A NOVEL SPECIES OF PARASITIC LOBULOMYCETALES INFECTING ASTERIONELLA FORMOSA: CULTIVATION, INFECTIOUS CYCLE AND CRYOPRESERVATION

Parasitic Chytridiomycota (chytrids) are ecologically significant in various aquatic ecosystems, notably through their roles in controlling bloom-forming phytoplankton populations. They are a proven link in the food web, facilitating the energy transfer from inedible algae to higher trophic levels. The diversity and study of these obligate parasites, whilst critical to understand the interactions between pathogens and their hosts in the environment, has been hindered by challenges inherent to their isolation and stable long-term maintenance in laboratory conditions.
In the present study, we isolated an obligate chytrid pathogen on its host, the freshwater bloom-forming diatom Asterionella formosa and characterised its infectious cycle in controlled conditions. Phylogenetic analyses based on 18S, ITS1-5.8S-ITS2 and 28S rDNA revealed that the chytrid is indeed a new species belonging to a novel clade within the Chytridiomycota. We describe the new species, investigate its host range and present a robust and simple assay that assesses both the infection success and the viability of the host. We also validate a cryopreservation method
for stable and cost-effective long term storage, and reach full recovery of the chytrid infectious characteristics post- cryopreservation. A quantitative assessment of the chytrid infectivity post-cryopreservation demonstrates its full recovery after thawing, thus guaranteeing the long term availability of material for future study of this interesting novel organism. All of the above tools establish a new gold standard for the isolation and long-term preservation of pathogenic aquatic chytrids, thus opening new perspectives to investigate the undiscovered diversity of these organisms and their physiology in a controlled laboratory
environment.