Investigation into the CO2 concentrating step rates within the carbon concentrating mechanism of Synechocystis sp. PCC6803 at various pH and light intensities reveal novel mechanistic properties

Flv3A facilitates O2 photoreduction and affects H2 photoproduction independently of Flv1A in diazotrophic Anabaena filaments

The model heterocyst-forming filamentous cyanobacterium Anabaena sp. PCC 7120 (Anabaena) is a typical example of a multicellular organism capable of simultaneously performing oxygenic photosynthesis in vegetative cells and O₂ -sensitive N₂ -fixation inside heterocysts. The flavodiiron proteins have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O₂ (a Mehler-like reaction). Here, we performed a phenotypic and biophysical characterization of Anabaena mutants impaired in vegetative-specific Flv1A and Flv3A in order to address their physiological relevance in the bioenergetic processes occurring in diazotrophic Anabaena under variable CO₂ conditions. We demonstrate that both Flv1A and Flv3A are required for proper induction of the Mehler-like reaction upon a sudden increase in light intensity, which is likely important for the activation of carbon-concentrating mechanisms and CO₂ fixation. Under ambient CO₂ diazotrophic conditions, Flv3A is responsible for moderate O₂ photoreduction, independently of Flv1A, but only in the presence of Flv2 and Flv4. Strikingly, the lack of Flv3A resulted in strong downregulation of the heterocyst-specific uptake hydrogenase, which led to enhanced H₂ photoproduction under both oxic and micro-oxic conditions. These results reveal a novel regulatory network between the Mehler-like reaction and the diazotrophic metabolism, which is of great interest for future biotechnological applications.

Hydrogen peroxide modified and bismuth vanadate decorated titanium dioxide nanocomposite (BiVO4@HMT) for enhanced visible light photocatalytic growth inhibition of harmful cyanobacteria in water

The persistence of harmful cyanobacterial algal blooms in aquatic ecosystems leads to health damage for various life forms. In this study, a photocatalyst active in the visible light range was prepared by combining BiVO4 with hydrogen peroxide modified titanium dioxide (BiVO4@HMT; for short), using an impregnation method. The catalyst was used to photocatalytically inhibit the growth of cyanobacteria collected from a bloom site. To infer the optimum pH for cyanobacterial growth, the effect of pH was studied. The growth of cyanobacteria was favoured in an alkaline environment at pH values in the range of 8-9.5 when analysed on the 20th day of incubation. Structural and chemical analysis of pristine and composite nano-powders was performed using XRD, SEM, TEM and XPS, confirming the heterojunction formation, while optical and band gap analysis revealed increased visible light absorption and reduced band gap of the composite. A small strawberry seed-like assembly of BiVO4 particles increased the light absorption in the 15%BiVO4@HMT composite and increased the inhibition efficiency up to 2.56 times compared to pristine HMT at an exposure time of 6 h and cell concentration at 0.1 g L-1 with an optimum catalyst dose of 1 g L-1. The amount of chlorophyll 'a' decreased due to the generation of catalytically reactive species, especially holes (h+), which caused oxidative damage to the cell wall, cell membrane and antioxidants in algal cells. This study reports that visible light active nanocatalysts can be used as a promising method for reducing algal blooms in water bodies.

Allophycocyanin A is a carbon dioxide receptor in the cyanobacterial phycobilisome

Light harvesting is fundamental for production of ATP and reducing equivalents for CO₂ fixation during photosynthesis. However, electronic energy transfer (EET) through a photosystem can harm the photosynthetic apparatus when not balanced with CO₂. Here, we show that CO₂ binding to the light-harvesting complex modulates EET in photosynthetic cyanobacteria. More specifically, CO₂ binding to the allophycocyanin alpha subunit of the light-harvesting complex regulates EET and its fluorescence quantum yield in the cyanobacterium Synechocystis sp. PCC 6803. CO₂ binding decreases the inter-chromophore distance in the allophycocyanin trimer. The result is enhanced EET in vitro and in live cells. Our work identifies a direct target for CO₂ in the cyanobacterial light-harvesting apparatus and provides insights into photosynthesis regulation.

The transfer of electronic energy through a photosystem can harm the photosynthetic apparatus when not balanced with CO₂ fixation. Here, the authors show that CO₂ modulates electronic energy transfer in cyanobacteria by binding to and enhancing the activity of the light-harvesting complex.

Regulation of carbon source metabolism in mixotrophic microalgae cultivation in response to light intensity variation

Microalgae are one of the promising sources for renewable energy production, and the light intensity variation can affect the biofuel generation and carbon assimilation of mixotrophic microalgae. To reveal the response of carbon assimilation to light intensity, the effect of light intensity on the carbon source metabolism of Chlorella vulgaris under mixotrophic cultivation was investigated in this study. Moreover, the optimal carbon source composition for mixotrophic microalgae cultivation was evaluated using bicarbonate (HCO₃^-) and carbonate (CO₃^2-) as inorganic carbon sources, and glucose and acetate as organic carbon sources. The optimal light intensity for Chlorella vulgaris growth was at the range of 8000-12000 lux. For the accumulation of biochemical components, low light intensity was beneficial to protein accumulation, and high light intensity was advantageous for carbohydrate and lipid accumulation. With HCO₃^- and glucose, the maximum lipid content reached 37.0% at a light intensity of 12000 lux. The citrate synthase activity was negatively correlated with light intensity, showing an opposite trend to biomass production. High light intensity had a positive impact on Rubisco expression, which promoted the microalgae growth and carbon fixing. The energy produced by heterotrophic metabolic activities increased at low light intensity, and the enhancement of biomass production with high light intensity was mainly caused by the improved photoreaction efficiency during the mixotrophic cultivation.

Functional redundancy between flavodiiron proteins and NDH-1 in Synechocystis sp. PCC 6803

In oxygenic photosynthetic organisms, excluding angiosperms, flavodiiron proteins (FDPs) catalyze light-dependent reduction of O₂ to H₂ O. This alleviates electron pressure on the photosynthetic apparatus and protects it from photodamage. In Synechocystis sp. PCC 6803, four FDP isoforms function as hetero-oligomers of Flv1 and Flv3 and/or Flv2 and Flv4. An alternative electron transport pathway mediated by the NAD(P)H dehydrogenase-like complex (NDH-1) also contributes to redox hemostasis and the photoprotection of photosynthesis. Four NDH-1 types have been characterized in cyanobacteria: NDH-1₁ and NDH-1₂ , which function in respiration; and NDH-1₃ and NDH-1₄ , which function in CO₂ uptake. All four types are involved in cyclic electron transport. Along with single FDP mutants (∆flv1 and Δflv3) and the double NDH-1 mutants (∆d1d2, which is deficient in NDH-1_1,2 and ∆d3d4, which is deficient in NDH-1_3,4 ), we studied triple mutants lacking one of Flv1 or Flv3, and NDH-1_1,2 or NDH-1_3,4 . We show that the presence of either Flv1/3 or NDH-1_1,2 , but not NDH-1_3,4 , is indispensable for survival during changes in growth conditions from high CO₂ /moderate light to low CO₂ /high light. Our results show functional redundancy between FDPs and NDH-1_1,2 under the studied conditions. We suggest that ferredoxin probably functions as a primary electron donor to both Flv1/3 and NDH-1_1,2 , allowing their functions to be dynamically coordinated for efficient oxidation of photosystem I and for photoprotection under variable CO₂ and light availability.

Towards a quantitative assessment of inorganic carbon cycling in photosynthetic microorganisms

Photosynthetic organisms developed various strategies to mitigate high light stress. For instance, aquatic organisms are able to spend excessive energy by exchanging dissolved CO2 (dCO2) and bicarbonate ( HCO 3 - ) with the environment. Simultaneous uptake and excretion of the two carbon species is referred to as inorganic carbon cycling. Often, inorganic carbon cycling is indicated by displacements of the extracellular dCO2 signal from the equilibrium value after changing the light conditions. In this work, we additionally use (i) the extracellular pH signal, which requires non- or weakly-buffered medium, and (ii) a dynamic model of carbonate chemistry in the aquatic environment to detect and quantitatively describe inorganic carbon cycling. Based on simulations and experiments in precisely controlled photobioreactors, we show that the magnitude of the observed dCO2 displacement crucially depends on extracellular pH level and buffer concentration. Moreover, we find that the dCO2 displacement can also be caused by simultaneous uptake of both dCO2 and HCO 3 - (no inorganic carbon cycling). In a next step, the dynamic model of carbonate chemistry allows for a quantitative assessment of cellular dCO2, HCO 3 - , and H+ exchange rates from the measured dCO2 and pH signals. Limitations of the method are discussed.