Ecophysiological Aspects and sxt Genes Expression Underlying Induced Chemical Defense in STX-Producing Raphidiopsis raciborskii (Cyanobacteria) against the Zooplankter Daphnia gessneri.

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.

Can small-bodied Daphnia control Raphidiopsis raciborskii in eutrophic tropical lakes? A mesocosm experiment.

Raphidiopsis raciborskii is being considered an expanding, invasive species all over the world. It is a potentially toxin producer cyanobacterium and form blooms specially in (sub)tropical lakes, causing concern to public health. Thus, controlling such phenomena are of vital importance. To test the hypothesis that a tropical clone of Daphnia laevis is able to reduce the biomass of R. raciborskii, we performed a mesocosm experiment simulating a bloom of this cyanobacterium in field conditions and exposing it to ecologically relevant densities of daphniids. In addition, we tested the hypothesis that omnivorous fish would be able to exert a top-down effect on Daphnia, decreasing the effectiveness of this control. We used treatments with (10 and 20 Daphnia L-1) or without Daphnia and fish (3 per mesocosm). Daphnia was able to significantly reduce the biomass of R. raciborskii only at the highest density tested. Fish had low effect on Daphnia biomass, but it is suggested that nutrient recycling by fish might have contributed to the higher R. raciborskii biomass in fish treatments. This is the first evidence of Daphnia control over saxitoxin-producing cyanobacteria in a tropical ecosystem.

Morphology and molecular phylogeny of a new PST-producing dinoflagellate species: Alexandrium fragae sp. nov. (Gonyaulacales, dinophyceae).

The genus Alexandrium comprises some of the most potentially toxic marine algae. A new toxic species of Alexandrium, A. fragae sp. nov., was found in Guanabara Bay, Rio de Janeiro, southern Brazil. The new species produces GTX2&3 and STX. The cell morphology of A. fragae resembles A. minutum in many characters, including the small size; the rounded-elliptical shape; and the shapes of the apical pore complex (APC), first apical plate (1'), sixth precingular plate (6″), and anterior and posterior sulcal plates (s.a. and s.p.). The main diagnostic characters of A. fragae are the ornamentation pattern, smooth epitheca and reticulated hypotheca, all of which were present in both natural populations and cultures. Phylogenies inferred from the ITS, LSU, and SSU rDNA of A. fragae showed that A. fragae clustered in a well-supported clade, distinct from other Alexandrium species. Morphology and molecular analyses based on ITS and LSU rDNA indicated that A. fragae strains and Alexandrium sp. from Japan (D163C5, D164C6) are a single species. Our findings suggest that the Alexandrium morphotype with a smooth epitheca and reticulated hypotheca, previously identified as A. minutum in different geographic regions, may corresponds to A. fragae.

Intraspecific variability in response to phosphorus depleted conditions in the cyanobacteria Microcystis aeruginosa and Raphidiopsis raciborskii.

Phosphorus loading plays an important role in the occurrence of cyanobacterial blooms and understanding how this nutrient affects the physiology of cyanobacteria is imperative to manage these phenomena. Microcystis aeruginosa and Raphidiopsis raciborskii are cyanobacterial species that form potentially toxic blooms in freshwater ecosystems worldwide. Blooms comprise numerous strains with high trait variability, which can contribute to the widespread distribution of these species. Here, we explored the intraspecific variability in response to phosphorus depleted conditions (P-) testing five strains of each species. Strains could be differentiated by cell volume or genetic profiles except for those of the same species, sampling location and date, though these presented differences in their response to (P-). Although differently affected by (P-) over 10 days, all strains were able to grow and maintain photosynthetic activity. For most M. aeruginosa and R. raciborskii strains growth rates were not significantly different comparing (P+) and (P-) conditions. After ten days in (P-), only one M. aeruginosa strain and two R. raciborskii strains showed reduction in biovolume yield as compared to (P+) but in most strains chlorophyll-a concentrations were lower in (P-) than in (P+). Reduced photosystem II efficiency was found for only one R. raciborskii strain while all M. aeruginosa strains were affected. Only two M. aeruginosa and one R. raciborskii strain increased alkaline phosphatase activity under (P-) as compared to (P+). Variation in P-uptake was also observed but comparison among strains yielded homogeneous groups comprised of representatives of both species. Comparing the response of each species as a whole, the (P-) condition affected growth rate, biovolume yield and chlorophyll yield. However, these parameters revealed variation among strains of the same species to the extent that differences between M. aeruginosa and R. raciborskii were not significant. Taken together, these results do not support the idea that R. raciborskii, as a species, can withstand phosphorus limitation better than M. aeruginosa and also point that the level of intraspecific variation may preclude generalizations based on studies that use only one or few strains.