Purification of an agent causing growth inhibition in anuran larvae and its identification as a unicellular unpigmented alga

Growth-inhibition mechanisms affecting amphibian larvae were investigated using five species of anurans (Rana temporaria, Rana esculenta, Bufo bufo, Bufo calamita, and Xenopus laevis) in the laboratory. Inhibition was selective for detritus-feeding rather than filter-feeding larvae, was not otherwise species-specific, and was mediated by unicellular organisms passed via coprophagy between tadpoles. These unicells were isolated and purified almost to homogeneity (in batches of > 107) by Percoll density gradient centrifugation and caused growth inhibition in a dose-dependent manner when added to tadpoles in culture. Ultrastructural, toxicological, and nucleic acid hybridisation studies indicated that the unicell was an unpigmented alga, probably of the genus Prototheca. Inhibitory cells were visualised by electron microscopy in the intestinal lumen, an environment in which no morphologically distinct life stages were identified. The relevance of these studies to interspecific competition between wild populations of anurans is discussed.