Non-photochemical quenching in an Antarctic unicellular green alga

Abstract

Chlorophyll-a PSII fluorescence was measured in a cell suspension of the unicellular green alga Koliella anctartica (temporary denomination) harvested in the Ross sea at the Italian base for Antarctic research. Brought to Italy, the alga was grown at 1 C and kept under salinity, temperature and light conditions characteristic of its original environment. PSII fluorescence emission was characterised by an unusually high Fo and low Fv. This indicates that, even when all the PSII reaction centers are open (dark adapted sample) only a small fraction of the excitation absorbed by the antenna is converted into charge separation (QA reduction). When actinic light was turned on, a maximum fluorescence level Fp was rapidly attained and a non-photochemical quencing was quickly built up bringing the fluorescence emission to a steady state level Fs well below the Fo level. When the actinic light was turned off, after a fast decrease to a Fo¿ level, the fluorescence increased back exponentially approaching Fo (t1/2 of the order of 3-4 s at 20 C). The time course of fluorescence induction described above is fully reversible in such that several cycles Fo ¿ Fm ¿ Fs ¿ Fo¿ ¿ Fo can be sequentially performed with little sign of irreversible photoinhibition. Giving the value of 100 to Fm (as obtained by a 1s saturating flash), the typical values observed for the fluorescence levels above, when a 680 nm actinic light of 38 m E m-2 s-1 is used, are the following: Fo = 75; Fp = 92; Fs = 55; Fo¿= 52. Generation of the described quencing is abolished or significantly reduced by addition of DCMU or of uncoupling agents such as ammonium sulphate, showing that the observed QNP is dependent on membrane energization (QE) in agreement with its relaxation time. It is tempting to interpret this behaviour as an adaptation strategy for an autothrophic organism which utilizes absorbed light energy to build up the necessary chemical free energy and reduction potential to bring about biosynthetic work, but also needs to use a large portion of the excitation absorbed by PSII antenna to locally and transiently increase its temperature to make biosynthetic activity faster as soon as photosynthesis is saturated.