First Report of Labyrinthula zosterae (Labyrinthulomycetes) as the Causal Pathogen of Wasting Disease in the Seagrass Zostera marina in Korea (2021)

Zostera marina L. plants have been seriously impacted by wasting disease along the Atlantic coasts of North America and Europe since the 1930s (Muehlstein 1989). Sudden declines in the population sizes of Zostera marina affect primary and secondary producers of different trophic levels in blue carbon ecosystems (Gleason et al. 2013). Muehlstein et al. (1991) first identified Labyrinthula zosterae (Labyrinthulomycetes) as the pathogen causing wasting disease in Zostera marina. However, there have been no reports of wasting disease pathogens affecting seagrass in Korea. In this study, we collected leaves of Z. marina showing symptoms of wasting disease in the southern region of South Korea (Dongdaeman, Namhae, Gyeongnam Province) during field monitoring (from April to September 2013). The pathogens of wasting disease, Labyrinthula zosterae has been isolated from the infected leaves of Z. marina and established as a culture strain (Supplementary Figure 1). Samples of Z. marina and L. zosterae were deposited at the Fisheries Seed and Breeding Research Institute (previous Seaweed Research Center, National Institute of Fisheries Science, South Korea). Microscopic examination of the infected leaf tissues revealed fusiform or spindle-shaped vegetative Labyrinthula cells (4-5 × 15-20 μm). These were similar in size and shape to those previously described for Labyrinthula species. The fusiform cells were cultured in 1% serum seawater agar medium, and they formed colonies and showed gliding motility along a network of hyaline slime filaments. To validate the pathogenicity, re-inoculation tests by L. zosterae were performed with the isolated strains in accordance with Koch's postulates. Healthy leaves of Z. marina collected from the field were used in the re-inoculation tests and were cultured at 15°C under white fluorescent irradiation of approximately 20 μmol·photons·m-2·s-1 and a 12:12-h light:dark cycle (Supplementary Figure 1). Labyrinthula zosterae re-isolated from artificially infected leaves of Z. marina was confirmed by DNA sequence similarity analysis. Total genomic DNA from the infected leaf cells and the culture strains was extracted using the QIAamp DNA Stool Mini Kit (Qiagen, Germany). Internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA were determined to identify Labyrinthula species. L. zosterae-specific primers (Lz2forward (5'- CTAAGACTAAACGAGGCGAAAGCCTAC-3') and Lz2reverse (5'-AGGTTTACAAAACACACTCGTCCACA-3') in Bergmann et al. (2011)) were used to confirm the infection of L. zosterae in the leaves from the field samples and the re-inoculation test samples. Next, PCR products were cloned using a pLUG-Prime® TA-cloning Vector (iNtRON Biotechnology, Korea) and commercially sequenced (SolGent, Korea). The ITS sequence of Korean L. zosterae (accession number MW357748) showed high sequence similarity (99.3-100%) with that of L. zosterae deposited in GenBank (National Center for Biotechnology Information) from BLAST searches. These findings confirm that this is the first report of L. zosterae as the causal pathogen of wasting disease in Z. marina in Korea.