Effects of temperature variation on the plasticity of carbon fixation in Microcystis

Microcystis aeruginosa PCC7806 is a bloom forming cyanobacteria, distributed globally in inland water. M. aeruginosa produces potent hepatotoxin microcystin. In eukaryotes, microcystin inhibits a class of enzymes known as protein phosphatases, especially PP-1 and PP-2A, leading to liver toxicity and tumour formation. Earlier studies have suggested both in-vivo and in-vitro binding of microcystin to specific proteins from M. aeruginosa such as the key enzyme of the Calvin-Benson cycle, RubisCO. High cell density, high light conditions as well as oxidative stress were shown to promote binding of the toxin to proteins. However, the effect of temperature on microcystin binding to proteins and carbon metabolism is still unknown. Recently M. aeruginosa cells were adjusted to low light conditions and subsequently exposed to varying temperature ranging from 20°C-35°C. We have observed striking differences in the dynamics of microcystin protein binding as well as secretion of microcystin. Furthermore, immunofluorescence microscopic analysis of subcellular localization of RubisCO and the carboxysome shell protein CcmK revealed pronounced variations at different temperatures and enhanced bio-condensate formation of carboxysomes at higher temperature when compared to its normal distribution in cytosol at low temperature. Our experiments suggest a substantial influence of elevated temperature on microcystin dynamics and carbon fixation and provide insights into the possible role of microcystin in adaptation to changing climatic conditions.