Cyanobacteria experience drastic changes in their carbon metabolism under daily light/dark cycles. During the day, the Calvin-Benson cycle fixes CO 2 and diverts excess carbon into glycogen storage. At night, glycogen is degraded to support cellular respiration. The dark/light transition represents a universal environmental stress for cyanobacteria and other photosynthetic lifeforms. Recent studies revealed the essential genetic background necessary for the fitness of cyanobacteria during diurnal growth. However, the metabolic processes underlying the dark/light transition are not well understood. In this study, we observed that glycogen metabolism supports photosynthesis in the cyanobacterium Synechococcus elongatus PCC 7942 when photosynthesis reactions start upon light exposure. Compared with the wild type, the glycogen mutant ΔglgC showed a reduced photosynthetic efficiency and a slower P700 1 rereduction rate when photosynthesis starts. Proteomic analyses indicated that glycogen is degraded through the oxidative pentose phosphate (OPP) pathway during the dark/light transition. We confirmed that the OPP pathway is essential for the initiation of photosynthesis and further showed that glycogen degradation through the OPP pathway contributes to the activation of key Calvin-Benson cycle enzymes by modulating NADPH levels. This strategy stimulates photosynthesis in cyanobacteria following dark respiration and stabilizes the Calvin-Benson cycle under fluctuating environmental conditions, thereby offering evolutionary advantages for photosynthetic organisms using the Calvin-Benson cycle for carbon fixation.

Sugar has the potential to create enough energy to power mobile electronics. Various sugar-powered fuel cells have been reported, however, most of them used pure glucose as substrate and enzymes/noble metals as catalysts. In this work, an alkaline fuel cell with cheap catalysts were constructed, and different sweet drinks were used as fuels for power generation. The influence of different substrates on the electrochemical performance was characterized under the controlled conditions. Our experimental results showed that the fuel cell fueled with carbonated soft drinks had the best performance under the conditions of 99.95 g/L chemical oxygen demand and 3M KOH. The power densities of the fuel cell fueled with different substrates decreased in the order of Pepsi (33.41 W/m2) > Sprite (28.38 W/m2) > apple juice (20.63 W/m2) > Coca (16.31 W/m2) > pear juice (15.31 W/m2) > orange juice (12.75 W/m2), which was consistent with linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) analysis. This is the first report on alkaline fuel cell (AFC) performance using different sweet drinks as substrate. These values are more than 10 times higher than those of reported microbial fuel cells. Our findings demonstrated that sweet drinks fueled alkaline fuel cells can be a promising energy source for low-power electronics.

1Cyanobacteria experience drastic changes in their carbon metabolism under daily light-2 dark cycles. In the light, the Calvin-Benson cycle fixes CO 2 and divert excess carbon into 3 glycogen storage. At night, glycogen is degraded to support cellular respiration. Dark-light 4 transition represents a universal environmental stress for cyanobacteria and other photosynthetic 5 lifeforms. Recent studies in the field revealed the essential genetic background necessary for the 6 fitness of cyanobacteria during diurnal growth. However, the metabolic engagement behind the 7 dark-light transition is not well understood. In this study, we discovered that glycogen 8 metabolism can jump-start photosynthesis in the cyanobacterium Synechococcus elongatus PCC 9 7942 when photosynthesis reactions start upon light. Compared to the wild type, the glycogen 10 mutant (∆glgC) showed much lower photosystem II efficiency and slower photosystem I-11 mediated cyclic electron flow rate when photosynthesis starts. Proteomics analyses indicated that 12 glycogen is degraded through the oxidative pentose phosphate pathway (OPPP) during dark-light 13 transition. We confirmed that the OPPP is essential for the initiation of photosynthesis, and 14 further showed that glycogen degradation through the OPPP is likely to contribute to the 15 activation of key Calvin-Benson cycle enzymes by modulating NADPH levels during the 16 transition period. This ingenious strategy helps jump-start photosynthesis in cyanobacteria 17 following dark respiration, and stabilize the Calvin-Benson cycle under fluctuating 18 environmental conditions. It has evolutionary advantages for the survival of photosynthetic 19 organisms using the Calvin-Benson cycle for carbon fixation. 20