Abstract
While the intracellular function of many toxic and bioactive cyanobacterial metabolites is not yet known, microcystins have been suggested to have a protective role in the cyanobacterial metabolism, giving advantage to toxic over nontoxic strains under stress conditions. The zooplankton grazer Daphnia reduce cyanobacterial dominance until a certain density, which may be supported by Daphnia exudates, affecting the cyanobacterial physiological state and metabolites’ production. Therefore, we hypothesized that D. magna spent medium will impact the production of cyanobacterial bioactive metabolites and affect cyanobacterial photosynthetic activity in the nontoxic, but not the toxic strain. Microcystin (MC-LR and des-MC-LR) producing M. aeruginosa PCC7806 and its non-microcystin producing mutant were exposed to spent media of different D. magna densities and culture durations. D. magna spent medium of the highest density (200/L) cultivated for the shortest time (24 h) provoked the strongest effect. D.magna spent medium negatively impacted the photosynthetic activity of M. aeruginosa PCC7806, as well as the dynamics of intracellular and extracellular cyanobacterial metabolites, while its mutant was unaffected. In the presence of Daphnia medium, microcystin does not appear to have a protective role for the strain. On the contrary, extracellular cyanopeptolin A increased in M. aeruginosa PCC7806 although the potential anti-grazing role of this compound would require further studies.
Highlights
Cyanobacterial blooms can occur in freshwater ecosystems due to eutrophication, often producing toxic and bioactive secondary metabolites that can affect the life traits of all organisms of the ecosystem [1].At the same time, zooplankton graze on phytoplankton, including cyanobacteria, and as such it has been suggested that Daphnia are able to reduce cyanobacterial dominance in aquatic ecosystems up to a certain density and toxicity [2,3,4]
Strain the photosynthetic activity showed a non-statistically significant decrease after being initially in the MC+ strain the photosynthetic activity showed a non-statistically significant decrease after significantly higher during the first three days of exposure and did not recover compared to the control being initially significantly higher during the first three days of exposure and did not recover (Figure 1C, Table 1). Both strains were exposed to a medium in which a higher density of Daphnia compared to the control (Figure 1C, Table 1)
To the best of our knowledge, this is the first study showing a lower physiological state of a MC+ strain when exposed to D. magna spent medium (DM) than its MC- mutant
Summary
Cyanobacterial blooms can occur in freshwater ecosystems due to eutrophication, often producing toxic and bioactive secondary metabolites that can affect the life traits of all organisms of the ecosystem [1].At the same time, zooplankton graze on phytoplankton, including cyanobacteria, and as such it has been suggested that Daphnia are able to reduce cyanobacterial dominance in aquatic ecosystems up to a certain density and toxicity [2,3,4]. Cyanobacterial blooms can occur in freshwater ecosystems due to eutrophication, often producing toxic and bioactive secondary metabolites that can affect the life traits of all organisms of the ecosystem [1]. Blocking the protein phosphatases typ 1 and 2A moves the critical balance between phosphorylated and dephosphorylated proteins towards the hyperphosphorylated state with consequences for cellular metabolism including energy allocation, gene expression and translation, and cytoskeletal components [13]. Oxidative stress is another mechanism, which impairs the cellular function on protein, lipid including membranes, and the nucleic acid level [14]. The biosynthesis of secondary metabolites occurs at the expense of energy [18], the understanding of metabolites or pathways that trigger cyanobacterial secondary metabolites production is of high importance for their management in aquatic environments
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