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A Widely Distributed Thraustochytrid Parasite of Diatoms Isolated from the Arctic Represents a Gen. and Sp. Nov Journal of Eukaryotic Microbiology ISSN 1066-5234 ORIGINAL ARTICLE A Widely Distributed Thraustochytrid Parasite of Diatoms Isolated from the Arctic Represents a gen. and sp. nov. Brandon T. Hassett UiT Norges arktiske universtiet, BFE, NFH bygget, Framstredet 6, 9019, Tromsø, Norway Keywords ABSTRACT Labyrinthulea; Labyrinthulomycetes; Thraus- tochytriidae. A unicellular, heterotrophic, eukaryotic parasite was isolated from nearshore Arctic marine sediment in association with the diatom Pleurosigma sp. The Correspondence parasite possessed ectoplasmic threads that could penetrate diatom frustules. B.T. Hassett, UiT Norges arktiske univer- Healthy and reproducing Pleurosigma cultures would begin to collapse within a stiet, BFE, NFH bygget, Framstredet 6, week following the introduction of this parasite. The parasite (2–10 lm diame- 9019 Tromsø, Norway ter) could reproduce epibiotically with biflagellate zoospores, as well as binary Telephone number: +4777645711; FAX division inside and outside the diatom host. While the parasite grew, diatom number: +19077824461; e-mail: bran- intracellular content disappeared. Evaluation of electron micrographs from co- [email protected] cultures revealed the presence of hollow tubular processes and amorphic cells that could transcend the diatom frustule, generally at the girdle band, as well Received: 18 September 2019; revised 23 as typical thraustochytrid ultrastructure, such as the presence of bothrosomes. March 2020; accepted March 25, 2020. After nucleotide extraction, amplification, and cloning, database queries of Early View publication April 29, 2020 DNA revealed closest molecular affinity to environmental thraustochytrid clone sequences. Testing of phylogenetic hypotheses consistently grouped this doi:10.1111/jeu.12796 unknown parasite within the Thraustochytriidae on a distinct branch within the environmental sequence clade Lab19. Reclassification of Arctic high-through- put sequencing data, with appended reference datasets that included this dia- tom parasite, indicated that the majority of thraustochytrid sequences, previously binned as unclassifiable stramenopiles, are allied to this new isolate. Based on the combined information acquired from electron microscopy, life history, and phylogenetic testing, this unknown isolate is described as a novel species and genus. THE Labyrinthulea are heterotrophic eukaryotic stra- the Thraustochytrida that circumscribes numerous genera: menopiles historically considered morphologically distinct Aurantiochytrium, Botryochytrium, Hondaea, Labyrinthu- by the presence of an ectoplasmic net (Tsui et al. 2009) lochytrium, Monorhizochytrium, Parietichytrium, Schizo- that is used to interface nutrient acquisition and motility chytrium, Sicyoidochytrium, Thraustochytrium, and Ulkenia (Bennett et al. 2017).The Labyrinthulea are generally (Bahnweg and Sparrow 1974; Dellero et al. 2018; Doi and divided into three morphologically distinct groups, based Honda 2017; Hassett and Gradinger 2018; Yokoyama and on the utility and function of the ectoplasmic net: the Honda 2007; Yokoyama et al. 2007), with additional gen- labyrinthulids, thraustochytrids, and the aplanochytrids era not yet represented in sequence databases (Pan et al. (Leander et al. 2004). The most recent molecular phyloge- 2017). nies suggest that Labyrinthulea is comprised of at least Labyrinthulea have been cultured and detected by four orders: Amphitremida, Labyrinthulida, Oblongichytrida, molecular methods throughout the global marine realm and Thraustochytrida (Pan et al. 2017), with the possibility (Bai et al. 2019; Bochdansky et al. 2017; Pan et al. 2017). of several additional orders that remain phylogenetically Ecologically, Labyrinthulea behave as bacteriovores, sapro- unresolved (Bennett et al. 2017). Within Thraustochytrida, trophs, and symbionts in marine ecosystems (Bennett there are at least two families: the Thraustochytriidae and et al. 2017) and are implicated as important degraders of Althorniidae (containing one genus, Althornia). Thraus- coastal detritus (Raghukumar 2002; Ueda et al. 2015) and tochytriidae is the most diverse taxonomic family within marine snow (Bochdansky et al. 2017). Members of © 2020 The Authors. Journal of Eukaryotic Microbiology published by Wiley Periodicals LLC on behalf of International Society of Protistologists 480 Journal of Eukaryotic Microbiology 2020, 67, 480–490 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Hassett Thraustochytrid Diatom Parasite Labyrinthulea can associate with photosynthetic organ- Cocultures were transferred to PmTG (peptonized milk, isms as substrate for development (Scholz et al. 2016). tryptone, and glucose) agar media, as well as f/2 nutrient However, parasitism of photosynthetic organisms by this media, and explored for the presence of diatom-indepen- group is atypically reported. Consequently, the ecological dent growth. In addition, a supplemental growth analysis role of Labyrinthulea as algal parasites is nebulous. Ambi- was conducted using various media (Daniel Powers, guity surrounding the parasitic nature of Labyrinthulea is unpublished thesis): GPY, Honda, KMV, and MC (Rosa heightened by observations of these organisms at the tips et al. 2011). of filamentous algae (Raghukumar 2006) and on senescent and moribund diatoms (Raghukumar 1986), suggesting DNA extraction, amplification, and sequencing these organisms are opportunists, as opposed to patho- gens that have co-evolved to parasitize. Select Labyrinthu- Biomass from liquid cocultures was concentrated on 0.2 lea members within the aplanochytrids and Labyrinthula lm filters (Sartorious, Gottingen,€ Germany). Genomic DNA can consume (Popova et al. 2020) and parasitize diatoms was extracted from filters using the DNeasy PowerSoil Kit with their ectoplasmic nets (Hamamoto and Honda 2019); (Qiagen, Hilden, Germany) and subsequently amplified however, few observations (Gaertner 1979) have ever using polymerase chain reaction (PCR) with Platinum Taq reported thraustochytrids parasitizing healthy diatom cells. (Thermo Fisher Scientific, Waltham, MA) and the 18S As a result, the relevance of Labyrinthulea algal parasites rRNA primer sets NS1/NS4 (50-GTAGTCATATGCTTGTCTC- to marine ecosystems remains unknown. 30/50-CTTCCGTCAATTCCTTTAAG-30) and NS5/NS8 (50-AA A coculture of a pennate diatom within the genus Pleu- CTTAAAGGAATTGACGGAAG-30/50-TCCGCAGGTTCACCTA rosigma was established with an unknown parasite that CGGA-30) (White et al. 1990). Amplicons were purified was isolated from Arctic marine sediment in Tromsø, Nor- with the PureLink Quick PCR Purification Kit (Thermo way. Analysis of this parasite revealed closest molecular Fisher Scientific). Purified amplicons were gel-eluted and affinity to environmental clone sequences generated in blunt-end cloned into the EcoRV restriction site of New York, USA, and the recently described Norwegian pUC57-Kan. Sanger sequencing was conducted using the isolate, Labyrinthulochytrium arktikum. Microscopic exami- M13R and M13F primers on a 3730xl DNA Analyzer nation of this isolate revealed a unique life history, primar- (Applied Biosystems, Foster City, CA). Sequencing was ily the ability to parasitize diatoms by penetrating their conducted bidirectionally to maximize confidence in base frustules with ectoplasmic threads. Phylogenetic analyses calls along the length of the amplicon. Chromatographs using deoxyribonucleic acid (DNA)-encoding regions of the were examined with MEGA v7.0.26 (Kumar et al. 2016) 18S small ribosomal subunit (18S rRNA) locus place this to assess the accuracy of automated base calls and to unknown isolate among the diverse thraustochytrids in explore the presence of single nucleotide polymorphisms the family Thraustochytriida. Based on phylogenetic test- (SNPs). The final sequence (NCBI accession MN382127) ing, contextualized with reference trees (Pan et al. 2017), was used for phylogenetic analysis. ultrastructure, and life history, this unknown isolate is described as a new genus, Phycophthorum (gen. nov.) Microscopy and species Phycophthorum isakeiti (sp. nov.). Transmission electron microscopy Cocultures were grown in fresh f/2 media for 2 wk and MATERIALS AND METHODS fixed at 4 °C with 0.075% ruthenium red, 4% formalde- hyde, and 2.5% glutaraldehyde in f/2 for 2 h. This fixation Isolation and culture cocktail was then exchanged for 4% formaldehyde and A surface sediment sample was collected from the near- 2.5% glutaraldehyde in PHEM buffer (PIPES, HEPES, shore marine environment outside of Tromsø, Norway EGTA, magnesium sulphate heptahydrate) and left over- (N69.632 E18.906), on January 21, 2019, at the end of the night at 4 °C. The samples were rinsed with PHEM, post- polar night. Sediment was aliquoted into Petri dishes and fixed with 1% OsO4 in ddH2O for 2 h, rinsed again, and diluted with unfiltered natural seawater. Diluted sediment dehydrated in a graded series of ethanol (30%, 60%, was explored for the presence of parasitized Pleurosigma 90%, 96%, and 100%), before a final embedding in epoxy sp. diatoms with an inverted microscope (Leica DM IL, resin (EPON). Sections were contrasted with uranyl acet- Wetzlar, Germany). Diatoms identified as Pleurosigma sp. ate and lead citrate. Imaging was conducted with a JEM- that were hosting parasites were harvested from sedi- 1010 electron microscope (JEOL, Tokyo, Japan). ment with a pipette, re-suspended in sterile
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