Wars in the sea: chemical weapons from microalgae, macroalgae and seagrasses

The competition in marine environment is extreme. Unicellular algae, like diatoms, are continuously eaten by microscopic grazers, the copepods. To secure their survival, the diatoms have elaborated an effective defence based on production of several different metabolites sharing common origin from the lipoxygenase-dependent oxidation of polyunsaturated fatty acids. Analogously, some green algae are chemically protected against many predators by chemicals. Nevertheless, this barrier is insufficient to prevent grazing of saglossan molluscs, which have also acquired the ability to use the algal metabolites for their own defence. Other molluscs of the same order avoid to sequester defensive compounds but, directly, biosynthesize protective polyketides structurally related to typical fungal metabolites. Finally, there is a stimulating international debate on the factors promoting marine invasions. In the Mediterranean Sea the seagrass Halophila stipulacea is worthy of particular attention. The plant is considered an invasive species that is quickly replacing endemic Mediterranean species. The ecological success of H. stipulacea is suggested to involve the metabolite patrimony which, however, is not enough to prevent the attacks of the anaspidean mollusc Siphonota geographica. In this contribution we will give a summary of our results concerning chemical characterization, origin and ecological role of the secondary metabolites involved in the interactions described above.