid (pCA). The cell-wide effects of pCA on P. inhibens remain unknown. Herein, we report a microarray-based transcriptomic study and find that genes related to the oxidative stress response and secondary metabolism are upregulated most, while those associated with energy production and motility are downregulated in the presence of pCA. Among genes upregulated is a previously unannotated biosynthetic gene cluster and, using a combination of gene deletions and metabolic profiling, we show that it gives rise to an unreported siderophore, roseobactin. The simultaneous production of algaecides and roseobactin in the parasitic phase allows the bacteria to take up any iron that is released from dying algal cells, thereby securing a limited micronutrient.