Sirolpidium bryopsidis, a parasite of green algae, is probably conspecific with Pontisma lagenidioides, a parasite of red algae

The genus Sirolpidium (Sirolpidiaceae) of the Oomycota includes several species of holocarpic obligate aquatic parasites. These organisms are widely occurring in marine and freshwater habitats, mostly infecting filamentous green algae. Presently, all species are only known from their morphology and descriptive life cycle traits. None of the seven species classified in Sirolpidium, including the type species, S. bryopsidis, has been rediscovered and studied for their molecular phylogeny, so far. Originally, the genus was established to accommodate all parasites of filamentous marine green algae. In the past few decades, however, Sirolpidium has undergone multiple taxonomic revisions and several species parasitic in other host groups were added to the genus. While the phylogeny of the marine rhodophyte-and phaeophyteinfecting genera Pontisma and Eurychasma, respectively, has only been resolved recently, the taxonomic placement of the chlorophyte-infecting genus Sirolpidium remained unresolved. In the present study, we report the phylogenetic placement of Sirolpidium bryopsidis infecting the filamentous marine green algae Capsosiphon fulvescens sampled from Skagaströnd in Northwest Iceland. Phylogenetic reconstructions revealed that S. bryopsidis is either conspecific or at least very closely related to the type species of Pontisma, Po. lagenidioides. Consequently, the type species of genus Sirolpidium, S. bryopsidis, is reclassified to Pontisma. Further infection trials are needed to determine if Po. bryopsidis and Po. lagenidioides are conspecific or closely related. In either case, the apparently recent host jump from red to green algae is remarkable, as it opens the possibility for radiation in a largely divergent eukaryotic lineage.