The question of why plants are green has been revisited in several articles recently. A common theme in the discussions is to explain why photosynthesis appears to absorb less of the available green sunlight than expected. The expectation is incorrect, however, because it fails to take the energy cost of the photosynthetic apparatus into account. Depending on that cost, the red absorption band of the chlorophylls may be closely optimized to provide maximum growth power. The optimization predicts a strong influence of Fraunhofer lines in the solar irradiance on the spectral shape of the optimized absorption band, which appears to be correct. It does not predict any absorption at other wavelengths.Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-009-9522-3) contains supplementary material, which is available to authorized users.

In Phaeodactylum tricornutum Photosystem II is unusually resistant to damage by exposure to high light intensities. Not only is the capacity to dissipate excess excitations in the antenna much larger and induced more rapidly than in other organisms, but in addition an electron transfer cycle in the reaction center appears to prevent oxidative damage when secondary electron transport cannot keep up with the rate of charge separations. Such cyclic electron transfer had been inferred from oxygen measurements suggesting that some of its intermediates can be reduced in the dark and can subsequently compete with water as an electron donor to Photosystem II upon illumination. Here, the proposed activation of cyclic electron transfer by illumination is confirmed and shown to require only a second. On the other hand the dark reduction of its intermediates, specifically of tyrosine Y(D), the only Photosystem II component known to compete with water oxidation, is ruled out. It appears that the cyclic electron transfer pathway can be fully opened by reduction of the plastoquinone pool in the dark. Oxygen evolution reappears after partial oxidation of the pool by Photosystem I, but the pool itself is not involved in cyclic electron transfer.

The functional role of the Ca (2+) ion in the oxygen-evolving complex of photosystem II is not yet clear. Current models explain why the redox cycle of the complex would be interrupted after the S 3 state without Ca (2+), but the literature shows that it is interrupted after the S 2 state. Reinterpretation of the literature on methods of Ca (2+) depletion [Miqyass, M., van Gorkom, H. J., and Yocum, C. F. (2007) Photosynth. Res. 92, 275-287] led us to propose that all S-state transitions require Ca (2+). Here we confirm that interpretation by measurements of flash-induced S-state transitions in UV absorbance. The results are explained by a cation exchange at the Ca (2+) binding site that, in the absence of the extrinsic PsbP and PsbQ polypeptides, can occur in minutes in low S-states and in seconds in high S-states, depending on the concentration of the substituting cation. In the S 2(K (+)) or S 2(Na (+)) state a slow conformational change occurs that prevents recovery of the slow-exchange situation on return to a lower S-state but does not inhibit the S-state cycle in the presence of Ca (2+). The ratio of binding affinities for monovalent vs divalent cations increases dramatically in the higher S-states. With the possible exception of S 0 to S 1, all S-state transitions specifically require Ca (2+), suggesting that Ca (2+)-bound H 2O plays an essential role in a H (+) transfer network required for H (+)-coupled electron transfer from the Mn cluster to tyrosine Z.