J.N. is a holder of a Royal Society University Research Fellowship. J.N. and J.B. thank the Biotechnology and Biological Research Sciences (BBSRC) for funding and the technical assistance of Karim Maghlaoui for preparation of the cyanobacterial cultures. N.C. is grateful to the Leverhulme Trust. We thank the staff at the SRS and Station 14.1 for their technical help, and to the UK Research Councils for the supply of beam-time.