Chemical defenses of the tropical, benthic marine cyanobacterium Hormothamnion enteromorphoides: Diverse consumers and synergisms

Abstract

The tropical, benthic marine cyanobacterium Hormothamnion enteromorphoides periodically dominates shallow reef habitats, forming erect tufts similar in size to the thalli of macroalgae. Although periodically very common and lacking structural defenses, H. enteromorphoides seems to be a low preference food item for marine herbivores. An organic extract of H. enteromorphoides deterred feeding by a natural assemblage of reef fishes, suggesting that H. enteromorphoides was chemically defended. Bioassay‐guided fractionation of the organic extract led to a mixture of related cyclic peptides, with the known compound laxaphycin A as the major component. The peptide mixture significantly and strongly deterred feeding at natural concentrations by the parrotfish Scarus schlegeli, the sea urchin Diadema savignyi, and the crabs Leptodius spp., but had a less striking and nonsignificant effect on feeding by the pufferfish Canthigaster solandri. Previous studies found that laxaphycins exhibited synergistic antifungal and cytotoxic effects. However, when tested at equal concentrations in feeding assays with S. schlegeli and C. solandri, laxaphycin A was as effective at deterring feeding as was the peptide mixture, indicating that the peptides do not interact synergistically to deter herbivory.