Rediscovering Zygorhizidium affluens Canter: Molecular Taxonomy, Infectious Cycle, and Cryopreservation of a Chytrid Infecting the Bloom-Forming Diatom Asterionella formosa

Parasitic Chytridiomycota (chytrids) are ecologically significant in various aquatic ecosystems, notably through their role in controlling bloom-forming phytoplankton populations, and in facilitating the transfer of nutrients 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. Here, we isolated an obligate chytrid parasite (CCAP 4086/1) on the freshwater bloom-forming diatom Asterionella formosa and characterised its infectious cycle in controlled conditions. Phylogenetic analyses based on 18S, 5.8S and 28S rDNA revealed that this strain belongs to the recently described clade SW-I within the Lobulomycetales. All morphological features observed agree with the description of the known Asterionella parasite Zygorhizidium affluens Canter. We thus provide a phylogenetic placement for this chytrid, 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 demonstrate its recovery after thawing. All the above tools establish a new gold standard for the isolation and long-term preservation of parasitic aquatic chytrids, thus opening new perspectives to investigate the diversity of these organisms and their physiology in a controlled laboratory environment. IMPORTANCE Despite their ecological relevance, parasitic aquatic chytrids are understudied especially due to the challenges associated with their isolation and maintenance in culture. Here we isolated and established a culture of a chytrid parasite infecting the bloom forming freshwater diatom Asterionella formosa . The chytrid morphology suggests it corresponds to the Asterionella parasite known as Zygorhizidium affluens . The phylogenetic reconstruction in the present study supports the hypothesis that our Z. affluens isolate belongs to the order Lobulomycetales, and clusters within the novel clade SW-I. We also validate a cryopreservation method for stable and cost-effective long-term storage of parasitic chytrid of phytoplankton. The establishment of a monoclonal pathosystem in culture and its successful cryopreservation opens the way to further investigate this ecologically-relevant parasitic interaction.