When water turns toxic: how climate change drives cyanotoxin biosynthesis-A mechanistic review.

Cyanobacterial harmful algal blooms (cHABs) represent an escalating global threat due to their ability to produce a wide range of potent cyanotoxins that threaten aquatic ecosystems, drinking water safety and public health. While eutrophication has long been recognized as a primary driver of bloom formation, increasing evidence suggests that climate change acts as a critical catalyst influencing not only bloom frequency and intensity but also toxin diversity and regulation. However, the mechanistic pathways through which climate-associated abiotic stressors regulate cyanotoxin biosynthesis remain insufficiently resolved. This review critically synthesizes current knowledge on how key environmental drivers, including rising temperatures, elevated CO₂ concentrations, nutrient enrichment, ultraviolet (UV) radiation, and hydrological variability regulate toxin production at physiological and molecular levels. Evidence supports that temperature influences toxin biosynthesis through complex regulation of gene expression, while elevated CO₂ has been proposed to alter intracellular carbon allocation and may shift toxin composition towards more bioactive variants. Nutrient availability, particularly nitrogen, modulates toxin synthesis through global regulatory networks such as NtcA in Microcystis, whereas UV radiation induces oxidative stress responses have been hypothesized to be linked to toxin release through programmed cell death pathways. Importantly, this review emphasizes that these stressors rarely act in isolation; instead, their interaction can produce synergetic or antagonistic effects that fundamentally reshape bloom toxicity. A key contribution of this review is the identification of persistent inconsistencies across studies, particularly regarding per-cell toxin quota responses under different environmental conditions. By integrating molecular mechanisms with ecological observations, this review provides a more nuanced framework for understanding how climate change drives cyanotoxin dynamics. Such mechanistic understanding is essential for improving predictive models of bloom risk and developing adaptive management strategies to mitigate the growing threats posed by cHABs under future climate scenarios.