Novel uncultivated labyrinthulomycetes revealed by 18S rDNA sequences from seawater and sediment samples

Labyrinthulomycetes (Labyrinthulea) are ubiquitous marine osmoheterotrophic protists that appear to be important in decomposition of both allochthonous and autochthonous organic matter. We used a cultivation-independent method based on the labyrinthulomycete-specific primer LABY-Y to PCR amplify, clone, and sequence 68 nearly full-length 18S rDNA amplicons from 4 sediment and 3 seawater samples collected in estuarine habitats around Long Island, New York, USA. Phylogenetic analyses revealed that all 68 amplicons belonged to the Labyrinthulea. Only 15 of the 68 amplicons belonged to the thraustochytrid phylogenetic group (Thraustochytriidae). None of these 15 were similar to cultivated strains, and 11 formed a novel group. The remaining 53 amplicons belonged either to the labyrinthulid phylogenetic group (Labyrinthulidae) or to other families of Labyrinthulea. that have not yet been described. Of these amplicons, 37 were closely related to previously cultivated Aplanochytrium spp. and Oblongichytrium spp. Members of these 2 genera were also cultivated from 1 of the sediment samples. The 16 other amplicons were not closely related to cultivated strains, and 15 belonged to 5 groups of apparently novel labyrinthulomycetes. Most of the novel groups of amplicons also contained environmental sequences from surveys of protist diversity using universal 18S rDNA primers. Because the primer LABY-Y is biased against several groups of labyrinthulomycetes, particularly among the thraustochytrids, these results may underestimate the undiscovered diversity of labyrinthulomycetes.

Metabolites derived from the tropical seagrass Thalassia testudinum are bioactive against pathogenic Labyrinthula sp

Temperate and tropical seagrasses are susceptible to wasting disease outbreaks caused by pathogenic protists of the genus Labyrinthula. Even though there is an increasing awareness of the environmental conditions that influence the etiology of seagrass-. Labyrinthula disease dynamics, the biochemical basis of seagrass defense responses, in particular chemical defenses, is still vastly understudied. Using an in vitro bioassay, we provide evidence that previously characterized phenolic and potentially novel, undescribed non-phenolic metabolites derived from Thalassia testudinum Banks ex Konig exhibit anti-labyrinthulid activity. All phenolic compounds tested displayed dose-dependent behavior and selected combinations interacted synergistically. The flavone glycoside thalassiolin B was roughly 20-100 times more active than any phenolic acid tested. Based upon values reported in the literature, it was calculated that infected specimens of T. testudinum contain natural concentrations of phenolic acids that are consistently greater than what is required to inhibit Labyrinthula growth. This suggests that while there may be an ample supply of phenolic-based derivatives available to inhibit Labyrinthula growth, they may not be readily bio-accessible.Using a bioactivity-guided approach, a semi-purified chemical fraction from T. testudinum was found to contain anti-labyrinthulid activity. 1H NMR spectra for this fraction lacked aromatic hydrogen signals, suggesting that the bioactive compound was non-aromatic in nature. Furthermore, the LC-MS fragmentation patterns were suggestive of the presence of glycosylated natural products of an unknown structural class. This has the potential to provide a foundation for future chemical investigations.