Abstract
Toxicity of cyanobacteria is the subject of ongoing research, and a number of toxic metabolites have been described, their biosynthesis pathways have been elucidated, and the mechanism of their action has been established. However, several knowledge gaps still exist, e.g., some strains produce hitherto unknown toxic compounds, while the exact dynamics of exerted toxicity during cyanobacterial growth still requires further exploration. Therefore, the present study investigated the toxicity of extracts of nine freshwater strains of Aphanizomenon gracile, an Aphanizomenon sp. strain isolated from the Baltic Sea, a freshwater strain of Planktothrix agardhii, and two strains of Raphidiopsis raciborskii obtained from 25- and 70-day-old cultures. An in vitro experimental model based on Cyprinus carpio hepatocytes (oxidative stress markers, DNA fragmentation, and serine/threonine protein activity) and brain homogenate (cholinesterase activity) was employed. The studied extracts demonstrated toxicity to fish cells, and in general, all examined extracts altered at least one or more of considered parameters, indicating that they possess, to some degree, toxic potency. Although the time from which the extracts were obtained had a significant importance for the response of fish cells, we observed strong variability between the different strains and species. In some strains, extracts that originated from 25-day-old cultures triggered more harmful effects on fish cells compared to those obtained from 70-day-old cultures, whereas in other strains, we observed the opposite effect or a lack of a significant change. Our study revealed that there was no clear or common pattern regarding the degree of cyanobacterial bloom toxicity at a given stage of development. This means that young cyanobacterial blooms that are just forming can pose an equally toxic threat to aquatic vertebrates and ecosystem functioning as those that are stable or old with a tendency to collapse. This might be largely due to a high variability of strains in the bloom.
Highlights
Cyanobacterial blooms have garnered increased scientific interest because of their potential threat to aquatic biota, ecosystem functioning, and adverse effects on human health
The observed response of parameters employed in the toxicity evaluation revealed a dependence on the stage of culture development at which the studied extracts were obtained, the cyanobacterial strain, as well as the interaction between the stage of culture development and the strains (Table 2)
Toxicological studies are usually focused on a single cyanotoxin, which allows exploring its exact mechanisms of action, whereas the massive occurrence of cyanobacteria in the aquatic environment is related to the presence of a mixture of various metabolites, some of which remain to be identified [33]
Summary
Cyanobacterial blooms have garnered increased scientific interest because of their potential threat to aquatic biota, ecosystem functioning, and adverse effects on human health. These blooms have occurred before the impacts of humans on the aquatic environment [1,2,3], their frequency, intensity, range, and time of persistence have increased over the last few decades [4,5]. Anthropogenic eutrophication and climate change are two of the most influential factors that promote cyanobacterial blooms [6,7,8] These blooms most often occur in months with the highest mean air temperature, winter blooms in eutrophic lakes have been reported with increasing frequency [9,10,11,12,13]. The nutritional value of cyanobacteria for consumers, such as zooplankton, is generally low because their cells are deficient in sterols and fatty acids [14]; at low biomass, they can be a valuable source of food for consumers [15,16]
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