Alkaloids are nitrogen-containing compounds that occur naturally not only in plants but also in microorganisms, marine organisms, and animals. Many kinds of alkaloids with extraordinary structures and significant biological activities have been isolated from marine organisms [1,2]. They continue to provide lead structures in the search for new drugs or biological probes for physiological studies. As new and more complicated diseases are encountered worldwide, the importance of novel bioactive alkaloids has increased due to their potential application in chemotherapy.

Many kinds of bioactive nitrogenous compounds, such as peptides, indols, oxazoles, and thiazoles, have been identified from marine invertebrates [3,4]. The true origins or progenitors ofthese metabolites have been suggested to be microorganisms, i.e., microalgae, bacteria, and fungi. These microorganisms are carried through symbiosis, association, a food chain, and other forms of nutrient-dependency with host animals [5-7]. Consequently, the isolation of bioactive metabolites from cultured marine microorganisms, such as symbiotic dinoflagellates and bacteria, as well as from their host animals, has been well investigated. Several alkaloidal metabolites isolated from cyanobacteria have been suggested to help to inhibit predation by marine herbivores, such as fish and sea urchins. However, the real role of most marine bioactive alkaloids in the ecosystem has not been well clarified.

In our ongoing search for bioactive metabolites from marine organisms, several novel heterocyclic alkaloids, such as pinnatoxins, norzoanthamine, pinnaic acids, zamamistatin, and symbioimine, have been isolated. This work features the structures, biological activities, and biogenesis of these bioactive heterocyclic marine alkaloids, along with up-to-date topics.

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