Introducing a new pathosystem for marine pathogens – the green alga Urospora neglecta and its pathogen Sirolpidium litorale sp. nov.

Holocarpic oomycetes, which had fallen into relative scientific obscurity for quite some time, recently regained scientific interest. Among these organisms, some stand out for infecting commercially valuable red algae, such as the nori algae of the genus Pyropia, rendering them economically significant. After having observed simple holocarpic pathogens of green algae in previous sampling trips, most of which we could not fully characterise due to their scarcity, we now discovered a parasite of the widespread littoral green algae Urospora neglecta. This previously unnoticed parasite turned out to be distinct from another green algae-infesting species, Sirolpidium bryopsidis. This finding led us to formally describe it as Sirolpidium litorale, marking the first new species of holocarpic oomycetes of green algae to be found on the shores of Blávík, East Iceland. The ease of cultivation of U. neglecta offers a unique opportunity to establish a genetically tractable system for studying how Sirolpidium interacts with algae. This could help uncover the underlying mechanisms behind the pathogenicity of the species of this genus and provide valuable insights into their ecological roles and evolutionary history.