Gradual Response of Cyanobacterial Thylakoids to Acute High-Light Stress—Importance of Carotenoid Accumulation

Abstract

Light plays an essential role in photosynthesis; however, its excess can cause damage to cellular components. Photosynthetic organisms thus developed a set of photoprotective mechanisms (e.g., non-photochemical quenching, photoinhibition) that can be studied by a classic biochemical and biophysical methods in cell suspension. Here, we combined these bulk methods with single-cell identification of microdomains in thylakoid membrane during high-light (HL) stress. We used Synechocystis sp. PCC 6803 cells with YFP tagged photosystem I. The single-cell data pointed to a three-phase response of cells to acute HL stress. We defined: (1) fast response phase (0–30 min), (2) intermediate phase (30–120 min), and (3) slow acclimation phase (120–360 min). During the first phase, cyanobacterial cells activated photoprotective mechanisms such as photoinhibition and non-photochemical quenching. Later on (during the second phase), we temporarily observed functional decoupling of phycobilisomes and sustained monomerization of photosystem II dimer. Simultaneously, cells also initiated accumulation of carotenoids, especially ɣ–carotene, the main precursor of all carotenoids. In the last phase, in addition to ɣ-carotene, we also observed accumulation of myxoxanthophyll and more even spatial distribution of photosystems and phycobilisomes between microdomains. We suggest that the overall carotenoid increase during HL stress could be involved either in the direct photoprotection (e.g., in ROS scavenging) and/or could play an additional role in maintaining optimal distribution of photosystems in thylakoid membrane to attain efficient photoprotection.

Molecular mechanisms of long-term light adaptation of an extremophilic alga Cyanidioschyzon merolae

“…The ST-related component of PSII fluorescence changes, observed in our study for eHL cells, is reminiscent ST in cyanobacteria, where illumination of dark adapted cells with low intensity blue light (mainly absorbed by Chla, light 1) leads to the similar fluorescence increase (Canonico et al, 2021). It has already been proven in cyanobacteria that this phenomenon (i.e., the fluorescence increases upon low-intensity blue light illumination) is caused by the induction of low fluorescence State 2 in the dark (Papageorgiou et al, 2007).…”

“…To this end, cells were initially exposed to low blue light (intensity 81 μmol photons m −2 s −1 , λ = 460 nm) and then high-intensity blue light (1374 μmol photons m −2 s −1 , λ = 460 nm), both accompanied by high-intensity red flashes (4000 μmol photons m −2 s −1 , duration 400 ms). The protocol followed the standard procedure used for PBS-containing cyanobacteria (Kirilovsky et al, 2014), whereby the first low-blue light illumination period is applied to induce State 2-to-State 1 transition (Canonico et al, 2021). Surprisingly, we did not observe a typical state transition behaviour (except for eHL-adapted cells) in the ‘low light’ phase of illumination.…”

“…The four values of maximal fluorescence were obtained: (1) maximal F m value of a dark adapted sample; (2) F m1 ’ value after 120 s of low intensity blue light illumination (81 μmol photons m −2 s −1 , λ = 460 nm) followed with intense red flashes illumination (4000 μmol photons m −2 s −1 , 400 ms long, λ = 620 nm); (3) F m2 ’ value after 200 s of high intensity blue light (1374 μmol photons m −2 s −1 , λ = 460 nm); (4) Recovery value F m2 ’’ after 420 s at low intensity blue light (81 μmol photons m −2 s −1 , λ = 460 nm). The used protocol followed the standard procedure used for cyanobacteria, as described in Canonico et al (2021). The maximal value of fluorescence was estimated by a multiple turnover flash (red light λ = 620 nm, 4000 μmol photons m −2 s −1 , duration 400 ms) every 10–30 s of the protocol.…”

“…We conducted confocal fluorescence imaging of intact cells adapted to LL, ML, and eHL conditions (35, 90 and 350 µE PAR, respectively). In the previous works (Steinbach et al, 2015;Konert et al, 2019;Strašková et al, 2019;Canonico et al, 2020;Canonico et al, 2021), two main types of microdomains were identified in cyanobacteriathose resembling higher plant grana (predominantly with PSII complexes and associated antenna, i.e. PBS in the case of cyanobacteria) and those resembling plant stromal lamellae (more PSI and fewer PSII complexes) (see Strašková et al, 2019 for details).…”

Exploitation of Hetero- and Phototrophic Metabolic Modules for Redox-Intensive Whole-Cell Biocatalysis

“…As mentioned above, phototrophs experience light excess under normal/natural conditions and have evolved multiple mechanisms for excessive energy dissipation ( Muramatsu and Hihara, 2012 ; Canonico et al, 2021 ). Approaches are needed to avoid such energy loss by overflow valves and to control electron flow.…”