Dynamics of the Short-Term and Long-Term Acclimation in Large Diatom Ditylum brightwellii (T. West) Grunow to High Light Intensity

A culture of the large diatom Ditylum brightwellii (T. West) Grunow isolated from the coastal waters of the Black Sea was grown under continuous low light (17.2 µmol photons m–2 s–1). The dynamics of its acclimation to high light intensities (172 and 425 µmol photons m–2 s–1) were studied. The process of short-term acclimation (within two hours) to 172 µmol photons m–2 s–1 was carried out mainly due to increase in non-photochemical quenching of chlorophyll fluorescence (NPQ) by 5 times after 90 min with its subsequent decrease and, to a lesser extent, due to a unidirectional increase in the C/Chl a ratio by 30% by the end of the second hour. Short-term acclimation to 425 μmol photons m–2 s–1 was achieved by a rapid (after 20 min) eight-fold increase in non-photochemical quenching (NPQ) followed by a return to baseline values, intracellular chloroplasts’ aggregation within 50 min followed by a return to uniform distribution, a 50% increase in the C/Chl a ratio. Long-term acclimation (within 48 h) to both light intensities was accompanied by a 2.4–3-fold increase in the C/Chl a ratio and a 2-fold decrease in the maximal efficiency of photosystem II (Fv/Fm) and the proportion of active reaction centers (RC/ABS), and some variability of NPQ. The gradual formation of chloroplast conglomerates as an additional photoacclimation mechanism was observed throughout the entire period of long-term acclimation only at 425 µmol photons m–2 s–1.