Upconversion Nanoparticles Expand the Photosynthetically Active Spectrum of Microalgae into the Near-Infrared for CO2 Biofixation.

The process of photosynthetic carbon fixation by microalgae represents a significant pathway for achieving green and sustainable carbon capture. However, the mismatch between the microalgal pigments' absorption spectrum and the solar spectrum severely limits light conversion efficiency. To mitigate this limitation, NaErF4:Tm(0.5%)@NaYF4 upconversion nanoparticles were employed to facilitate the conversion of near-infrared (NIR) photons into photosynthetically active radiation. This extended the usable spectral range of microalgae beyond the conventional photosynthetically active region of 400-700 nm by converting near-infrared photons into red light that can be used for photosynthesis, thereby enhancing light conversion efficiency. The biohybrid system of microalgae integrating NaErF4:Tm(0.5%)@NaYF4 exhibited a 157.41% enhancement in light absorption in the 950-1000 nm region and a 27.98% increase in the 1500-1600 nm region, accompanied by red emission at 650-700 nm that can be efficiently utilized by microalgae. The maximum electron transport rate (ETRmax) of microalgae increased by 20.18%, while ATP and NADPH contents rose by 47.06% and 19.82%, respectively, and the maximum CO2 fixation rate increased by 94.31%. Consequently, compared with the control group, the density of the microalgal biomass rose by 19.67%, while the light conversion efficiency increased by 34.99% to reach 8.4%. This work develops a biohybrid system that enhances NIR light harvesting and photosynthetic carbon fixation, elucidates the role of NIR photon upconversion in regulating photosynthetic electron transport and metabolism in microalgae, and thereby provides a pathway for efficient microalgal photosynthetic carbon capture.