and dropping to low values at night, further stressing the ecosystem. In addition, many cyanobacteria species produce potent cyanotoxins which concentrate during a bloom to the point where they become lethal to nearby aquatic organisms and any other animals in direct contact with the bloom, including birds, livestock, domestic animals and sometimes humans. In 1991 a harmful cyanobacterial bloom affected 1,000 km of the Darling-Barwon River in Australia at an economic cost of $10M AUD. Chemical structure Cyanotoxins usually target the nervous system (neurotoxins), the liver (hepatotoxins) or the skin (dermatoxins). The chemical structure of cyanotoxins falls into three broad groups: cyclic peptides, alkaloids and lipopolysaccharides (endotoxins). Most cyanotoxins have a number of variants (analogues). As of 1999, altogether over 84 cyanotoxins were known and only a small number have been well studied. Cyclic peptides A peptide is a short polymer of amino acids linked by peptide bonds. They have the same chemical structure as proteins, except they are shorter. In a cyclic peptide, the ends link to form a stable circular chain. In mammals this stability makes them resistant to the process of digestion and they can bioaccumulate in the liver. Of all the cyanotoxins, the cyclic peptides are of most concern to human health. The microcystins and nodularins poison the liver, and exposure to high doses can cause death. Exposure to low doses in drinking water over a long period of time may promote liver and other tumours. Microcystins As with other cyanotoxins, microcystins were named after the first organism discovered to produce them, Microcystis aeruginosa. However it was later found other cyanobacterial genera also produced them. There are about 60 known variants of microcystin, and several of these can be produced during a bloom. The most reported variant is microcystin-LR, possibly because the earliest commercially available chemical standard analysis was for microcystin-LR. Blooms containing microcystin are a problem worldwide in freshwater ecosystems. Microcystins are cyclic peptides and can be very toxic for plants and animals including humans. They bioaccumulate in the liver of fish, in the hepatopancreas of mussels, and in zooplankton. They are hepatotoxic and can cause serious damage to the liver in humans. In this way they are similar to the nodularins (below), and together the microcystins and nodularins account for most of the toxic cyanobacterial blooms in fresh and brackish waters. In 2010, a number of sea otters were poisoned by microcystin. Marine bivalves