Abstract
Microalgae and cyanobacteria are considered as important model organisms to investigate the biology of photosynthesis; moreover, they are valuable sources of biomolecules for several biotechnological applications. Understanding the species-specific traits of photosynthetic electron transport is extremely important, because it contributes to the regulation of ATP/NADPH ratio, which has direct/indirect links to carbon fixation and other metabolic pathways and thus overall growth and biomass production. In the present work, a cuvette-based setup is developed, in which a combination of measurements of dissolved oxygen, pH, chlorophyll fluorescence and NADPH kinetics can be performed without disturbing the physiological status of the sample. The suitability of the system is demonstrated using a model cyanobacterium Synechocystis sp. PCC6803, as well as biofuel-candidate microalgae species, such as Chlorella sorokiniana, Dunaliella salina and Nannochloropsis limnetica undergoing inorganic carbon (Ci) limitation. Inorganic carbon limitation, induced by photosynthetic Ci uptake under continuous illumination, caused a decrease in the effective quantum yield of PSII (Y(II)) and loss of oxygen-evolving capacity in all species investigated here; these effects were largely recovered by the addition of NaHCO3. Detailed analysis of the dark-light and light-dark transitions of NADPH production/uptake and changes in chlorophyll fluorescence kinetics revealed species- and condition-specific responses. These responses indicate that the impact of decreased Calvin-Benson cycle activity on photosynthetic electron transport pathways involving several sections of the electron transport chain (such as electron transfer via the QA-QB-plastoquinone pool, the redox state of the plastoquinone pool) can be analyzed with high sensitivity in a comparative manner. Therefore, the integrated system presented here can be applied for screening for specific traits in several significant species at different stages of inorganic carbon limitation, a condition that strongly impacts primary productivity.
Highlights
Microalgae are phototrophic organisms that play an essential role in the biogeochemical cycling of macro and microelements and are key components of freshwater and marine ecosystems
The light reactions of photosynthesis, performed by several components of the photosynthetic electron transport chain, including Photosystem II (PSII), Cytochrome b6/f, Plastocyanin, Photosystem I (PSI), together with the subsequent carbon fixation processes provide rapid regulatory mechanisms that can fine-tune the energetic balance to mitigate the detrimental effects of highly variable stress conditions
The M55 mutant has been thoroughly characterized in the past (e.g. [37, 68,69,70] reviewed in [71, 72]), applying this mutant in our studies was an important validation of the suitability of real-time monitoring of several photosynthetic parameters (changes in O2 and Y(II) dynamics), along with alterations of NADPH synthesis/ uptake kinetics, the redox state of the PQ pool and forward electron transfer kinetics in different Ci conditions
Summary
Microalgae are phototrophic organisms that play an essential role in the biogeochemical cycling of macro and microelements and are key components of freshwater and marine ecosystems. CEF involves transferring electrons from PSI to the PQ pool, mediated by the NAD(P)H PQ oxidoreductase pathway, namely the type 1 NADH dehydrogenase (NDH-1) or the type 2 NADPH dehydrogenase (NDH-2), or by the Proton Gradient Regulation/PGR5-Like Photosynthetic Phenotype 1 (PGR5/PGRL1) pathway (reviewed e.g. in [3]). This cyclic electron transfer mechanism is coupled with proton translocation from the stroma to the lumen during the Q-cycle [4], and the translocated protons are utilized by ATP synthase to produce ATP without the net synthesis of NADPH
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