Secondary metabolites from marine organisms.

Marine macroorganisms and microorganisms, like terrestrial species, produce a dizzying array of secondary metabolites, including terpenes, steroids, polyketides, peptides, alkaloids and porphyrins. Most of the marine metabolites are found in terrestrial species as such or have close counterparts in land-based species, but some are sufficiently unusual to constitute a separate class (e.g. marine sterols). Although in many cases the functions of these secondary metabolites in the marine species themselves are unclear, other compounds play well-defined roles-for example as trail markers, sexual attractants, antifouling substances or antifeedants. What is clear is that many of the most interesting marine secondary metabolites have potent activities largely unrelated to their in situ roles. Examples abound of antitumour, antiviral, immunosuppressive and antimicrobial agents, as well as neurotoxins, hepatotoxins and cardiac stimulants. Relatively few biosynthetic studies of marine secondary metabolites have been done because of the logistical problems of working under water and the primitive state of techniques for growing marine invertebrates in culture. However, recent studies indicate that many compounds isolated from marine macrospecies (e.g. sponges) may instead be produced by microorganisms (e.g. bacteria or phytoplankton). Studies of these symbionts may facilitate efforts to understand the biosyntheses of these metabolites.