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

• Seagrass phenolic acids were found to inhibit Labyrinthula growth in vitro.
• Phenolics reduced Labyrinthula growth at levels ≥2.5× lower than found in planta.
• Combinations of phenolic acids synergistically inhibited Labyrinthula.
• Chemical extracts from Thalassia testudinum possessed anti labyrinthulid activity.

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.