Photoadaptation in the green alga Spongiochloris sp. A three-fluorometer study

Abstract

The dark-adapted cells of the green alga Spongiochloris sp. were exposed to 'white light' of 1000 μmol(photon) m-2 s-1 for 2 h and then dark adapted for 1.5 h. Changes of photochemical activities during photoadaptation were followed by measurement of chlorophyll (Chl) fluorescence kinetics, 77 K emission spectra, photosynthetic oxygen evolution, and pigment composition. We observed a build-up of slowly-relaxing non-photochemical quenching which led to a decrease of the F(v)/F(m) parameter and the connectivity. In contrast to the depression of F(v)/F(m) (35 %) and the rise of non-photochemical quenching (~ 1.6), we observed an increase in effective absorption cross-section (20 %), Hill reaction (30 %), photosynthetic oxygen evolution (80 %), and electron transport rate estimated from the Chl fluorescence analysis (80 %). We showed an inconsistency in the presently used interpretation schemes, and ascribe the discrepancy between the increase of effective absorption cross-section and the photosynthetic activities on one side and the effective nonphotochemical quenching on the other side to the build-up of a quenching mechanism which dissipates energy in closed reaction centres. Such a type of quenching changes the ratio between thermal dissipation and fluorescence without any effect on photochemical yield. In this case the F(v)/F(m) ratio cannot be used as a measure of the maximum photochemical yield of PS2.