| Summary: | Cyanobacterial blooms pose a serious threat to water quality and human health due to the production of the potent hepatotoxin microcystin. In microcystin-producing strains of the widespread genus Microcystis, the toxin is largely constitutively produced, but there are fluctuations between the cellular and extracellular pool and between free microcystin and protein-bound microcystin. Here we addressed the question of how different temperatures affect the growth and temporal dynamics of secondary metabolite production in the strain Microcystis aeruginosa PCC7806 and its microcystin-deficient ΔmcyB mutant. While the wild-type strain showed pronounced growth advantages at 20°C, 30°C, and 35°C, respectively, the ΔmcyB mutant was superior at 25°C. We further show that short-term incubations at 25°C–35°C result in lower amounts of freely soluble microcystin than incubations at 20°C and that microcystin congener ratios differ at the different temperatures. Subsequent assessment of the protein-bound microcystin pool by dot blot analysis and subcellular localization of microcystin using immunofluorescence microscopy showed re-localization of microcystin into the protein-bound pool combined with an enhanced condensation at the cytoplasmic membrane at temperatures above 25°C. This temperature threshold also applies to the condensate formation of the carbon-fixing enzyme RubisCO thereby likely contributing to reciprocal growth advantages of wild type and ΔmcyB mutant at 20°C and 25°C. We discuss these findings in the context of the environmental success of Microcystis at higher temperatures.
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