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.