(311b) Metabolic Modeling and Pulse Amplitude Modulation Fluorometry Elucidate the Interplay between Photosynthesis and Carbon Fixation in a Purple Non-Sulfur Bacterium

Rhodopseudomonas Palustris is a metabolically versatile Purple Non-Sulfur Bacterium (PNSB). Depending on growth conditions, R. palustris can operate on either one of the four different forms of metabolism: photoautotrophic, photoheterotrophic, chemoautotrophic, and chemoheterotrophic. R. palustris is also a facultative anaerobe, meaning it can operate both aerobically and anaerobically. Furthermore, the organism is capable of fixing nitrogen and subsequently producing hydrogen. This metabolic flexibility and plethora of functionalities have made R. palustris a model organism for studying the network of interacting reactions and how it’s altered in response to changing conditions. Recently, several works have identified the roles of carbon fixation as a redox balancing mechanism in photoheterotrophically grown PNSB. Furthermore, the involvement of global regulators such as RegA/PrrA was shown to affect a number of metabolic pathways including the electron-sink pathways of CO2 and N2 fixation, hydrogen metabolism, and the energy generating photosynthetic pathway. The aim of this study was to determine the metabolic bottlenecks associated with the use of cyclic photophosphorylation. Mainly, our focus was on how the use of various carbon sources with different oxidation states affected the photosynthetic efficiency. To achieve this aim, a genome-scale metabolic model of R palustris was reconstructed, curated, and validated against experimental data. The model was used to generate hypotheses on the effect of growth conditions (i.e. carbon source and light intensity) on the activity of the Electron Transport Chain (ETC), carbon dioxide fixation rate, and Quinone oxidoreductase reaction rates¹. These hypotheses were then experimentally tested using Pulse Amplitude Modulation (PAM) fluorometry techniques to measure the Electron Transport Rate (ETR) as well as qPCR to determine expression levels of the CO2 fixing components under various growth conditions. This study illustrates (i) the predictive power of genome scale metabolic modeling, especially for the case of metabolically versatile organisms and (ii) the viability of PAM fluorometry for measuring photosynthetic rates in PNSB.

References

1. Alsiyabi, A., Immethun, C.M., Saha, R., 2019. Modeling the Interplay between Photosynthesis, CO2 Fixation, and the Quinone Pool in a Purple Non-Sulfur Bacterium. Sci. Rep. 9, 12638. https://doi.org/10.1038/s41598-019-49079-z