Grazer‐induced toxin production is energetically costly and significantly reduces growth of cylindrospermopsin‐producing cyanobacteria

Toxins have been proposed as a defense mechanism for phytoplankton against zooplankton grazing. However, the associated costs are not understood. Here, we aim to reveal the costs in terms of both physiological and molecular responses in the cylindrospermopsin (CYN)‐producing cyanobacterium Raphidiopsis raciborskii. Exposure of R. raciborskii to Daphnia magna resulted in a significant increase in its CYN cell quota which was grazer cell density‐dependent, indicating an inducible effect on CYN production. A negative and significant relationship between cellular growth rate and CYN production suggested a growth‐defense trade‐off, highlighting the costly nature of toxin production. Despite strong transcriptional responses (121 differentially expressed genes) in R. raciborskii under grazing pressure, the CYN gene cluster remained largely unaffected, implying post‐transcriptional regulation of CYN biosynthesis. Significant variation was observed in transcripts for phosphorus metabolism and photosynthesis‐related genes, and evidenced that energy (ATP) metabolic processes were stimulated. This study is the first to demonstrate the high energy requirements for CYN production and provide insight into the molecular basis underlying the costs of toxin production.