Miracula nipponica: a new marine oomycete parasitizing the diatom Leptocylindrus sp. in Sagami Bay, Japan

Parasitic oomycetes are ubiquitous in marine ecosystems, yet their phylogeny remains poorly characterized. While metabarcoding confirms their high diversity, the lack of formal descriptions leaves most of the sequences unclassified. In this study, we describe Miracula nipponica sp. nov., a parasitic oomycete from Sagami Bay, infecting the bloom-forming diatom Leptocylindrus sp. Phylogenetic analysis based on 18S rDNA sequences showed that all isolated strains were placed as a distinct lineage within the genus Miracula. Detailed microscopic observations revealed the formation of primary and secondary zoospores in the new species life cycle, with an occasional cyst cluster formation at the orifice of the discharge tube. Notably, the encysted stage demonstrated a long-term viability in the absence of hosts. Finally, laboratory experiments revealed a broad host range with low infection prevalence that includes other common diatoms from the study area. As the first parasitic oomycete infecting phytoplankton described in Japanese coastal waters, this study highlights their overlooked diversity and widespread presence of fungi-like organisms in marine ecosystems.