Pythium polare, a new heterothallic oomycete causing brown discolouration of Sanionia uncinata in the Arctic and Antarctic

Macroalgal eukaryotic microbiome composition indicates novel phylogenetic diversity and broad host spectrum of oomycete pathogens

Seaweeds are important components of marine ecosystems with emerging potential in aquaculture and as sources of biofuel, food products and pharmacological compounds. However, an increasingly recognised threat to natural and industrial seaweed populations is infection with parasitic single-celled eukaryotes from the relatively understudied oomycete lineage. Here we examine the eukaryomes of diverse brown, red and green marine macroalgae collected from polar (Baffin Island), cold-temperate (Falkland Islands) and tropical (Ascension Island) locations, with a focus on oomycete and closely related diatom taxa. Using 18S rRNA gene amplicon sequencing, we show unexpected genetic and taxonomic diversity of the eukaryomes, a strong broad-brush association between eukaryome composition and geographic location, and some evidence of association between eukaryome structure and macroalgal phylogenetic relationships (phylosymbiosis). However, the oomycete fraction of the eukaryome showed disparate patterns of diversity and structure, highlighting much weaker association with geography and no evidence of phylosymbiosis. We present several novel haplotypes of the most common oomycete Eurychasma dicksonii and report for the first time a cosmopolitan distribution and absence of host specificity of this important pathogen. This indicates rich diversity in macroalgal oomycete pathogens and highlights that these pathogens may be generalist and highly adaptable to diverse environmental conditions.

Potential origins of fish toti-like viruses in invertebrates

The virus family Totiviridae had originally been considered to include only viruses which infected fungal and protist hosts, but since 2006 a growing number of viruses found in invertebrates and fish have been shown to cluster phylogenetically within this family. These Totiviridae-like, or toti-like, viruses do not appear to belong within any existing genera of Totiviridae, and whilst a number of new genus names have been suggested, none has yet been universally accepted. Within this growing number of toti-like viruses from animal hosts, there exists emerging viral threats particularly to aquaculture, namely Infectious myonecrosis virus in whiteleg shrimp and Piscine myocarditis virus (PMCV) in Atlantic salmon (Salmo salar). PMCV in particular continues to be an issue in salmon aquaculture as a number of questions remain unanswered about how the virus is transmitted and the route of entry into host fish. Using a phylogenetic approach, this study shows how PMCV and the other fish toti-like viruses probably have deeper origins in an arthropod host. Based on this, it is hypothesized that sea lice could be acting as a vector for PMCV, as seen with other RNA viruses in Atlantic salmon aquaculture and in the toti-like Cucurbit yellows-associated virus which is spread by the greenhouse whitefly Trialeurodes vaporariorum.

Whole genome sequencing and phylogenomic analysis show support for the splitting of genus Pythium

The genus Pythium (nom. cons.) sensu lato (s.l.) is composed of many important species of plant pathogens. Early molecular phylogenetic studies suggested paraphyly of Pythium, which led to a formal proposal by Uzuhashi and colleagues in 2010 to split the genus into Pythium sensu stricto (s.s.), Elongisporangium, Globisporangium, Ovatisporangium (= Phytopythium), and Pilasporangium using morphological characters and phylogenies of the mt cytochrome c oxidase subunit 2 (cox2) and D1-D2 domains of nuc 28S rDNA. Although the split was fairly justified by the delineating morphological characters, there were weaknesses in the molecular analyses, which created reluctance in the scientific community to adopt these new genera for the description of new species. In this study, this issue was addressed using phylogenomics. Whole genomes of 109 strains of Pythium and close relatives were sequenced, assembled, and annotated. These data were combined with 10 genomes sequenced in previous studies. Phylogenomic analyses were performed with 148 single-copy genes represented in at least 90% of the taxa in the data set. The results showed support for the division of Pythium s.l. The status of alternative generic names that have been used for species of Pythium in the past (e.g., Artotrogus, Cystosiphon, Eupythium, Nematosporangium, Rheosporangium, Sphaerosporangium) was investigated. Based on our molecular analyses and review of the Pythium generic concepts, we urge the scientific community to adopt the generic names Pythium, Elongisporangium, Globisporangium, and their concepts as proposed by Uzuhashi and colleagues in 2010 in their work going forward. In order to consolidate the taxonomy of these genera, some of the recently described Pythium spp. are transferred to Elongisporangium and Globisporangium.

Pythium intermedium, a species complex consisting of three phylogenetic species found in cool-temperate forest ecosystems

Pythium intermedium plays a vital role in the carbon cycle of cool-temperate forests and is widely distributed in Japan's forest soils. In this study, we performed a phylogenetic analysis of the P. intermedium species complex using DNA sequences from multiple loci. The study included 35 isolates from cool-temperate forest soils, seven known P. intermedium isolates, and six known Pythium attrantheridium isolates. We also performed morphological observations and mating tests. Our results showed that all the isolates formed one large clade but were divided into three subclades. Furthermore, we observed many mating reactions between isolates from different subclades, including between P. attrantheridium and P. intermedium. Therefore, we suggest that P. intermedium, P. attrantheridium, and another phylogenetic species belong to one species complex. This is the first report of a species complex within P. intermedium and will be helpful in understanding the evolution of Pythium species in natural ecosystems.

Fairy ring disease affects epiphytic algal assemblages associated with the moss Sanionia uncinata (Hedw.) Loeske (Bryophyta) on King George Island, Antarctica

Since the nineteenth century, a ring-forming disease attacking Antarctic mosses has been reported. However, to date, only the effects on the mosses themselves are known. In this study, we used DNA metabarcoding to investigate the effects on the moss epiphytic algal community at different stages of disease progression. As the disease progressed, algal species richness decreased, although overall abundance was not significantly affected. Prasiolales appeared unaffected, whereas Ulotrichales were more sensitive. Trebouxiales dominated the advanced disease stage, suggesting a possible benefit from the disease, either through the elimination of competition or creation of new niches. Infection is responsible for moss death, leading to habitat loss for other organisms, but pathogenic effects on algae cannot be ruled out. Our data indicate that the disease not only impacts mosses but also other groups, potentially resulting in loss of Antarctic biodiversity. This study provides the first report of the disease effects on epiphytic algal communities of Antarctic bryophytes.

Identification and Isolation Pattern of Globisporangium spp. from a Sanionia Moss Colony in Ny-Ålesund, Spitsbergen Is., Norway from 2006 to 2018

Globisporangium spp. are soil-inhabiting oomycetes distributed worldwide, including in polar regions. Some species of the genus are known as important plant pathogens. This study aimed to clarify the species construction of Globisporangium spp. and their long-term isolation pattern in Sanionia moss in Ny-Ålesund, Spitsbergen Is., Norway. Globisporangium spp. were isolated at two-year intervals between 2006 and 2018 at a Sanionia moss colony, Ny-Ålesund, Spitsbergen Is., Norway. The isolates were obtained by using three agar media and were identified based on sequences of the rDNA-ITS region and cultural characteristics. Most of the Globisporangium isolates obtained during the survey were identified into six species. All six species were grown at 0 °C on an agar plate and used to infect Sanionia moss at 4 and/or 10 °C under an in vitro inoculation test. The total isolation frequency of Globisporangium gradually decreased throughout the survey period. The isolation frequency varied among the six species, and four of the species that showed a high frequency in 2006 were rarely isolated after 2016. The results suggested that Globisporangium inhabiting Sanionia moss in Ny-Ålesund has a unique composition of species and that most of the species reduced their population over the recent decade.

Ecological succession of fungal and bacterial communities in Antarctic mosses affected by a fairy ring disease

We evaluated fungal and bacterial diversity in an established moss carpet on King George Island, Antarctica, affected by 'fairy ring' disease using metabarcoding. A total of 127 fungal and 706 bacterial taxa were assigned. Ascomycota dominated the fungal assemblages, followed by Basidiomycota, Rozellomycota, Chytridiomycota, Mortierellomycota and Monoblepharomycota. The fungal community displayed high indices of diversity, richness and dominance, which increased from healthy through infected to dead moss samples. A range of fungal taxa were more abundant in dead rather than healthy or fairy ring moss samples. Bacterial diversity and richness were greatest in healthy moss and least within the infected fairy ring. The dominant prokaryotic phyla were Actinobacteriota, Proteobacteria, Bacteroidota and Cyanobacteria. Cyanophyceae sp., whilst consistently dominant, were less abundant in fairy ring samples. Our data confirmed the presence and abundance of a range of plant pathogenic fungi, supporting the hypothesis that the disease is linked with multiple fungal taxa. Further studies are required to characterise the interactions between plant pathogenic fungi and their host Antarctic mosses. Monitoring the dynamics of mutualist, phytopathogenic and decomposer microorganisms associated with moss carpets may provide bioindicators of moss health.

Sunagoke Moss (Racomitrium japonicum) Used for Greening Roofs Is Severely Damaged by Sclerotium delphinii and Protected by a Putative Bacillus amyloliquefaciens Isolate

Mosses are ecologically important plants also used for greening, gardening, and decorative purposes. Knowledge of the microbial flora associated with mosses is expected to be important for control and preservation of global and local environments. However, the moss-associated microbial flora is often poorly known. Moss-associated fungi and bacteria may promote plant growth and pest control, but they may be alternative hosts for pathogens of vascular plants. In this study, the fungus Sclerotinia delphinii was identified for the first time as a pathogen that causes severe damage to Sunagoke moss (Racomitrium japonicum). This moss is used for greening roofs and walls of buildings in urban environments owing to its notable tolerance of environmental stresses. Inoculation with the S. delphinii strain SR1 of the mono- and dicotyledonous seed plants Hordeum vulgare, Brassica rapa var. pekinensis, Lactuca sativa, and Spinacia oleracea, in addition to the liverwort Marchantia polymorpha and the moss Physcomitrella patens, showed that the fungus has a wide host range. Colonization with SR1 progressed more rapidly in non-vascular than in vascular plant species. Studies with P. patens under controlled conditions showed that SR1 secreted a fluid during colonization. Treatment with the secretion induced production of reactive oxygen species in the moss. Endogenous peroxidase partially inhibited SR1 colonization of P. patens. A bacterial isolate, most likely Bacillus amyloliquefaciens, that coexists with R. japonicum was antagonistic to SR1 growth. Taken together, the present results suggest that fungal colonization of mosses may be prevented by a peroxidase secreted by the moss and an antagonistic bacterium coexisting in the moss habitat. The findings suggest that there is potential to apply biological control measures for protection of mosses against fungal pathogens.

Typhula cf. subvariabilis, new snow mould in Antarctica

We collected snow blight of moss, Polytrichum juniperinum on King George Island, maritime Antarctica. Host died in a circle of about 10-30 cm after snow melts. Clamp connected hyphae and no sclerotia were observed on tip of host leaves. DNA sequence of ITS region from moss symptoms were perfectly matched with fruit bodies of Typhula sp. on Macquarie Island in the maritime Antarctica and high homology with Typhula cf. subvariabilis from Iran. Therefore, we suggested that T. cf. subvariabilis caused snow blight on moss in Antarctica, and this is first record of Typhula snow blight in Southern Hemisphere. These results also suggested that fungi in same genera gained similar ecological niche in both Polar Regions.

Pythium rishiriense sp. nov. from water and P. alternatum sp. nov. from soil, two new species from Japan

In an investigation of Pythium species in natural ecosystems of Rishiri Island in Northern Japan, two new species, Pythium rishiriense and P. alternatum, were identified based on morphological and molecular analyses. Pythium rishiriense differed morphologically from other Pythium species by its characteristic oogonial formation which occasionally arranged in chains. Pythium alternatum differed morphologically from other Pythium species by its distinguishing sexual organs where oogonia occasionally arranged alternately with antheridia in chains. Pythium rishiriense is a fast growing, high-temperature loving species, while P. alternatum is a slow growing species. Phylogenetic analyses based on the internal transcribed spacer region and cytochrome c oxidase 1 gene sequences showed that these two species are clearly separate from morphologically similar species.