Combined effects of CO₂ and nitrogen on the stoichiometry of toxin synthesis in a harmful cyanobacterium

The increase in frequency and intensity of harmful cyanobacterial blooms in freshwater ecosystems over past decades has been attributed to anthropogenic influence, notably eutrophication and climate change. Microcystis is among the most widespread cyanobacterial bloom-forming genera, some strains of which can produce a range of microcystin variants having different toxicities. The synthesis of microcystins (MCs) is closely linked to carbon and nitrogen metabolism as microcystin variants differ in their nitrogen:carbon ratio. Thus, changes in availability of both CO₂ and nitrogen may impact microcystin production and composition. While the separate effects of CO₂ and nitrogen have been documented, their combined effect is less understood. We therefore assessed the effects of a CO₂ gradient at both nitrogen-replete and -deplete conditions on cellular nitrogen and carbon contents, N:C stoichiometry and microcystin synthesis in three Microcystis aeruginosa strains. We observed an interactive effect of increasing CO₂ concentrations with nitrogen availability across strains. Specifically, with increasing CO₂ availability, cellular N:C stoichiometry decreased under nitrogen-deplete conditions from 0.14 to 0.07 but increased under nitrogen-replete conditions from 0.11 to 0.17. Although total cellular microcystin content remained largely unaffected by both CO₂ and nitrogen despite shifts in N:C stoichiometry, changes in variant composition were consistent across strains and followed a stoichiometrically predictable pattern. N-rich but less toxic microcystin variants were favored at high cellular N:C ratios (e.g. MC-RR reached up to 44 % of total MC at highest N:C ratios), whereas relatively less N-containing but more toxic variants became more prevalent at low N:C ratios (e.g. MC-LW shifted from 9 % to 36 % of total MC for one of the strains). This study shows that shifts in CO₂ and nitrogen availability affect cellular N:C stoichiometry and alter microcystin composition, which may cause changes in the toxicity of cyanobacterial blooms.