| Many plants and prokaryotes from both aquatic and terrestrial environments respond to competition, herbivory, and infection by producing antagonistic chemical signals. Cyanobacteria, responsible for toxic aquatic blooms world-wide, produce chemicals that inhibit phototrophic growth, zooplankton grazing rate and survivorship, and bacterial growth. While the chemical defenses of terrestrial and marine plants are inducible by herbivory and infection, the role of biological interactions on cyanobacterial toxin production is not known. Furthermore, the effect of these toxins on the outcome of competition among cyanobacteria and aquatic phototrophs has not been established. This dissertation explores antagonistic chemical interactions between the co-occurring freshwater phototrophs, Anabaena flos-aquae, a toxic cyanobacterium, and Chlamydomonas reinhardtii, a motile green alga. In growth experiments, A. flos-aquae extracellular products containing both anatoxin and microcystin significantly reduced the yield of C. reinhardtii. A. flos-aquae produced anatoxin constitutively when grown alone. However, anatoxin concentrations were nearly twice as high in the presence of hydrophilic, heat- and proteinase-sensitive extracellular products harvested from late growth phase C. reinhardtii cells. While microcystin accumulation depended on the growth phase of the cyanobacterium, high concentrations of green algal extracellular products completely inhibited microcystin accumulation. Purified toxins and cyanobacterial extracellular products also paralyzed C. reinhardtii cells, causing them to settle faster in toxic than in nontoxic media. The presence of either the cyanobacterium or its extracellular products also increased the accumulation rate of C. reinhardtii at the bottom of microcosms. In addition to producing a chemical that influences toxin production, C. reinhardtii extracellular products also suppressed A. flos-aquae heterocyst formation. Heterocysts are specialized cells within A. flos-aquae filaments that fix nitrogen. Heterocyst frequencies were reduced in the presence of soluble proteinase- and heat-resistant extracellular products of C. reinhardtii. While ammonium and nitrate are known inhibitors of heterocyst development they were not responsible for this change in phenotype. These results describe complex chemical interactions between prokaryotic and eukaryotic phototrophs that influence the outcome of competition. These signals may help to explain the appearance and persistence of toxic cyanobacterial blooms in lake ecosystems. |
