Abstract
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.
Highlights
Infochemicals released by D. gessneri significantly reduced growth but enhanced sxt Infochemicals by D. gessneri growth but enhanced sxt gene expression andreleased toxin production in R. significantly raciborskii T3,reduced evidencing a predator-induced gene expression toxin production in R. using raciborskii
T3, evidencing a predator-induced chemical defense. and. We observed these effects a density of cladocerans near to those chemical defense. We observed these effects using a density of cladocerans those naturally occurring, and we detected an approximately two-fold increase near in sxttogene naturally occurring, and we detected an approximately two-fold increase in sxt gene transcript levels and cellular STX quotas of the cyanobacterium in response to zooplankton transcript alarm cues. levels and cellular STX quotas of the cyanobacterium in response to zooplankton alarm have cues. characterized the physiological responses underlying predatorSeveral studies
[5,51,52,53], including gene expression an increase in STX production mediated by predator genehas expression an increase
Summary
In addition to the direct effect provided by active foraging, zooplankton release infochemicals that induce functional responses in phytoplankton species, such as increased toxin production [4,5,6,7] and colony formation or trichome stretching, which in turn may limit grazing pressure [8,9,10,11]. These infochemicals are referred to as kairomones or predator cues, which are chemical signals released by predators that indicate a threat and can benefit prey defense. In Daphnia, these chemicals were found to be aliphatic sulfates and sulfamates [12,13]
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