Significantly fewer thraustochytrid protists (zoosporic fungi) were observed in association with healthy leaf
tissue of the marine angiosperm Thalassia testudinum than in association with sterilized samples that were
returned to the collection site for 48 h. In support of the hypothesis that sea grass secondary metabolites were
responsible for these differences, extracts of healthy T. testudinum leaf tissues inhibited the growth of the
co-occurring thraustochytrid Schizochytrium aggregatum and deterred the attachment of S. aggregatum motile
zoospores to an extract-impregnated substrate. By using S. aggregatum for bioassay-guided chemical fractionation,
a new flavone glycoside was isolated and structurally characterized as luteolin 7-O-b-D-glucopyranosyl-
2(-sulfate. Whole-leaf tissue concentrations of this metabolite (4 mg/ml of wet leaf tissue) inhibited S. aggregatum
attachment, and a significantly lower concentration (270 mg/ml) reduced thraustochytrid growth by 50%,
suggesting that natural concentrations are at least 15 times greater than that needed for significant microbiological
effects. These results offer the first complete chemical characterization of a sea grass sulfated flavone
glycoside and provide evidence that a secondary metabolite chemically defends T. testudinum against fouling
microorganisms.