A Review on the Study of Cyanotoxins in Paleolimnological Research: Current Knowledge and Future Needs

Abstract

Cyanobacterial metabolites are increasingly studied, in regards to their biosynthesis, ecological role, toxicity, and potential biomedical applications. However, the history of cyanotoxins prior to the last few decades is virtually unknown. Only a few paleolimnological studies have been undertaken to date, and these have focused exclusively on microcystins and cylindrospermopsins, both successfully identified in lake sediments up to 200 and 4700 years old, respectively. In this paper, we review direct extraction, quantification, and application of cyanotoxins in sediment cores, and put forward future research prospects in this field. Cyanobacterial toxin research is also compared to other paleo-cyanobacteria tools, such as sedimentary pigments, akinetes, and ancient DNA isolation, to identify the role of each tool in reproducing the history of cyanobacteria. Such investigations may also be beneficial for further elucidation of the biological role of cyanotoxins, particularly if coupled with analyses of other abiotic and biotic sedimentary features. In addition, we identify current limitations as well as future directions for applications in the field of paleolimnological studies on cyanotoxins. Key Contribution:This review provides updated information on paleolimnological studies of cyanotoxins and highlights their value for the understanding of the history and occurrence of toxic cyanobacteria, as well as understanding the potential environmental drivers of cyanotoxin production.Toxins 2020, 12, 6 2 of 15 of cylindrospermopsin production also exist [3]. These timescales imply that cyanobacterial toxins play ecological role(s) other than grazer defense, and that their toxicity towards zooplankton may only be an indirect effect of their production and release into the water column. Cyanotoxin biological function still remains a subject of debate and various hypotheses, derived mostly from experimental observations, on their potential intra-and extracellular roles have been put forward [1,[12][13][14].Like the ecological role of cyanobacterial metabolites, the environmental triggers causing toxin production lack definite identification in experimental and monitoring investigations. Nutrients, frequently associated with cultural eutrophication (nitrogen and phosphorus) in freshwaters, have been identified as key drivers of toxin production [15], but cyanobacteria in selected hypereutrophic systems do not produce cyanotoxins [1]. Nutrients that generally have less influence on trophic state in freshwater ecosystems, such as S and Fe, have also been associated with toxin occurrence [16], as have changes in N/P ratios [15]. Biological drivers, such as zooplankton grazing pressure and allelopathy [1], have been linked to cyanobacteria toxin production; along with abiotic factors, such as temperature and light intensity [17,18]. While recent genetic, experimental, and monitoring efforts have provided extensive knowledge of cyanobacterial metabolites, placing these data into a historic context and determining whether toxi...

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