J.B. thanks the Biotechnology and Biological Sciences Research Council for financial support. J.N. holds a University Research Fellowship of The Royal Society. F.P. and I.M. were supported by the EU FPS program Margenes, and I.M. was awarded a Ph.D. grant from Region Bretagne. We thank F. Le Gall for preparing large volumes of Prochlorococcus cultures.

Correspondence and requests for materials should be addressed to J.B. (j.barber@imperial.ac.uk).