Ichthyophthirius multifiliis and Tetrahymena thermophila tolerate glyphosate but not a commercial herbicidal formulation

Freshwater habitats are routinely exposed to glyphosate because it is useful in controlling aquatic weeds and is a broad-spectrum, non-selective algacide. In the United States, approximately 848,000 kg (9,350 tons) of glyphosate in a variety of formulations-including Roundup registered -are applied each year. Glyphosate activity is reduced by adsorption to sediment. Glyphosate by-products are found when soil contains live microorganisms, although cultured microorganisms have a limited tolerance for glyphosate. Ciliated protozoa (Ciliophora) are prevalent in freshwater aquatic systems, and their sensitivity to glyphosate is not known. One such ciliate, Tetrahymena pyriformis, has been selected for standard determinations of substance toxicity in European freshwater ecosystems. T. pyriformis sensitivity to glyphosate per se is unknown (http://www.epa.gov/ecotox/), but in laboratory tests it avoids swimming into capillaries that contain Roundup (Roberts and Berk 1993). Another ciliate found in freshwater aquaculture systems is the obligate fish parasite Ichthyophthirius multifiliis. Few effective chemotherapeutic agents are available for use against Ichthyophthirius in farmed fish. Parasitic ciliates Plasmodium, Toxoplasma, and Cryptosporidium, which cause disease in humans, are sensitive to glyphosate and, like these parasites, I. multifiliis has a complex developmental cycle and infects vertebrate hosts. Thus, glyphosate may be useful for controlling I. multifiliis in aquaculture. The sensitivity of I. multifiliis to glyphosate is unknown. This study tests the sensitivity of Tetrahymena and I. multifiliis to glyphosate and Roundup.