**Acknowledgements**

This work was supported by grants from the DFG (Fo195/9), the research training group GRK 1708 and the Baden-Württemberg foundation grant 7533-10-5-92B. We thank Iris Maldener for provision of the electron micrographs of *Anabaena* sp. PCC 7120 and *Synechocystis* sp. PCC 6803. We would also like to give thanks to Rebeca Pérez for provision of light micrographs of *Anabaena variabilis ATCC 29413* and *Nostoc punctiforme ATCC 29133.*
