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Fungi encompass, in addition to classically well‐studied lineages, an ever‐expanding diversity of poorly known lineages that include, among others, zoosporic chytrid‐like parasites. According to recent phylogenetic analysis based on 18S + 28S rRNA concatenated genes two unusual chytrid‐like fungi Amoeboradix gromovi and Sanchytrium tribonematis form a monophyletic group, the family Sanchytriaceae, which represents a new divergent taxon that remains incertae sedis within Fungi. Zoospores of Amoeboradix gromovi contain one of the longest kinetosomes known in eukaryotic cells, which are composed of microtubular singlets or doublets. However, the ultrastructure of S. tribonematis, the type species of the genus had not been yet studied. Here, we provide the results of TEM investigations of zoospores and sporangia from two strains of S. tribonematis. The two strains are endowed with unusual features. Like in A. gromovi, amoeboid zoospores of S. tribonematis contain a long kinetosome composed of microtubular singlets, and the two orthogonal centrioles in their sporangia have nine microtubular singlets with an internal ring. The morphological and ultrastructural features of S. tribonematis are now included in the improved taxonomic diagnosis for this species. [ABSTRACT FROM AUTHOR]

The aphelids (phylum Aphelida) are phagotrophic parasitoids of algae and represent the most basal branch in superphylum Opisthosporidia, which contains the Microsporidia, Rozellosporidia and Aphelida. Being the closest group to traditional fungi, the aphelids should have ancestral features of both phyla. As in chytrids and other zoosporic fungi, the structure of zoospores is the most informative and important morphological feature for the phylogeny and taxonomy of aphelids. Though a general zoospore description exists for some aphelid species, their flagellar apparatus (kinetid) structure, which contains pivotal taxonomic and phylogenetic characters, has not been studied. Here we represent the kinetid structure in two genera, Aphelidium and Paraphelidium, and demonstrate independent reduction in the kinetid in each genus. The kinetid‐mitochondrion connection found in Aphelidium and Paraphelidium is rare for opisthokonts in general, but present in the most basal branches of Fungi and Opisthosporidia. We suggest, therefore, that this connection represents an ancestral character for both these phyla. [ABSTRACT FROM AUTHOR]

• Diverse zoosporic parasites are detected during a Kryptoperidinium foliaceum bloom in the Baltic Sea. • The new chytrid genus and species Paradinomyces triforaminorum is described, showing specificity for K. foliaceum. • The new species co-occurs with three other Chytridiomycota and Perkinsea parasites. • Parasites compete for dinoflagellate hosts, impacting the development of the bloom. A new chytrid genus and species was isolated and cultured from samples obtained in the Baltic Sea during a dinoflagellate bloom event. This species is characterized by having a spherical sporangium without papillae and zoospores of 2–3 µm in diameter that are released through 3 discharge pores. Molecular phylogeny based on ribosomal operon showed its sister position to the Dinomyces cluster in Rhizophydiales. Zoospores lack fenestrated cisternae but contain a paracrystalline inclusion, found in a Rhizophydiales representative for the first time. Additionally, the kinetid features are uncommon for Rhizophydiales and only observed in Dinomyces representatives so far. These morphological features and its phylogenetic relationships justify the description of the new genus and species Paradinomyces triforaminorum gen. nov. sp. nov. belonging to the family Dinomycetaceae. The chytrid was detected during a high-biomass bloom of the dinoflagellate Kryptoperidinium foliaceum. Laboratory experiments suggest this species is highly specific and demonstrate the impact it can have on HAB development. The chytrid co-occurred with three other parasites belonging to Chytridiomycota (Fungi) and Perkinsea (Alveolata), highlighting that parasitic interactions are common during HABs in brackish and marine systems, and these multiple parasites compete for similar hosts. [ABSTRACT FROM AUTHOR]