Photoinactivation of Photosystem II in <i>Prochlorococcus</i> and <i>Synechococcus</i>

<div><p>The marine picocyanobacteria <i>Synechococcus</i> and <i>Prochlorococcus</i> numerically dominate open ocean phytoplankton. Although evolutionarily related they are ecologically distinct, with different strategies to harvest, manage and exploit light. We grew representative strains of <i>Synechococcus</i> and <i>Prochlorococcus</i> and tracked their susceptibility to photoinactivation of Photosystem II under a range of light levels. As expected blue light provoked more rapid photoinactivation than did an equivalent level of red light. The previous growth light level altered the susceptibility of <i>Synechococcus</i>, but not <i>Prochlorococcus</i>, to this photoinactivation. We resolved a simple linear pattern when we expressed the yield of photoinactivation on the basis of photons delivered to Photosystem II photochemistry, plotted versus excitation pressure upon Photosystem II, the balance between excitation and downstream metabolism. A high excitation pressure increases the generation of reactive oxygen species, and thus increases the yield of photoinactivation of Photosystem II. Blue photons, however, retained a higher baseline photoinactivation across a wide range of excitation pressures. Our experiments thus uncovered the relative influences of the direct photoinactivation of Photosystem II by blue photons which dominates under low to moderate blue light, and photoinactivation as a side effect of reactive oxygen species which dominates under higher excitation pressure. <i>Synechococcus</i> enjoyed a positive metabolic return upon the repair or the synthesis of a Photosystem II, across the range of light levels we tested. In contrast <i>Prochlorococcus</i> only enjoyed a positive return upon synthesis of a Photosystem II up to 400 μmol photons m^-2 s^-1. These differential cost-benefits probably underlie the distinct photoacclimation strategies of the species.</p>