Ecology and molecular phylogenetic analysis of marine algicidal labyrinthulids

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Labyrinthulids are known to live on the surface of marine macroalgae and aquatic plants and to be causative organisms for eelgrass wasting disease. Furthermore, they are considered to intake organic matters from the diatom or bacteria cells which are lysed extracellularly by their lytic activities. In this study, distirbution, growth response, and molecular phylogeny of marine labyrinthulids were investigated.Algicidal microorganisms were isolated by using double layered agar plates containing diatom cells as plaque-forming organisms from marine macroalgae and aquatic plants. As the expanding plaque-forming organisms, amoeba, labyrinthulids, and filamentous bacteria were observed. Labyrinthulid isolates formed plaques on the diatom double layered agar plates and colonies on the double layered agar plates containing heated bacterial cells. It was clear that phospholipids derived from diatom, bacteria, or serum stimulate colony growth of labyrinthulid isolates. Docosahexaenoic acid (DHA) content was very high (60-70%) in cellular lipids of labyrinthulid isolates.From molecular phylogenetic analysis of labyrinthulids L95-1 and L95-2 based on 18s rDNA sequences, they were found to be close to Labyrinthula sp. AN-1565 (87% homology) and belong to labyrinthulid phylogenetic group.