Drought Resistant Resting Cysts of Paraphysoderma sedebokerense Preserves the Species Viability and Its Virulence

The blastocladialean fungus P. sedebokerense is a facultative parasite of economically important microalgae and for this reason it has gained a lot of interest. P. sedebokerense has a complex life cycle which includes vegetative and resting stages. The resting cysts were assumed to play an essential role in survival by resisting drought, but this ability was never tested and the factors that trigger their formation were not evaluated. This study was aimed to induce resting cyst formation and germination in P. sedebokerense. At first, we tested the survival of P. sedebokerense liquid cultures and found that infectivity is retained for less than two months when the cultures were stored on the bench at room temperature. We noticed that dry cultures retained the infectivity for a longer time. We, thus, developed a method, which is based on dehydration and rehydration of the biomass, to produce, maintain, and germinate resting cysts of P. sedebokerense in both saprophytic and parasitic modes of growth. When the dry cultures were rehydrated and incubated at 30 °C, resting cysts asynchronously germinated after 5 h and the "endosporangium" was protruding outside of the cyst. Our method can be used to preserve P. sedebokerense for research purposes with the advantage of no need for expensive equipment.