Cyanobacteria produce a range of toxins with different modes of action including liver toxins (e.g., microcystins), neurotoxins (e.g. saxitoxins and anatoxins), and cytotoxins (e.g. cylindrospermopsin). Some of these toxins have been well characterised, particularly with regard to their effects on vertebrates, although much less is known about the effects of cyanotoxins on plants. However, comparisons of the effects of pure toxins with crude extracts, and investigations of unusual patterns of toxicity, have suggested the presence of other unidentified toxins in cyanobacteria. Toxic effects on plants may have implications for food crops irrigated with cyanobacteria-contaminated water, and if toxins are taken up into plant tissues there is potential for them to enter the food chain.
In this study we have found evidence of toxicity to Medicago sativa (alfalfa) seedlings from extracts of a strain of Anabaena circinalis (now Dolichospermum circinale) containing no detectable saxitoxins, and of Microcystis aeruginosa with no detectable microcystins. Further studies are under way to investigate these effects on alfalfa and other species, and if possible identify the compounds responsible. This information will aid in setting water quality guidelines for cyanobacteria-contaminated waters, and also provide evidence about the risks of contaminated waters to agriculture and food supplies.