Fig. 1. Fluorescence excitation spectrum of the SoNar protein-based biosensor with two typical peaks at 420 nm and 490 nm. When NADH is added, fluorescence intensity increases at 420 nm and weakens at 490 nm. When NAD+ is added, signal intensity increases at 490 nm. The SoNar biosensor has one emission peak with emission maximum at 518 nm

Fig. 2. Changes in the NAD+/NADH ratio and pH in the tissues of one-day old D. rerio embryos before and after pyruvate and lactate injections into the yolk sac. (A and D) Images of a D. rerio embryo body before and after injections of 1 nl 200 mM lactate (A) and pyruvate (D) solutions. Images are shown in pseudo-colors corresponding to the SoNar signal values (F420/F490). Zones 1 and 2 circled in white in the first picture indicate regions on the embryo’s body where F420/F490 fluctuations were registered. These regions were the same for all inspected fish. (B and E) Changes in the NAD+/NADH ratio measured using the SoNar biosensor in the yolk sac (red line) and body (black line) of the D. rerio embryo before and after injections of lactate (B) and pyruvate (E) solutions. (C and F) Changes in pH registered using the SypHer2 in the yolk sac (red line) and body (black line) of a D. rerio embryo before and after injections of lactate (C) and pyruvate (F). For all graphs (B, C, E, F) the error bar corresponds to the standard deviation. Graph (C) was constructed based on the imaging of 10 fish. Graph (E) — based on the imaging of 11 fish. Graphs (C) and (F) — based on the imaging of 6 animals (per graph)