Harmful algal blooms in both freshwater and marine environments pose significant challenges for humans, livestock and aquatic food webs. Over the last 50 years, several authors proposed that small mol. iron chelators (siderophores) could be responsible for such blooms, but these studies were never evaluated on a chem., mol. level. In this communication, we present that the iron chelator anachelin, isolated from the cyanobacterium Anabaena cylindrica, is promoting the growth of cyanobacteria while concurrently decreasing the growth of competing phytoplankton (green algae). This dual mode of action (growth promotion and allelopathic activity) is caused by two different fragments of anachelin, as shown through chem. synthesis and biol. evaluation. Anachelin thus constitutes a natural product hybrid blending two bioactivities in a single compound.