Fate of intracellular microcystins in the cyanobacterium Microcystis aeruginosa (Chroococcales, Cyanophyceae)

The aim of the present study was to evaluate the importance of microcystin loss processes in cells of Microcystis aeruginosa, either as export from the cells or intracellular breakdown, and to draw conclusions about the function of microcystins to their producers. By supplying radioactive inorganic carbon to M. aeruginosa cultures and suppressing a considerable production of radioactive microcystin after a certain period of time, an intracellular pool of labelled microcystin was built up. The fate of this pool was studied by following its radioactivity in different time-course experiment. No evidence for losses from the intracellular microcystin pool could be found. All radioactive carbon that had been incorporated into microcystin molecules remained in the intracellular microcystin pool irrespective of whether the cyanobacterial cells were subjected to low light or high light. This suggests that under the conditions tested here, microcystins are not subjects of significant loss processes such as export from the cells or intracellular breakdown. The present study therefore failed to produce evidence for an involvement of microcystins in quorum sensing, defence against epiphytic organisms, or other functions requiring an export of the compounds into the surrounding medium. The lack of an intracellular breakdown suggests that microcystins are not involved in metabolic cellular processes.