Cyanobacteria stand out among phytoplankton when they form massive blooms and produce toxins. Because cyanotoxin genes date to the origin of metazoans, the hypothesis that cyanotoxins function as a defense against herbivory is still debated. Although their primary cellular function might vary, these metabolites could have evolved as an anti-predator response. Here we evaluated the physiological and molecular responses of a saxitoxin-producing Raphidiopsis raciborskii to infochemicals released by the grazer Daphnia gessneri. Induced chemical defenses were evidenced in R. raciborskii as a significant increase in the transcription level of sxt genes, followed by an increase in saxitoxin content when exposed to predator cues. Moreover, cyanobacterial growth decreased, and no significant effects on photosynthesis or morphology were observed. Overall, the induced defense response was accompanied by a trade-off between toxin production and growth. These results shed light on the mechanisms underlying zooplankton–cyanobacteria interactions in aquatic food webs. The widespread occurrence of the cyanobacterium R. raciborskii in freshwater bodies has been attributed to its phenotypic plasticity. Assessing the potential of this species to thrive over interaction filters such as zooplankton grazing pressure can enhance our understanding of its adaptive success.

The problem of artificial eutrophication, together with the effects of climate changes has led to an increase in the frequency of the occurrence of cyanobacterial blooms

Changes in food quality can dramatically impair zooplankton fitness, especially in eutrophic water bodies where cyanobacteria are usually predominant. Cyanobacteria are considered a food with low nutritional value, and some species can produce bioactive secondary metabolites reported as toxic to zooplankton. Considering that cyanobacteria can limit the survival, growth and reproduction of zooplankton, we hypothesized that the dietary exposure of neotropical Daphnia species (D. laevis and D. gessneri) to saxitoxin-producing cyanobacteria impairs Daphnia feeding rates and fitness regardless of a high availability of nutritious algae. Life table and grazing assays were conducted with different diets: (1) without nutritional restriction, where neonates were fed with diets at a constant green algae biomass (as a nutritious food source), and an increasing cyanobacterial concentration (toxic and poor food source), and (2) with diets consisting of different proportions of green algae (nutritious) and cyanobacteria (poor food) at a total biomass 1.0 mg C L⁻¹. In general, the presence of high proportions of cyanobacteria promoted a decrease in Daphnia somatic growth, reproduction and the intrinsic rate of population increase (r) in both diets with more pronounced effects in the nutritionally restricted diet (90% R. raciborskii). A two-way ANOVA revealed the significant effects of species/clone and treatments in both assays, with significant interaction between those factors only in the second assay. Regarding the grazing assay, only D. laevis was negatively affected by increased cyanobacterial proportions in the diet. In the life table assay with constant nutritious food, a reduction in the reproduction and the intrinsic rate of the population increase (r) of all species were observed. In conclusion, we found adverse effects of the toxic cyanobacterial strain R. raciborskii on Daphnia fitness, regardless of the constant amount of nutritious food available, proving the toxic effect of R. raciborskii and that the nutritional quality of the food has a greater influence on the fitness of these animals.

Cyanobacterial blooms are recurrent phenomena in several water bodies, mostly eutrophic. They are considered a public health problem, especially considering harmful species like Raphidiopsis raciborskii, a bloom-forming cyanobacteria recorded as a producer of neurotoxic and cytotoxic alkaloids. The present study aimed to assess the acute toxicity of a saxitoxin-producing strain Raphidiopsis raciborskii ITEP-A1 on the zooplankter Macrothrix spinosa. Cladoceran clones isolated from an eutrophic system with cyanobacterial blooms records and an oligotrophic one with no bloom record; both from Pernambuco (Northeastern Brazil), were tested for R. raciborskii ITEP-A1 toxicity. Acute toxicity assays were carried out with newborns (<24h, n=10) and the animals were exposed to three concentrations of cyanobacterial cell biomass (μgC L-1). Quantification of saxitoxins (STX and neoSTX) was done by high-performance liquid chromatography coupled with fluorescence detector (HPLC-RF). The EC50(48h) for the Duas Unas and Prata clones were 359 and 189 μgC L-1, respectively. Obtained results showed that M. spinosa Prata was more sensitive to R. raciborskii than Duas Unas, reinforcing the importance of analyzing the previous life history of the test organisms regarding their native environments. However, both clones demonstrated high sensitivity to R. raciborskii, which enhances their potential for biomonitoring toxic cyanobacterial blooms in tropical reservoirs.