Functional metabolites and macroalgal-herbivore interactions
Over 2400 secondary metabolites have been described from marine red, brown and green algae, the majority of which are produced by tropical algae. Although these compounds generally occur in low concentrations, some compounds such as the polyphenolics in brown algae can occur at concentrations as high as 15% of algal dry mass. The majority of macroalgal compounds are terpenoids, especially sesqui- and diterpenoids, acetogenins (acetate-derived compounds), amino acid derivates, and polyphenols. Apparent differences in the secondary chemistry of seaweeds and terrestrial plants include the relative scarcity of nitrogen containing algal metabolites and the higher proportion of halogenated compounds in seaweeds, probably reflecting relative differences in availability of nitrogen and halides such as bromine and chlorine in terrestrial versus marine systems.
Many defensive functions for algal secondary metabolites have been reported including antimicrobial, antifouling, and antifeeding, the last of which has been most studied. An array of strategies to cope with herbivory have been described, including tolerance through compensatory growth, escape through spatial, temporal, or associational refuges, and structural, morphological, or chemical defenses (see reviews in McClintock and Baker 2001). Several of these strategies may be used simultaneously by seaweeds in order to reduce herbivory. A number of benthic herbivores are trophic specialists that consume one or a few algal species including those that are chemically defended, but in comparison with terrestrial ecosystems feeding specialization among marine herbivores is rare. Metabolites that are toxic for generalist herbivores may be selectively consumed by feeding specialists such as nudibranch molluscs that concentrate these metabolites and use them as defenses against their own enemies.
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