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

Diatoms are single-celled microalgae extremely abundant in the oceans and are responsible for producing 20% of the oxygen on our planet. Together with the other microalgae, they are the basis of the trophic web of our seas, a role similar to that of plants on earth.

The diatoms are covered with a thin rigid wall of silicon - the main component of glass - which forms a sort of 'box' around the cell. This siliceous coating is adorned with microscopic pores that allow the exchange of nutrients, oxygen and carbon dioxide with the environment surrounding the cell. Some diatoms produce quiescent cells - called 'spores' - lined with an extremely thick siliceous wall. The spores settle in the deep layers of the water column, where they can remain quiescent for decades in the bottom sediments, and are able to germinate if resuspended to light.

The factors that regulate the formation of spores are still almost completely unexplored. The research group coordinated by Dr. M. Montresor, in collaboration with two colleagues from Rutgers University (USA), has studied the possible role of viruses in inducing the transition from vegetative cells to spores. As we all know in this period of Covid-19 emergency, all living organisms can be victims of viral attacks, including bacteria and unicellular microalgae. The concentration of viruses in the sea is extremely high, but to date very few viruses capable of infecting diatoms have been identified. One of these was isolated and characterized some years ago in Japan: it is an ssRNA virus that infects the diatom Chaetoceros socialis.

Cultures of C. socialis isolated from the Gulf of Naples were inoculated with this virus and, within a few days, high cell mortality coupled with a significant increase in viral particles in the culture medium was observed. This shows that the virus is able to reproduce inside the cells. The presence of the virus has also been demonstrated in ultra-thin sections of infected cells observed in transmission electron microscopy.

The surprising result of our study was the finding of a high concentration of spores in the cultures infected by the virus, proof that the viral attack is able to induce the transformation between these two stages of the diatom's life cycle. Will these spores be able to propagate the viral infection when they germinate, producing a new population of vegetative cells? To answer this question, the spores were separated from dead cells and placed in environmental conditions favorable to their germination. The resulting cultures did not show evidence of viral infection.

The results of this study suggest that the transformation of vegetative cells into spores represents a defense mechanism against the virus. A possible scenario - to be tested in the future in the natural environment - is that viral infection occurs in the superficial portion of the sea, where Chaetoceros socialis can reach very high concentrations in certain periods of the year. The spores are heavy and sink at high speeds, helping to preserve the diatom population in the deep layers of the water column and, at the same time, limiting the transmission of the virus to future populations that will originate from the germination of the quiescent cells.

Pelusi Angela, De Luca Pasquale, Manfellotto Francesco, Thamatrakoln Kim, Bidle Kay D. & Montresor M. 2020. Virus-induced spore formation as a defense mechanism in marine diatoms. New Phytologist doi: 10.1111/nph.16951

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