Non-photochemical chlorophyll fluorescence quenching and structural rearrangements induced by low pH in intact cells of Chlorella fusca (Chlorophyceae) and Mantoniella squamata (Prasinophyceae)

We have used circular dichroism (CD) spectroscopy and chlorophyll fluoresce nce induction measurements in order to examine low-pH-induced changes in th e chiral macro-organization of the chromophores and in the efficiency of no n-photochemical quenching of the chlorophyll a fluorescence (NPQ) in intact , dark-adapted cells of Chlorella fusca (Chlorophyceae) and Mantoniella squ amata (Prasinophyceae). We found that: (i) high proton concentrations enhan ced the formation of chiral macrodomains of the complexes, i.e. the formati on of large aggregates with long-range chiral order of pigment dipoles; thi s was largely independent of the low-pH-induced accumulation of de-epoxidiz ed xanthophylls; (ii) lowering the pH led to NPQ; however, efficient energy dissipation, in the absence of excess light, could only be achieved if a s ubstantial part of violaxanthin was converted to zeaxanthin and antheraxant hin in Chlorella and Mantoniella, respectively; (iii) the low-pH-induced ch anges in the chiral macro-organization of pigments were fully reversed by t itrating the cells to neutral pH; (iv) at neutral pH, the presence of anthe raxanthin or zeaxanthin did not bring about a sizeable NPQ. Hence, low-pH-i nduced NPQ in dark adapted algal cells appears to be associated both with t he presence of de-epoxidized xanthophylls and structural changes in the chi ral macrodomains. It is proposed that the macrodomains, by providing a suit able structure for long-distance migration of the excitation energy, in the presence of quenchers associated with de-epoxidized xanthophylls, facilita te significantly the dissipation of unused excitation energy.