Defense metabolites from the marine sponge Verongia aerophoba

Abstract

The marine sponge Verongia aerophoba (syn. Aplysina aerophoba) accumulates isofistularin-3 and aerophobin-2 as major brominated isoxazoline alkaloids. Following disrupture of the compartmentation (e.g. by wounding) both isofistularin-3 and aerophobin-2 are enzymatically converted into aeroplysinin-1 which in turn gives rise to a dienone. Aeroplysinin-1 and dienone were shown to exhibit pronounced biological activities in various bioassays with marine organisms (bacteria, algae and molluscs) whereas their biogenetic precursors isofistularin-3 and aerophobin-2 were either inactive or exhibited only marginal activity. In the agar plate diffusion assay, aeroplysinin-1 and dienone were antibiotically active against eight different Gram-positive or Gram-negative marine bacteria including Alteromonas, Moraxella and Vibrio spp. Towards the marine Photobacterium phosphoreum the EC50S of aeroplysinin-1 and dienone were 3.45 and 1.37 μM, respectively. Both compounds inhibited also the growth of the marine microalgae Coscinodiscus wailesii and Prorocentrum minimum. Towards the former, the EC50s of aeroplysinin-1 and dienone were 5.6 and 27.9 μM, respectively. In addition to their growth inhibitory activity, aeroplysinin-1 and dienone were algicidal as evident by their damaging effects on the algal cellular membranes. The polyphagous marine gastropod Littorina littorea was repelled when exposed to either aeroplysinin-1 or dienone that were added to seawater. The EC50 of the most active compound aeroplysinin-1 was observed at 0.1 mM. It is suggested that the enzymatically catalysed conversion of brominated metabolites in V. aerophoba represents a wound-induced defense mechanism hitherto unreported from the marine environment. Copyright © 1996 Elsevier Science Ltd.