Photosynthetic vs Photovoltaic Efficiency of Limnospira Indica, Perspective Cyanobacteria Strain for Space Mission Live Support Systems.

Due to their excellent photosynthetic capability, which can be directly and efficiently applied in O₂ generation, CO₂ utilization, food and chemical production, as well as the photoelectrochemical production of fuels, cyanobacteria are being utilized by space agencies to establish life support systems in future space stations, planetary stations, and long-haul space missions [1].<br/>In the harsh conditions of space, the use of live cyanobacteria is preferred over cellular components because cyanobacteria can adapt to and cope with stress, including ionizing radiation, enabling long-term autonomous operation.<br/>Protein-pigment complexes located in intracellular structures (the thylakoid membranes) capture and transform photonic energies (in the wavelength range of 400-700 nm), leading to the liberation of electrons from water into an intricate electron flow that can be utilized for photosynthesis or driven into an external circuit of a photovoltaic or photoelectrochemical cell. Excess energy is also dissipated as heat. The productivity of photosynthesis can be estimated through a combination of electrochemical and spectroscopic methods. In an attempt to achieve the highest current output, electrical polarization of bio photovoltaic devices is required, either through external polarization bias or by implementing living cells into a self-biased device. Since photosynthesis, in a nutshell, is a process of electron transport involving a series of Redox reactions, it can be influenced by polarization, and electrical bias must be considered an environmental stress for cyanobacteria.<br/>Pulse-Amplitude-Modulation (PAM) fluorometry, in conjunction with the saturation pulse method, has been successfully employed for studying the induction and quenching of chlorophyll fluorescence in plant physiological studies [2]. Here, we have employed PAM to investigate cyanobacterial photosynthetic performance under various polarization conditions and conducted in-situ spectroelectrochemistry to analyze the dependence of cyanobacterial pigment absorbance on electrical polarization.<br/>We have found that negatively polarized bioelectrodes based on intact Limnospira, cyanobacteria reported to be resistant to Gamma irradiation and perspective for space applications [3, 4], demonstrate higher absorbance. Relative increase of absorbance in red region of visible light spectrum is higher compared to blue. As a result, biophotoelectrodes exhibit higher cathodic photocurrents under monochromatic red light than what can be expected from absorbance spectrum analysis. Furthermore, embedding cyanobacteria in either a conductive polymer matrix or a non-conductive matrix with a charge carrier facilitates higher photosynthetic efficiency, as measured by PAM, resulting in higher photocurrent output and improved efficiency in light-limited conditions.<br/>This work was supported by the Research Foundation Flanders (FWO-Flanders) and the Swiss National Scientific Foundation under the Senior Lead Agency Project G0D4920N/189455.<br/>[1] Poughon, L. et al, Life Sci Space Res, 25, 53-65 (2020).<br/>[2] Miao, A. J. et al, Environ.Toxicol., 24, 2603-2611 (2005).<br/>[3] Yadav, A. et al, Microorganisms, 9, 1626 (2021)<br/>[4] Poughon, L. et al, Life Sci. Space Res. 25, 53-65 (2020)