Bacteria induce an amoeboid phase in coccolithophores that persists after bloom collapse
Coccolithophores, who contribute approximately 1-10% of phytoplankton biomass, are crucial players in the oceans biogeochemical cycles. Significant contributions come from bloom-forming species including Gephyrocapsa huxleyi (formerly Emiliania huxleyi), which has served as a model system for investigating algal-bacterial and agal-viral interactions as well as algal fitness responses to environmental changes. Coccolithophores follow a biphasic lifecycle, existing as motile haploid and non-motile diploid phases. Here we characterize a third, amoeboid phase, using a combination of light, electron and phase amplitude modulation (PAM) microscopy. Using time-resolved imaging we captured the rapid morphological transition from a spherical haploid to an elongated motile amoeboid cell. Cell tracking revealed slower and more directional swimming compared to haploid cells. Amoeboid metamorphosis was triggered by exposure to bacteria, including strains isolated from G. huxleyi blooms and known G. huxleyi pathogens, but not by a range of classical phytoplankton stressors, including viral infection and oxidative stress. Further, the sub-population of haploids which switched to the amoeboid phase persisted past the rapid crash of the haploid population. This amoeboid phase was only observed in the primary bloom-forming coccolithophore species G. huxleyi and Geopharycapsa oceanica in stationary phase when exposed to high bacterial concentrations, typical of late-stage algal bloom events. Photophysiology of amoeboid cells was confirmed to be unaltered via PAM microscopy, indicating these cells are metabolically active. These findings highlight a previously uncharacterized morphotype in this important phytoplankton species and suggest that the amoeboid phase could be a bacteria-resistant morphotype following algal bloom collapse.