Mode of action and toxicity of major cyanobacterial toxins and corresponding chemical variants

Toxic cyanobacterial blooms are often found in aquatic ecosystems, spanning from fresh to brackish waters and marine waters, and may reflect the increased eutrophication of these environments and alterations in climate. Cyanobacterial toxins (cyanotoxins) are secondary metabolites, with very different chemical structures, and highly reactive to various biological molecules. Scientists and public health and environmental agencies have recognized that contamination by cyanobacterial toxins is a global and serious environmental and health threat. Notwithstanding, it is notorious the efforts done so far from the scientific community that resulted in the isolation, purification, identification, structure elucidation of several of these groups of toxins from many ecosystems worldwide as well as the identification of respective molecular targets and biological activities. The chemical variability of cyanotoxins is a critical determinant of the biological activity leading to a need to classify the different groups but also to distinguish the several chemical variants within each group. Cyanotoxins are thus currently classified on the basis of their chemical composition and toxic activity. This chapter aims to review and summarize key information concerning this class of natural compounds produced by cyanobacteria, as perceived by the authors being critical for understanding the impact of these compounds in the environment, and thus necessary for carrying out and validating risk assessment studies.