THE XANTHOPHYLL CYCLE OF MANTONIELLA-SQUAMATA CONVERTS VIOLAXANTHIN INTO ANTHERAXANTHIN BUT NOT TO ZEAXANTHIN - CONSEQUENCES FOR THE MECHANISM OF ENHANCED NONPHOTOCHEMICAL ENERGY-DISSIPATION

The prasinophycean alga Mantoniella squamata uses in vivo an incomplet e violaxanthin cycle. Although the violaxanthin cycle in Mantoniella i s capable of converting violaxanthin to zeaxanthin, in intact cells on ly antheraxanthin accumulates during periods of strong illumination. A ntheraxanthin enhances non-photochemical quenching of chlorophyll fluo rescence. Inhibition of antheraxanthin synthesis by the de-epoxidase i nhibitor dithiothreitol abolishes increased thermal energy dissipation . Antheraxanthin-dependent non-photochemical quenching, like zeaxanthi n-mediated non-photochemical quenching in higher plants, is uncoupler- sensitive. Mantoniella squamata cells cultivated at high light intensi ties contain higher amounts of violaxanthin than cells grown at low li ght. The increased violaxanthin-cycle pool size in high-light-grown Ma ntoniella cells is accompanied by higher de-epoxidation rates in the l ight and by a greater capacity to quench chlorophyl fluorescence non-p hotochemically. Antheraxanthin-dependent amplification of non-photoche mical quenching is discussed in the light of recent models developed f or zeaxanthin- and diatoxanthin-mediated enhanced heat dissipation.