Acclimation of Chlamydomonas reinhardtii to extremely strong light
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ORIGINAL ARTICLE
Acclimation of Chlamydomonas reinhardtii to extremely strong light Olli Virtanen1 · Sergey Khorobrykh1 · Esa Tyystjärvi1 Received: 24 April 2020 / Accepted: 17 November 2020 © The Author(s) 2020
Abstract Most photosynthetic organisms are sensitive to very high light, although acclimation mechanisms enable them to deal with exposure to strong light up to a point. Here we show that cultures of wild-type Chlamydomonas reinhardtii strain cc124, when exposed to photosynthetic photon flux density 3000 μmol m−2 s−1 for a couple of days, are able to suddenly attain the ability to grow and thrive. We compared the phenotypes of control cells and cells acclimated to this extreme light (EL). The results suggest that genetic or epigenetic variation, developing during maintenance of the population in moderate light, contributes to the acclimation capability. EL acclimation was associated with a high carotenoid-to-chlorophyll ratio and slowed down PSII charge recombination reactions, probably by affecting the pre-exponential Arrhenius factor of the rate constant. In agreement with these findings, EL acclimated cells showed only one tenth of the 1O2 level of control cells. In spite of low 1O2 levels, the rate of the damaging reaction of PSII photoinhibition was similar in EL acclimated and control cells. Furthermore, EL acclimation was associated with slow PSII electron transfer to artificial quinone acceptors. The data show that ability to grow and thrive in extremely strong light is not restricted to photoinhibition-resistant organisms such as Chlorella ohadii or to high-light tolerant mutants, but a wild-type strain of a common model microalga has this ability as well. Keywords Chlamydomonas reinhardtii · Photosystem II · PSII · High light · Extreme light · Acclimation · Light stress
Introduction Light is the driving force of photosynthesis but also a stress factor affecting both photosystems. Photosystem II (PSII) is particularly susceptible to light-induced damage, and the rate of damage is proportional to light intensity (Tyystjärvi and Aro 1996). Photoinhibition is counteracted by concurrent repair, and several biochemical mechanisms offer partial protection (for review, see Tyystjärvi 2013), but in spite of the protective mechanisms, light intensities far above saturation are expected to lower the number of active PSII units and thereby cause decrease in the photosynthetic rate. Furthermore, reactive oxygen species produced in very high light are also expected to cause oxidative damage, and oxidative repression of translational elongation (Nishiyama Y et Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11120-020-00802-2) contains supplementary material, which is available to authorized users. * Esa Tyystjärvi [email protected] 1
Department of Biochemistry/Molecular Plant Biology, University of Turku, 20014 Turku, Finland
al. 2004) directly interfering with the repair of photoinhibitory damage. Green microalgae that live in surface waters are expos
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