Abstract
Many bacterial genes and virulence factors are regulated by interbacterial and/or host–parasite chemical signals. We demonstrate that toxin production by a free‐living freshwater cyanobacterium is regulated in part by the presence of extracellular products of a eukaryotic green alga. In growth experiments, extracellular products made by the cyanobacterium Anabaena flos‐aquae contained both anatoxin and microcystin, and significantly reduced the yield of Chlamydomonas reinhardtii , a green alga. Based on experiments in which we added purified toxins to C. reinhardtii cultures, we believe that microcystin was responsible for the growth reduction. A. flos‐aquae produced anatoxin constitutively when grown alone, but anatoxin concentration increased in the presence of C. reinhardtii elicitors. Microcystin accumulation depended on the growth phase; however, high concentrations of C. reinhardtii extracellular products completely inhibited microcystin accumulation. Our results demonstrate that cyanobacterial toxin production may be regulated by complex growth phase‐dependent and environmental chemical cues, and suggest that secreted chemicals can mediate the outcome of competition between the cyanobacterium A. flos‐aquae and the green alga C. reinhardtii .