Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.

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Dissolved microcystins (MCs) are regularly present in water dominated by microcystin-producing, bloom-forming cyanobacteria. In vitro experiments with environmentally feasible concentrations (5 x 10(-7) M) of the three most common microcystins, MC-LR, MC-RR, and MC-YR, revealed that they influence the metabolism of different representative phytoplanktons. At light intensities that are close to the cyanobacterial bloom environment (50 micromol m(-2) s(-1)), they produce morphological and physiological changes in both microcystin-producing and -nonproducing Microcystis aeruginosa strains and also have similar effects on the green alga Scenedesmus quadricauda that is frequently present in cyanobacterial blooms. All three microcystin variants tested induce cell aggregation, increase in cell volume, and overproduction of photosynthetic pigments. All three effects appear to be related to each other but are not necessarily caused by the same mechanism. The biological activity of microcystins toward the light-harvesting complex of photobionts can be interpreted as a signal announcing the worsening of light conditions due to the massive proliferation of cyanobacteria. Although the function of microcystins is still unknown, it is evident that they have numerous effects on phytoplankton in nature. These effects depend on the individual organism as well as on the various intracellular and extracellular signaling pathways. The fact that dissolved microcystins also influence the physiology of microcystin-producing cyanobacteria leads us to the conclusion that the role of microcystins in the producing cells differs from the role in the water environment.

The presence of planktopeptin BL1125, anabaenopeptin B and anabaenopeptin F, two types of "non-toxic" cyclic peptide produced in bloom forming cyanobacteria, can provoke lysis of different non-axenic Microcystis aeruginosa cell lines via the induction of virus-like particles. The resulting particles are also able to infect the axenic M. aeruginosa cell line without lytic effects. Nevertheless, the presence of "non-toxic" cyclic peptides of cyanobacterial origin can induce lysis of these previously infected cells. This effect implies that a possible role of these peptides in the natural environment is the control of cyanobacterial population density. Lysogenic cyanobacteria can consequently act as hot-spots that, in the presence of cyanobacterial cyclic peptides, release numerous infectious particles. The process can be self-augmented with the simultaneous release of additional cyclic peptides from the producing lysogens, starting a forest fire effect that ends in collapse of cyanobacterial blooms.

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