Relationship Between Glycerolipids and Photosynthetic Components During Recovery of Thylakoid Membranes From Nitrogen Starvation-Induced Attenuation in Synechocystis sp. PCC 6803

DesC1 and DesC2, Δ9 Fatty Acid Desaturases of Filamentous Cyanobacteria: Essentiality and Complementarity

DesC1 and DesC2, which are fatty acid desaturases found in cyanobacteria, are responsible for introducing a double bond at the Δ9 position of fatty-acyl chains, which are subsequently esterified to the sn-1 and sn-2 positions of the glycerol moiety, respectively. However, since the discovery of these two desaturases in the Antarctic cyanobacterium Nostoc sp. SO-36, no further research has been reported. This study presents a comprehensive characterization of DesC1 and DesC2 through targeted mutagenesis and transformation using two cyanobacteria strains: Anabaena sp. PCC 7120, comprising both desaturases, and Synechocystis sp. PCC 6803, containing a single Δ9 desaturase (hereafter referred to as DesCs) sharing similarity with DesC1 in amino acid sequence. The results suggested that both DesC1 and DesC2 were essential in Anabaena sp. PCC 7120 and that DesC1, but not DesC2, complemented DesCs in Synechocystis sp. PCC 6803. In addition, DesC2 from Anabaena sp. PCC 7120 desaturated fatty acids esterified to the sn-2 position of the glycerol moiety in Synechocystis sp. PCC 6803.

Sustainability of in vitro light-dependent NADPH generation by the thylakoid membrane of Synechocystis sp. PCC6803

Background

NADPH is used as a reductant in various biosynthetic reactions. Cell-free bio-systems have gained considerable attention owing to their high energy utilization and time efficiency. Efforts have been made to continuously supply reducing power to the reaction mixture in a cyclical manner. The thylakoid membrane (TM) is a promising molecular energy generator, producing NADPH under light. Thus, TM sustainability is of major relevance for its in vitro utilization.

Results

Over 70% of TMs prepared from Synechocystis sp. PCC6803 existed in a sealed vesicular structure, with the F₁ complex of ATP synthase facing outward (right-side-out), producing NADPH and ATP under light. The NADPH generation activity of TM increased approximately two-fold with the addition of carbonyl cyanide-p-(trifluoromethoxy) phenylhydrazone (FCCP) or removal of the F₁ complex using EDTA. Thus, the uncoupling of proton translocation from the electron transport chain or proton leakage through the F_o complex resulted in greater NADPH generation. Biosilicified TM retained more than 80% of its NADPH generation activity after a week at 30°C in the dark. However, activity declined sharply to below 30% after two days in light. The introduction of engineered water-forming NADPH oxidase (Nox^m) to keep the electron transport chain of TM working resulted in the improved sustainability of NADPH generation activity in a ratio (Nox^m to TM)-dependent manner, which correlated with the decrease of singlet oxygen generation. Removal of reactive oxygen species (ROS) by catalase further highlighted the sustainable NADPH generation activity of up to 80% in two days under light.

Conclusion

Reducing power generated by light energy has to be consumed for TM sustainability. Otherwise, TM can generate singlet oxygen, causing oxidative damage. Thus, TMs should be kept in the dark when not in use. Although NADPH generation activity by TM can be extended via silica encapsulation, further removal of hydrogen peroxide results in an improvement of TM sustainability. Therefore, as long as ROS formation by TM in light is properly handled, it can be used as a promising source of reducing power for in vitro biochemical reactions.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01825-1.

Photosynthesis and Cell Growth Trigger Degradation of Phycobilisomes during Nitrogen Limitation

Under nitrogen (N)-limited conditions, the non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803) actively grows during early stages of starvation by performing photosynthesis but gradually stops the growth and eventually enters dormancy to withstand long-term N limitation. During N limitation, Synechocystis 6803 cells degrade the large light-harvesting antenna complex phycobilisomes (PBSs) presumably to avoid excess light absorption and to reallocate available N to essential functions for growth and survival. These two requirements may be driving forces for PBS degradation during N limitation, but how photosynthesis and cell growth affect PBS degradation remains unclear. To address this question, we examined involvements of photosynthesis and cell growth in PBS degradation during N limitation. Treatment of photosynthesis inhibitors and shading suppressed PBS degradation and caused non-bleaching of cells during N limitation. Limitations of photosynthesis after initial gene responses to N limitation suppressed PBS degradation, implying that photosynthesis affects PBS degradation in a post-translational manner. In addition, limitations of cell growth by inhibition of peptidoglycan and fatty acid biosynthesis, low growth temperature and phosphorous starvation suppressed PBS degradation during N limitation. Because decreased photosynthetic activity led to decreased cell growth, and vice versa, photosynthesis and cell growth would inseparably intertwine each other and affect PBS degradation during N limitation in a complex manner. Our data shed light on the coordination mechanisms among photosynthesis, cell growth and PBS degradation during N limitation.

Differential Membrane Lipid Profiles and Vibrational Spectra of Three Edaphic Algae and One Cyanobacterium

The membrane glycerolipids of four phototrophs that were isolated from an edaphic assemblage were determined by UPLC-MS after cultivation in a laboratory growth chamber. Identification was carried out by 18S and 16S rDNA sequencing. The algal species were Klebsormidium flaccidum (Charophyta), Oocystis sp. (Chlorophyta), and Haslea spicula (Bacillariophyta), and the cyanobacterium was Microcoleus vaginatus (Cyanobacteria). The glycerolipid profile of Oocystis sp. was dominated by monogalactosyldiacylglycerol (MGDG) species, with MGDG(18:3/16:4) accounting for 68.6%, whereas MGDG(18:3/16:3) was the most abundant glycerolipid in K. flaccidum (50.1%). A ratio of digalactosyldiacylglycerol (DGDG) species to MGDG species (DGDG/MGDG) was shown to be higher in K. flaccidum (0.26) than in Oocystis sp. (0.14). This ratio increased under high light (HL) as compared to low light (LL) in all the organisms, with its highest value being shown in cyanobacterium (0.38-0.58, LL-HL). High contents of eicosapentaenoic acid (EPA, C20:5) and hexadecenoic acid were observed in the glycerolipids of H. spicula. Similar Fourier transform infrared (FTIR) and Raman spectra were found for K. flaccidum and Oocystis sp. Specific bands at 1629.06 and 1582.78 cm-1 were shown by M. vaginatus in the Raman spectra. Conversely, specific bands in the FTIR spectrum were observed for H. spicula at 1143 and 1744 cm-1. The results of this study point out differences in the membrane lipid composition between species, which likely reflects their different morphology and evolutionary patterns.

Membrane lipid remodeling is required for photosystem II function under low CO2

Membrane lipid remodeling in plants and microalgae has a crucial role in their survival under nutrient-deficient conditions. Aquatic microalgae have low access to CO₂ , an essential carbon source for photosynthetic assimilates; however, 70-90 mol% of their membrane lipids are sugar-derived lipids (glycolipids) such as monogalactosyldiacylglycerol (MGDG). In this study, we discovered a new system of membrane lipid remodeling responding to CO₂ in Synechocystis sp. PCC 6803, a unicellular, freshwater cyanobacterium. As compared with higher CO₂ (HC; 1% CO₂ ), under ambient air (lower CO₂ : LC), phosphatidylglycerol (PG) content was increased at the expense of MGDG content. To explore the biological significance of this alteration in content, we generated a transformant of Synechocystis sp. PCC 6803 overexpressing sll0545 gene encoding a putative phosphatidic acid phosphate (oxPAP), which produces diacylglycerol that is used for the synthesis of glycolipids, and examined the effect on membrane lipid remodeling and phototrophic growth responding to LC. Photosystem II (PSII) activity and growth rate were inhibited under LC in oxPAP cells. PG content was substantially reduced, and MGDG and sulfoquinovosyldiacylglycerol contents were increased in oxPAP cells as compared with control cells. These phenotypes in oxPAP cells were recovered under the HC condition or PG supplementation. Increased PG content may be required for proper functioning of PSII under LC conditions.

Lipids Composition in Plant Membranes

The paper focuses on the selected plant lipid issues. Classification, nomenclature, and abundance of fatty acids was discussed. Then, classification, composition, role, and organization of lipids were displayed. The involvement of lipids in xantophyll cycle and glycerolipids synthesis (as the most abundant of all lipid classes) were also discussed. Moreover, in order to better understand the biomembranes remodeling, the model (artificial) membranes, mimicking the naturally occurring membranes are employed and the survey on their composition and application in different kind of research was performed. High level of lipids remodeling in the plant membranes under different environmental conditions, e.g., nutrient deficiency, temperature stress, salinity or drought was proved. The key advantage of lipid research was the conclusion that lipids could serve as the markers of plant physiological condition and the detailed knowledge on lipids chemistry will allow to modify their composition for industrial needs.