No abstract

Zoosporic fungi play an important role in aquatic environments, but their diversity, especially that of parasitic fungi of phytoplankton, has still not been fully revealed. We conducted monthly analyses of the community structure of zoosporic fungi at a pelagic site in Lake Biwa, Japan, from May to December 2016. Metabarcoding analysis, targeted to a large subunit region of ribosomal DNA in the nano-size fraction of particles (2-20 µm), was carried out on the samples. We also counted large phytoplankton and chytrid sporangia attached to the hosts. We detected 3 zoosporic fungal phyla (Blastocladiomycota, Chytridiomycota and Cryptomycota) within 107 operational taxonomic units (OTUs), in which Chytridiomycota was the most diverse and abundant phylum. Few fungal OTUs overlapped between months, and specific communities were detected in each month. These results showed that diverse zoosporic fungi with high temporal variability inhabited the lake. Five large phytoplankton species were found to be infected by chytrids: Staurastrum dorsidentiferum, S. rotula, Closterium aciculare, Asterionella formosa and Aulacoseira granulata. Some chytrids were detected by metabarcoding analysis: Zygophlyctis asterionellae infecting A. formosa, Staurastromyces oculus infecting S. dorsidentiferum and Pendulichytrium sphaericum infecting A. granulata. One OTU detected in association with infected C. aciculare by microscopic counting might have been an obligate parasitic chytrid of C. aciculare. The results indicated that a combination of metabarcoding and microscopic analysis revealed more information on zoosporic fungi, including those that are parasitic.

Scolionema suvaense is a medusa belonging to the order Limnomedusae, and commonly lives on seaweed and seagrass. The benthic polyp and planktonic medusa are both stages of the life cycle of S. suvaense. Most of the planulae produced by sexual reproduction by the medusae do not metamorphose into polyps but to motile frustules moving to the bottom, which then metamorphose into polyps. The polyps can asexually produce not only medusae but also motile frustules. At a low temperature (15°C), polyps mostly asexually produce frustules. Extension of the habitat area by motile frustules is suggested to occur during winter and early spring when the temperatures are low. With increasing temperature, the reproduction type of S. suvaense polyps changes to asexual production of medusae. In the medusa stage, S. suvaense can reproduce both sexually and asexually. The relative percentages of asexually-reproducing medusae vs. sexually-reproducing medusae were highest at 20°C. This coincides with the highest abundance recorded for S. suvaense medusae, which showed an abrupt increase between May and June. Moreover, with increasing temperature (over 25°C), the reproductive mode of S. suvaense medusae changes from asexual to sexual, and asexually reproducing medusae are not observed at such high temperatures. Changes in the reproduction type and fluctuations in the subsequent population dynamics of S. suvaense depend on the in situ temperatures.