Establishing a Computational Model of the CO2-Concentrating Mechanism in Chlamydomonas reinhardtii

Abstract

Due to population growth and dietary shifts, larger crop plant yields are needed to provide food to people across the globe. While many advances in crop yields have reached theoretical plateaus, improving the efficiency of photosynthesis remains a novel path forward, offering new gains not attainable through selective breeding. Limits to photosynthetic efficiency involve Rubisco, the enzyme responsible for fixing carbon in photosynthetic organisms. Rubisco has a relatively slow catalytic rate and catalyzes a competing reaction with oxygen that is wasteful to the organism. The eukaryotic alga Chlamydomonas reinhardtii has a CO2-concentrating mechanism (CCM) that increases the efficiency of Rubisco by increasing the concentration of CO2 near the enzyme. The Jonikas Lab plans to engineer the Chlamydomonas CCM into higher plants with the goal of increasing efficiency. In this thesis project, I worked with colleagues to create a multi-compartmental reaction-diffusion model of the Chlamydomonas CCM to guide engineering efforts. We establish metrics to evaluate the ability of the chloroplast to concentrate carbon near Rubisco and the cost of doing so to the cell. We find that the model chloroplast requires a barrier to CO2 diffusion in order to achieve a working CCM. We show that the CCM can be achieved with two different strategies at low CO2: by employing only channel transporters and enzymes or by actively pumping inorganic carbon into the cell, and the former is of lower cost to the cell. However, at very low CO2 we find that active transport is necessary to achieve a working CCM. The new insights gained through our model advance our understanding of how the CCM functions and can be used to guide the CCM engineering process.