ABSTRACT
Microcystins are some of the most common and most toxic cyanobacterial toxins in freshwaters. These cyclic peptides are synthesized nonribosomally and are powerful inhibitors of eukaryotic protein phosphatases (type 1 and 2A), which have several regulatory roles in eukaryotes [1,2]. From the chemical point of view, microcystins are cyclic heptapeptides with variable aminoacids at seven different positions [3]. All microcystins contain two amino acids that are unique to this type of compound: Adda ((2S, 3S, 8S) S)-3-amino-9-methoxy-2, 6, 8-trimethyl-10-phenyldeca-4, 6 dienoic acid) and ADMAdda (O-acetyl-O-demethylAdda) [4]. The toxicity of microcystins in vertebrates is mediated through their active transport into hepatocytes by the bile acid organic anion transport system, and acute poisoning may provoke death due to massive hepatic hemorrhage in both animals and humans [5]. The main target of acute intoxication is the liver, although a role for these compounds in tumor promotion and carcinogenesis has also been revealed in laboratory animals [6]. The exposure routes are several: skin contact, inhalation, hemodialysis, ingestion, drinking and
17.1 Introduction .......................................................................................................................... 321 17.2 Biological Effects of Microcystins ....................................................................................... 322
17.2.1 Macroinvertebrates ................................................................................................... 323 17.2.2 Fish............................................................................................................................ 323 17.2.3 Birds .......................................................................................................................... 323 17.2.4 Algae and Bacteria.................................................................................................... 323 17.2.5 Angiosperms ............................................................................................................. 325
17.3 Toxin Production in Benthic Communities .......................................................................... 326 17.3.1 Dynamics of Benthic Communities ......................................................................... 326 17.3.2 Limnological Characteristics of Water and Relation with Toxin Production ........... 326 17.3.3 Nutrients ................................................................................................................... 326 17.3.4 Role of Mucilage ....................................................................................................... 327
17.4 Geographical Distribution .................................................................................................... 328 17.4.1 Standing and Flowing Aquatic Systems of Mediterranean Spanish
Semiarid Regions ...................................................................................................... 329 17.4.2 Reservoirs ................................................................................................................. 329 17.4.3 Streams ..................................................................................................................... 331
17.5 Climate Change and the Impact on Prevalence of Toxin Producers .................................... 332 17.6 Proposals for the Future........................................................................................................ 334 Acknowledgments .......................................................................................................................... 335 References ...................................................................................................................................... 335
recreational water, food, or dietary supplements [7]. It should be borne in mind that more than one exposure route may operate simultaneously and the implications of the presence of the toxins for industry (food manufacture, agricultural products, aquaculture, etc.), especially in the predicted scenario of increased aridity, are not to be taken lightly.
