Virus-induced spore formation as a defense mechanism in marine diatoms

Diverse organisms have evolved a reversible state of reduced metabolism, dormancy or quiescence, to cope with stressful conditions. Spores, or resting cells, are dormant stages produced by diatoms, unicellular siliceous microalgae responsible for approximately 40% of global carbon fixation in marine environments. We show that viral infection of the bloom-forming, planktonic diatom Chaetoceros socialis induces the mass formation of resting spores, a heavily silicified life cycle stage associated with carbon export due to rapid sinking. Although viral RNA was detected within spores, mature virions were not observed. ‘Infected’ spores were capable of germinating, but did not propagate or transmit infectious viruses. These results demonstrate that diatom spore formation is a potential defense strategy against viral-mediated mortality. They provide a possible mechanistic link between viral infection, bloom termination, and mass carbon export events and highlight an unappreciated role of viruses in regulating diatom life cycle transitions and ecological success.