Chemical defense against predators and bacterial fouling in the mediterranean sponges axinella polypoides and A. verrucosa

The diverse natural products found within sponges mediate many of their ecological interactions, including predator feeding deterrence and defense against fouling. Pyrrole-imidazole compounds, often involved in such ecological interactions, are exclusively found in marine sponges, mainly from the genera Axinella and Agelas. Employing field and laboratory assays, we studied the chemical defense of 2 co-occurring Mediterranean sponges, Axinella polypoides and A. verrucosa, collected from the Gulf of Naples, Italy, against microbial fouling, as well as their feeding deterrence against the generalist shrimp Palaemon elegans. These assays revealed no activity in the extracts of A. polypoides, while the n-butanol part of the A. verrucosa extract showed activity in all assays at natural concentrations. This fraction of the extract contains the bromopyrrole hymenidin, and our assays revealed antibacterial and feeding deterrence activity for this pure compound. ¹H nuclear magnetic resonance quantification confirmed the presence of hymenidin and debromo-carteramine A, another previously isolated antibacterial compound, at ecologically relevant concentrations in the n-butanol part of the A. verrucosa extract. Our results indicate different antimicrobial and feeding deterrence defense strategies in the 2 examined sympatric sponge species, and multiple defensive roles of hymenidin in the defense of A. verrucosa, similar to those of other bromopyrroles isolated from taxonomically related Caribbean sponges.