Strong-light-induced yellowing of green microalgae Chlorella: A study on molecular mechanisms of the acclimation response

Wojciech Grudziński, Izabela Krzemińska, Rafał Luchowski, Artur Nosalewicz, Wiesław I. Gruszecki

Two strains of unicellular microalgae, Chlorella protothecoides and Chlorella vulgaris, subjected to strong light conditions (400 µmol photons m-2s-1) turn yellow, as compared to the green control cells cultured at moderate light intensity (80 µmol photons m-2s-1). Such a mechanism is typically interpreted in terms of an adaptive response of algae to overexcitation conditions. In the present work, fluorescence spectroscopy and molecular imaging techniques: fluorescence lifetime imaging microscopy and Raman imaging microscopy, were applied to readdress the problem: whether this process is associated with an acclimation or rather is a manifestation of a photo-degradation process. Yellow coloration of the algal culture exposed to strong light, was found to be associated with accumulation of xanthophyll pigments, predominantly zeaxanthin. The results show that carotenoids newly synthesized in response to strong light conditions are not energetically coupled to chlorophylls and therefore are not photosynthetically active. On the other hand, over-synthesized xanthophylls can be potentially active as antioxidants, membrane stabilizing agents and, importantly, in shielding cells from intensive radiation, via the “molecular sunglasses” mechanism. The latter mechanism has been identified in a cell nucleus and concluded to protect a genetic material against photodamage. Culturing Chlorella at elevated light intensities may also be considered as an alternative source of zeaxanthin, one of the macular pigments protecting human eyes against the age related macular degeneration.