Effects of Cyanotoxins: Sea and Freshwater Toxins

Abstract

Cyanobacteria produce a wide range of toxins with different chemical composition and molecular targets. Cyanobacterial neurotoxins comprise a variety of compounds acting either on acetylcholine receptors or acetylcholinesterase, on voltage-gated sodium channels and on excitatory neuronal synapses. Acute intoxications can be lethal and they mainly present as paralytic shellfish poisoning. Chronic exposure may lead to neurodegenerative disorders. Hepatotoxins and neurotoxins present two main groups of cyanobacterial toxins that affect human health. Nodularin and microcystins are hepatotoxins characterized by the presence of non-proteinogenic b-amino acid ADDA in cyclic pentaor heptapeptide structure. Best known acute lethal intoxications of humans have occurred in Brazil, but chronic exposure to these toxins leading to multi-organ failure is a more serious and widespread health problem. Microcystins are also tumor promoters, and there are several reports showing harmful effects of long-term exposure to microcystins in highly populated regions of China and other parts of the world. Cyanobacterial toxins must be considered as a serious threat, and high safety measures must be followed in monitoring the quality of water and food used in human nutrition, medical care, and recreational activities. Introduction: Cyanobacteria and Their Toxins Cyanobacteria developed several billions of years ago, and at that time the oxygen they released into the atmosphere was the most devastating toxic substance to life on earth, although there is no evidence that this gave any advantage to cyanobacteria. Presently, this function of cyanobacteria is, from the human point of view, beneficial as they bind nitrogen, trap CO2, and contribute to our oxygen supply. On the other hand, cyanobacteria present serious hazard to human health because they release potent cyanobacterial toxins. About 2000 species of cyanobacteria inhabit aqueous bodies worldwide, and it seems they can survive under most environmental conditions (Fig. 1). Substances that they synthesize, including cyanotoxins, serve a role in cyanobacterial bloom growth and competition with other organisms in surface waters. Cyclic cyanopeptides comprise a large and heterogeneous group of biologically active substances that often contain unusual amino acids in their structures. The importance of many of these substances for cyanobacteria still remains to be uncovered, but it seems that the interplay of the effects of cyanobacterial neurotoxins and cyclic hepatotoxic and non-hepatotoxic cyanopeptides is important for the regulation of cyanobacterial bloom growth and decay as well as for the interactions with other photosynthetic organisms. Under favorable conditions such as water eutrophication, these planktonic cyanobacteria readily multiply in marine and freshwater environments forming toxic and nontoxic blooms. From anthropocentric view, cyanobacteria are harmful organisms producing two main classes of toxins: neurotoxins and hepatotoxins. Acute intoxications with neurotoxins are relatively common and *Email: dusan.suput@mf.uni-lj.si Marine and Freshwater Toxins DOI 10.1007/978-94-007-6650-1_15-1 # Springer Science+Business Media Dordrecht 2015