Multi-omics reveals mechanisms of total resistance to extreme illumination of a desert alga

The desert green algaeChlorella ohadiithrives at excessively high light intensities by exceptionally enhancing the mechanisms that protect photosynthesis from photoinhibition

“…In order to prevent self-shading of the cells, the OD750 was maintained below 0.7 for the entire growth time (about 6 generations) by diluting the culture. C. ohadii is one of the most rapidly reproducing photosynthetic eukaryotes, with a reported division time as rapid as 2 h, but was found to be about 4 h in our growth system (Table 1) (Treves et al, 2017;Treves et al, 2020). Under both LL and HL, the cells grew rapidly, displaying the same generation time of about 4 h, demonstrating the high tendency of C. ohadii to thrive under extreme HL (Table 1 and Figure S1).…”

“…Notably, the LHCSR genes are present in another Chlorella species, C. vulgaris (Cecchin et al, 2019). However and as noted above, two psbS genes are present and their transcripts are elevated in high light treated cells (Treves et al, 2020). The corresponding proteins are presumably involved in the significant NPQ observed in HL cells.…”

“…Although the non-photochemical chlorophyll fluorescence quenching (NPQ) is very low as compared to plants and other green algae, presumably due to the lack of the stress-related LHC (LHCSR) proteins in this strain (Treves et al, 2016), it was three-fold higher in HL as compared to LL cells (Table 1 and Figure 1). Two genes encoding psbS, another component of the NPQ system, are present in the genome and their transcripts accumulated in HL (Treves et al, 2020). The HL cells and thylakoids were more yellowish and olive-colored than that of deep green color of LL cells, reflecting the significant accumulation of carotenoids (see below).…”

“…It should be noted that the graph of the QA relaxation kinetics of the HL cells was expanded and normalized to that of LL in order to determine the half-life of the relaxation time of the forward electron flow (Figure 1d). Since C. ohadii lacks the light-harvesting stress-related (LHCSR) complexes, the quenching can be ascribed to the significant accumulation of carotenoids, and/or the activity of psbS, which is over-expressed in high light (Girolomoni et al, 2019;Pinnola and Griffiths, 2019;Larkum et al, 2020;Rochaix and Bassi, 2019;Nicol et al, 2019;Głowacka et al, 2018;Tian et al, 2019;Treves et al, 2020). Even though the fluorescence signal was quenched in HL cells, the rate of linear PSII electron flow continued to be high, as observed by the similar Fv/Fm, QA relaxation half-life time and PSII electron transport rates, indicating the resistance of C. ohadii to PI while growing for many generations at HL.…”

“…Proteomics analysis and immunoblotting were performed to analyze protein profiles of HL and LL thylakoids. Since the C. ohadii genome sequences were not yet available (NCBI accession PRJNA573576) (Treves et al, 2020), the proteomics analysis and the identification of the proteins was accomplished using the genomic nucleotide sequence of its close relative, C. sorokiniana (ENA accession LHPG02000000) (Arriola et al, 2018). For the immunoblotting analysis, we used commercial antibodies generated against Arabidopsis LHCII and LHCI proteins.…”