EVIDENCE THAT A NEW ANTIBIOTIC FLAVONE GLYCOSIDE CHEMICALLY DEFENDS THE SEA GRASS THALASSIA-TESTUDINUM AGAINST ZOOSPORIC FUNGI

Significantly fewer thraustochytrid protists (zoosporic fungi) were ob served in association with healthy leaf tissue of the marine angiosper m Thalassia testudinum than in association with sterilized samples tha t were returned to the collection site fur 48 h. In support of the hyp othesis that sea grass secondary metabolites were responsible for thes e differences, extracts of healthy T. testudinum leaf tissues inhibite d the growth of the co-occurring thraustochytrid Schizochytrium aggreg atum 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 an d structurally characterized as luteolin 7-O-beta-D-glucopyranosyl-2 ' '-sulfate. Whole-leaf tissue concentrations of this metabolite (4 mg/m l of wet leaf tissue) inhibited S. aggregatum attachment, and a signif icantly lower concentration (270 mu g/ml) reduced thraustochytrid grow th by 50%, suggesting that natural concentration are at least 15 times greater than that needed for significant microbiological effects. The se results offer the first complete chemical characterization of a sea grass sulfated flavone glycoside and provide evidence that a secondar y metabolite chemically defends T. testudinum against fouling microorg anisms.