Metabolite variability in Caribbean sponges of the genus Aplysina

Sponges of the genus Aplysina are among the most common benthic animals on reefs of the Caribbean, and display a wide diversity of morphologies and colors. Tissues of these sponges lack mineralized skeletal elements, but contain a dense spongin skeleton and an elaborate series of tyrosine-derived brominated alkaloid metabolites that function as chemical defenses against predatory fishes, but do not deter some molluscs. Among the earliest marine natural products to be isolated and identified, these metabolites remain the subject of intense interest for commercial applications because of their activities in various bioassays. In this study, crude organic extracts from 253 sponges from ten morphotypes among the species Aplysina archeri, Aplysina bathyphila, Aplysina cauliformis, Aplysina fistularis, Aplysina fulva, A. insularis, and Aplysina lacunosa were analyzed by liquid chromatography–mass spectrometry (LC–MS) to characterize the pattern of intra- and interspecific variabilities of the twelve major secondary metabolites present therein. Patterns across Aplysina species ranged from the presence of mostly a single compound, fistularin-3, in A. cauliformis, to a mixture of metabolites present in the other species. These patterns did not support the biotransformation hypothesis for conversion of large molecular weight molecules to smaller ones for the purpose of enhanced defense. Discriminant analyses of the metabolite data revealed strong taxonomic patterns that support a close relationship between A. fistularis, A. fulva and A. insularis, while two morphotypes of A. cauliformis (lilac creeping vs. brown erect) were very distinct. Two morphotypes of A. lacunosa, one with hard tissue consistency, the other soft and thought to belong to a separate genus (Suberea), had very similar chemical profiles. Of the twelve metabolites found among samples, variation in fistularin-3, dideoxyfistularin-3 and hydroxyaerothionin provided the most predictive influence in decreasing order. Except for one morphotype, weak relationships were found from within-morphotype analyses of metabolite concentrations as a function of geographic location (Florida, N Bahamas, S Bahamas) and depth (<10 m, 10–20 m, >20 m). Our data suggest that metabolite profiles are strongly influenced by sponge phenotype rather than by the diverse microbiome which many Aplysina species share.