New record and phylogenetic affinities of the oomycete Olpidiopsis feldmanni infecting Asparagopsis sp. (Rhodophyta)

Morphology, phylogeny, and host range of the novel early-diverging oomycete Sirolpidium dinoletiferum sp. nov. parasitizing marine dinoflagellates

Oomycetes are fungus-like heterotrophic organisms with a broad environmental distribution, including marine, freshwater, and terrestrial habitats. They function as saprotrophs that use the remains of other organisms or as parasites of a variety of eukaryotes, including protists, diatoms, dinoflagellates, macroalgae, plants, fungi, animals, and even other oomycetes. Among the protist hosts, the taxonomy, morphology, and phylogenetic positions of the oomycete parasitoids of diatoms have been well studied; however, this information concerning the oomycete parasitoids of dinoflagellates is poorly understood. During intensive sampling along the east and west coasts of Korea in May and October 2019, a new species of oomycetes was discovered and two strains of the new parasitoid were successfully established in cultures. The new oomycete parasitoid penetrated the dinoflagellate host cell and developed to form a sporangium, which was very similar to the perkinsozoan parasitoids that infect marine dinoflagellates. The most distinctive morphological feature of the new parasitoid was a central large vacuole forming several long discharge tubes. The molecular phylogenetic tree inferred based on the small subunit (SSU) ribosomal DNA (rDNA) revealed that the new parasitoid forms a distinct branch unrelated to other described species belonging to early-diverging oomycetes. It clustered with species belonging to the genus Sirolpidium with strong support values in the cytochrome c oxidase subunit 2 (cox2) tree. Cross-infection experiments showed that infections by the new parasitoid occurred in only six genera belonging to dinoflagellates among the protists tested in this study. Based on the morphological and molecular data obtained in this study, we propose to introduce a new species, Sirolpidium dinoletiferum sp. nov., for this novel parasitoid, conservatively within the genus Sirolpidium.

Pontisma blauvikense sp. nov. the first member of the early-diverging oomycete genus Pontisma parasitizing brown algae

Holocarpic oomycetes have been neglected over several decades, until interest in these organisms has recently resurged. One of the most widespread genera of holocarpic oomycetes is Pontisma, parasitic to red seaweeds throughout all oceans. Recently, the genus Sirolpidium (parasitic to green algae) was found to be congeneric with Pontisma. This hinted at a high pathogenic versatility and prompted the screening of other macroalgae on the coastline of Iceland. During this survey a parasite of the brown algae Pylaiella littoralis was found, which formed anisolpidium-like thalli, but produced biflagellate zoospores. Phylogenetic investigations revealed that the parasite was placed in the genus Pontisma. In reconstructions based on partial nrSSU sequences, it grouped with some sequences of parasitoids of the diatom genus Licmophora, but the more variable mitochondrial cox2 sequences were divergent. Based on phylogenetic evidence and the unique parasitism of brown algae, the parasitoid is described as Pontisma blauvikense in this study. Pontisma blauvikense is the fourth oomycete species parasitic to Pylaiella, which is also parasitised by Euychasma dicksonii and two Anisolpidium species. For a better understanding of the ecology and evolution of holocarpic oomycetes, further research is necessary to investigate the host spectrum of Pontisma in general and Pontisma blauvikense in particular.

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 phaeophyte-infecting 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.

Citation: Buaya AT, Scholz B, Thines M (2021). Sirolpidium bryopsidis, a parasite of green algae, is probably conspecific with Pontisma lagenidioides, a parasite of red algae. Fungal Systematics and Evolution7: 223–231. doi: 10.3114/fuse.2021.07.11

Microeukaryotic Communities Associated With the Seagrass Zostera marina Are Spatially Structured

Epibiotic microorganisms link seagrass productivity to higher trophic levels, but little is known about the processes structuring these communities, and which taxa consistently associate with seagrass. We investigated epibiotic microeukaryotes on seagrass (Zostera marina) leaves, substrates, and planktonic microeukaryotes in ten meadows in the Northeast Pacific. Seagrass epibiotic communities are distinct from planktonic and substrate communities. We found sixteen core microeukaryotes, including dinoflagellates, diatoms, and saprotrophic stramenopiles. Some likely use seagrass leaves as a substrate, others for grazing, or they may be saprotrophic organisms involved in seagrass decomposition or parasites; their relatives have been previously reported from marine sediments and in association with other hosts such as seaweeds. Core microeukaryotes were spatially structured, and none were ubiquitous across meadows. Seagrass epibiota were more spatially structured than planktonic communities, mostly due to spatial distance and changes in abiotic conditions across space. Seawater communities were relatively more similar in composition across sites and more influenced by the environmental component, but more variable over time. Core and transient taxa were both mostly structured by spatial distance and the abiotic environment, with little effect of host attributes, further indicating that those core taxa would not show a strong specific association with Z. marina.

Bolbea parasitica gen. et sp. nov., a cultivable holocarpic parasitoid of the early-diverging Saprolegniomycetes

Holocarpic oomycetes convert their entire cytoplasm into zoospores and thus do not form dedicated sporangia or hyphal compartments for asexual reproduction. The majority of holocarpic oomycetes are obligate parasites and parasitoids of a diverse suite of organisms, among them green and red algae, brown seaweeds, diatoms, fungi, oomycetes and invertebrates. Most of them are found among the early diverging oomycetes or the Peronosporomycetes, and some in the early-diverging Saprolegniomycetes (Leptomitales). The obligate parasitism renders it difficult to study some of these organisms. Only a few members of the genus Haliphthoross. l. have been cultured without their hosts, and of the parasitoid Leptomitales, some transient cultures have been established, which are difficult to maintain. Here, the cultivation of a new holocarpic oomycete genus of the Leptomitales, Bolbea, is presented. Bolbea is parasitic to ostracods, is readily cultivable on malt extract agar, and upon contact with water converts its cytoplasm into zoospores. Its morphology and phylogenetic relationships are reported. Due to the ease of cultivation and the ready triggering of zoospore development, similar to some lagenidiaceous oomycetes, the species could be a promising model to study sporulation processes in detail.

Holocarpic oomycete parasitoids of red algae are not Olpidiopsis

Olpidiopsis is a genus of obligate holocarpic endobiotic oomycetes. Most of the species classified in the genus are known only from their morphology and life cycle, and a few have been examined for their ultrastructure or molecular phylogeny. However, the taxonomic placement of all sequenced species is provisional, as no sequence data are available for the type species, O. saprolegniae, to consolidate the taxonomy of species currently placed in the genus. Thus, efforts were undertaken to isolate O. saprolegniae from its type host, Saprolegnia parasitica and to infer its phylogenetic placement based on 18S rDNA sequences. As most species of Olpidiopsis for which sequence data are available are from rhodophyte hosts, we have also isolated the type species of the rhodophyte-parasitic genus Pontisma, P. lagenidioides and obtained partial 18S rDNA sequences. Phylogenetic reconstructions in the current study revealed that O. saprolegniae from Saprolegnia parasitica forms a monophyletic group with a morphologically similar isolate from S. ferax, and a morphologically and phylogenetically more divergent species from S. terrestris. However, they were widely separated from a monophyletic, yet unsupported clade containing P. lagenidioides and red algal parasites previously classified in Olpidiopsis. Consequently, all holocarpic parasites in red algae should be considered to be members of the genus Pontisma as previously suggested by some researchers. In addition, a new species of Olpidiopsis, O. parthenogenetica is introduced to accommodate the pathogen of S. terrestris.

O gênero Olpidiopsis (Oomycota) no Nordeste do Brasil

Resumo Estudos sobre a diversidade de oomicetos foram realizados de 1998 a 2016, na Bahia, Maranhão e Piauí, e parasitas biotróficos holocárpicos zoospóricos de quitrídias, glomeromicetos e oomicetos foram identificados como Olpidiopsis achlyae, O. aphanomycis, O. fusiformis, O. karlingae, Olpidiopsis sp1 e Olpidiopsis sp2. A ocorrência de O. karlingae é novo relato para a América do Sul e de O. aphanomycis, para o Brasil. Os táxons encontrados foram descritos, ilustrados e comentados. A diversidade de termos da literatura especializada para identificação das estruturas morfológicas de Olpidiopsis foi padronizada. São propostas as sinonímias de O. braziliensis (= Pseudolpidium achlyae) com O. fusiformis e de Cornumyces karlingae com O. karlingae.