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.