Cyanobacteria represent an extremely rich source ofbioac-tive natural products. They produce some ofthe most toxic compounds apparently to defend their living space against other organisms (Figure 3). In choice experiments, crustaceans avoid toxic cyanobacteria indicating that toxic cyanobacteria benefit from their secondary metabolites. The alkaloid anatoxin a 14 from Anabaena flos-aquae has been characterized as a neurotoxin blocking the
Zosteric acid 13
acetylcholine receptor which results in paralysis and rapid death. Also, saxitoxins 4 and related gonyautoxins produced by cyanobacteria act as ion channel blockers. Such compounds cause severe poisoning of shellfish (PSP, paralytic shellfish poisoning). Nonribosomal cyclic peptides such as microcystin LR 15 are potent hepatotox-ins, that inhibit protein phosphatases. The macrocycle cryptophycin 1 (16) is an example of a powerful fungicide.
Many cyanobacteria live in symbiosis with other organisms, for example, with dinoflagellates or sponges, and contribute to the protection of their hosts.
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