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

Abstract

Coccolithophores, including bloom-forming species, <i>Gephyrocapsa huxleyi</i> (formerly <i>Emiliania huxleyi</i>), contribute ~1 to 10% of phytoplankton biomass and are critical for oceanic biogeochemical cycles. <i>G. huxleyi</i> 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 <i>G. huxleyi</i> 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 <i>G. huxleyi</i> and <i>Gephyrocapsa oceanica</i> 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.