Bacteria induce an amoeboid phase in coccolithophores that persists after bloom collapse.

Coccolithophores, including bloom-forming species, (formerly ), contribute ~1 to 10% of phytoplankton biomass and are critical for oceanic biogeochemical cycles. is a model system for investigating algal-bacterial-viral interactions and responses to environmental changes and follows a biphasic lifecycle with motile haploid and nonmotile diploid phases. Here, we report a third, "amoeboid" phase: Light and electron microscopy revealed haploid cells rapidly transitioning to an elongated amoeboid cell with reduced motility. Metamorphosis was triggered by exposure to bacteria isolated from mesocosm blooms, but not by classical phytoplankton stressors including viral infection. The amoeboid phase persisted beyond the collapse of the haploid population and was only observed in the bloom-forming coccolithophore species and under conditions reminiscent of late-stage algal blooms. These findings highlight a previously uncharacterized life phase in this ubiquitous phytoplankton and suggest a bacteria-resilient morphotype following algal bloom collapse.