Temperature dependent response of microcystin-LR in acclimated Microcystis aeruginosa: highest content expected near the growth optimum

As climate change raises global temperatures and increases the frequency of cyanobacterial blooms, understanding how rising mean temperatures affect cyanotoxin content is crucial. However, no clear consensus exists, and the use of different methodologies, including different units of measurement and experimental conditions could significantly alter the yield of the relationship between temperature and toxins content. In this study, we assessed free microcystin content and cell volume in Microcystis aeruginosa PCC 7806 acclimated to seven temperatures spanning its entire thermal niche. This experimental design firstly highlighted the significant reduction in cell volume with rising temperatures between 17{degrees}C and 29{degrees}C. As a result, when the microcystin concentration was normalized by its cell volume, its temperature response was transformed from a negative correlation to a bell-shaped curve, with higher free MC-LR content measured at an estimated optimum temperature of 26.2{degrees}C, close to the thermal growth optimum of Microcystis aeruginosa. These findings provide new insights into the effects of climate warming on microcystin content. ImportanceMicrocystin-LR is a widespread cyanotoxin, originally known for its liver toxicity. In freshwater environments, cyanotoxins are an increasing concern as harmful cyanobacterial blooms become more frequent with rising global temperatures. Microcystis aeruginosa, a common bloom-forming species found worldwide, is a major producer of microcystin-LR. Understanding how environmental factors such as temperature influence toxin content in this species is essential for predicting bloom toxicity under future climate scenarios. However, current knowledge remains fragmented due to numerous factors that can influence its production and also to different way of measuring toxins and expressing their concentrations (cell or {micro}m3). Confirming that temperature greatly modifies biovolume of M. aeruginosa, this study offers new insights by highlighting the importance of considering cell volume when evaluating toxin content. Integrating changes in cell size helps reconcile earlier conflicting results and contributes to a more accurate understanding of how temperature affects toxin production in cyanobacteria. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=176 HEIGHT=200 SRC="FIGDIR/small/660678v1_ufig1.gif" ALT="Figure 1"> View larger version (31K): org.highwire.dtl.DTLVardef@7a591org.highwire.dtl.DTLVardef@6eb39org.highwire.dtl.DTLVardef@3d0f19org.highwire.dtl.DTLVardef@aa6be0_HPS_FORMAT_FIGEXP M_FIG C_FIG