Diel Variations of Extracellular Microcystin Influence the Subcellular Dynamics of RubisCO in Microcystis aeruginosa PCC 7806

The cyanobacterium Microcystis is widely known for the production of the hepatotoxin microcystin. While the aspects regarding its toxicity have been studied extensively, only little is known about the natural function of this compound. Here we show our latest findings on how microcystin interferes with the inorganic carbon metabolism in the model strain M. aeruginosa PCC 7806. Both intra- and extracellular functions as a signaling molecule are discussed, as microcystin can interact with proteins of the photosynthetic apparatus, especially with RubisCO. Diel experiments showed a direct link between microcystin and its intracellular targets, as fluctuations in the extracellular microcystin content correlated with changes in the microcystin binding pattern to intracellular proteins. Concomitantly, alterations in the accumulation of RubisCO products are occurring. Interestingly, we also observed changes in the subcellular localization of RubisCO associated with high levels of extracellular microcystin. Microcystin addition experiments demonstrated that effects of externally added microcystin appear strongest at high cell densities and high light intensities. This gives further insight into how microcystin could be part of a possible fast response mechanism to environmental changes like high light and high cell density and thus contribute to the incomparable success of Microcystis in the field.