Reviewer 1's Review

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Picoplanktonic cyanobacteria are of immense importance in driving the primary productivity of large areas of the open ocean. In a circulating water column, phytoplankton cells can be subjected to large variations in photon flux and it is critical that they optimise their capacity to absorb light, but minimise the chances of photoinhibition.

One way to minimise damage from excess light is to have active repair processes, and the net effect of excess light (whether it be UVB radiation or PAR) is a balance between damage and repair processes. This paper seeks to examine the photophysiological tolerance of Synechococcus and Prochlorococcus isolates from different habitats.

The work is nicely designed and carried out. The results are clearly presented and provide some useful insights into the differential responses of Synechococcus vs Prochlorococcus to high photon fluxes, and their ecological significance - these are discussed logically and thoroughly.

The concept of photodamage being a dynamic balance between damage and repair is not however novel in itself, though it is used to good effect here, and dates back to the work of Kok. It has since been used to describe PAR and UVB photoinhibition in a number of algae, and it was a little curious to see papers from the higher plant literature, rather than algal based papers, cited in the references. It might have also been possible to analyse the data obtained using the Kok model, which yields rate constants for damage and repair processes. Nonetheless, the manuscript as it stands makes a useful contribution to the literature and is deserving of publication.

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N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.