The organisation of photosynthetic pigments in a cryptophyte alga: A linear dichroism study

Linear dichroism spectra have been obtained for cells, thylakoid membranes, light-harvesting complex and the Photosystem I of Chroomonas. In the light-harvesting complex, a form of chlorophyll c (chl c) shows strong dichroism at 648 nm, the O_y chlorophyll a (chl a) transition is split at low temperature but only that absorbing at 674 nm is strongly dichroic. We conclude that the 660 nm component of the Q_y transition of chl a is oriented close to 55° to the membrane normal (or the direction of the applied force) whereas that at 674 nm is at 55°, i.e., ciose to the plane of the membrane and the plane of largest cross-section of the particle. The orientation of these transitions is in agreement with that observed in the intact cells. Phycoerythrin within the thylakoid lumen is not preferentially oriented. The orientation of chl a in Photosystem I resembles that previously reported for other photosynthesis organisms, i.e., the long-wavelength Q_y transitions are oriented parallel to the membrane plane.