Anabaena and Aphanizomenon species may contain the low-molecular-weight alkaloid anatoxin-a. This toxin mimics the action of acetylcholine. Death results from respiratory arrest. Blooms of some Aphanizomenon species may produce small amounts of the neurotoxin (saxitoxin) and at least three related toxic compounds of unknown structure. Saxitoxin blocks sodium conductance through excitable membranes, which subsequently stops nerve action potential. Death resulting from saxitoxin is caused by respiratory neuromuscular paralysis.Anatoxin-a(s) can also be produced by these organisms, whose action is to inhibit acetylcholinesterase activity. Microcystis species of cyanobacteria can produce the cyclic hepatotoxin microcystin. The immediate cause of death is hemorrhagic shock resulting from massive hepatocellular necrosis and collapse of the hepatic parenchyma.