Diffusion barriers and adaptive carbon uptake strategies enhance the modeled performance of the algal CO₂-concentrating mechanism

Abstract

Many photosynthetic organisms enhance the performance of their CO2-fixing enzyme Rubisco by operating a CO2-concentrating mechanism (CCM). Most CCMs in eukaryotic algae supply concentrated CO2 to Rubisco in an organelle called the pyrenoid. Ongoing efforts seek to engineer an algal CCM into crops that lack a CCM to increase yields. To advance our basic understanding of the algal CCM, we develop a chloroplast-scale reaction-diffusion model to analyze the efficacy and the energy efficiency of the CCM in the green alga Chlamydomonas reinhardtii. We show that achieving an effective and energetically efficient CCM requires a physical barrier such as thylakoid stacks or a starch sheath to reduce CO2 leakage out of the pyrenoid matrix. Our model provides insights into the relative performance of two distinct inorganic carbon uptake strategies: at air-level CO2, a CCM can operate effectively by taking up passively diffusing external CO2 and catalyzing its conversion to HCO3-, which is then trapped in the chloroplast; however, at lower external CO2 levels, effective CO2 concentration requires active import of HCO3-. We also find that proper localization of carbonic anhydrases can reduce futile carbon cycling between CO2 and HCO3-, thus enhancing CCM performance. We propose a four-step engineering path that increases predicted CO2 saturation of Rubisco up to seven-fold at a theoretical cost of only 1.5 ATP per CO2 fixed. Our system-level analysis establishes biophysical principles underlying the CCM that are broadly applicable to other algae and provides a framework to guide efforts to engineer an algal CCM into land plants.

Chloroplast Methyltransferase Homolog RMT2 is Involved in Photosystem I Biogenesis

“…ROS generated in HL or conditions of nutrient deprivation may trigger changes in gene expression that are responsible for altering the pyrenoid structure and further physically isolating it from O2. A peripheral pyrenoid mesh may enhance its physical compartmentalization and create the 'cement' that holds the starch plates in place, which may allow it to serve as a more effective diffusion barrier [55][56][57] . Additionally, recent proteomic work found that proteins involved in chloroplast translation may have physical associations with the pyrenoids 54,58 .…”

Interplay between LHCSR proteins and state transitions governs the NPQ response in intact cells of Chlamydomonas during light fluctuations

“…2 ). The energetic requirement (and thus its proton gradient consumption) of the CCM depends on the inorganic carbon (C i ) availability (Fei et al ., 2021). Since the high cell concentration in the TCSPC sample leads to strong C i consumption, this could deplete the C i concentration even in the presence of bubbling, leading to a C i level sensed by cells in the TCSPC sample being lower than what is experienced by cells in the PAM sample.…”

Engineered Accumulation of Bicarbonate in Plant Chloroplasts: Known Knowns and Known Unknowns

“…The modeling did not consider additional ATP requirements for synthesis and maintenance of C i uptake complexes though, since protein accumulation and turnover rates are unknown in both native organisms and chloroplasts, which is a modest pressure onto ATP production compared to the overall daily expenditure in living cells. Recent modeling of proposed pyrenoid-based systems also highlights ATP costs to chloroplastic CCMs; however, these can be limited depending on the engineering strategy ( Fei et al, 2021 ).…”