The localization of exogenous microcystin LR taken up by a non-microcystin producing cyanobacterium

•The manuscript contains data to support microcystin internalization.•The manuscript reports on localization of internalized exogenous microcystin.•The locale of internalized microcystin relates to its proposed function.•The function of internalized microcystin is identical to that of endogenous microcystin.

The effect of exogenous microcystin on non-microcystin producing cyanobacteria has not yet been extensively studied. Existing evidence for internalization of microcystin by cyanobacteria is based only on the presence of internalized radioisotopic label. Where a function or physiological role for microcystin has been proposed based on the molecule acting as a signalling molecule, the hypothetical function has not been demonstrated at the site of action in receiving cells. We therefore exposed Synechocystis PCC6803 to microcystin LR and showed that the microcystin-LR was both taken up by Synechocystis PCC6803 and localised in the thylakoid membranes, where it caused a decrease in photosystem II activity as has been shown for endogenous microcystin, without any negative effects on the cell's survival.