One of the compounds suggested to be responsible for the cyanobacterial dominance over competing green algae is identified. Evidence is provided on the molecular, chemical level that the iron chelator anachelin from the cyanobacterium Anabaena cylindrica promotes both the growth of cyanobacteria and reduces the growth of competing chlorophytes. These results illustrate a molecular strategy of addressing two challenges (nutrient availability and algal competition) by one molecule. Such strategies could be implied in harmful algal blooms in marine and freshwater environments.