CO2 transport across biological membranes

The movement of physiologically relevant gases, including O₂, CO₂, and N₂, across cell membranes is fundamental for sustaining diverse forms of life. In particular, efficient CO₂ transport is essential for photosynthesis, the fundamental biological process that converts light energy into chemical energy through carbon fixation and thereby forms the primary basis of nearly all life on Earth.

Research on CO₂ transporters is gaining increasing attention due to their potential in developing advanced biomimetic membranes for industrial carbon capture and climate change mitigation, which are global concerns today. CO₂transporters regulate the movement of carbon dioxide across cell membranes, directly affecting critical processes such as photosynthesis, respiration, and pH balance. While CO₂ can diffuse passively through membranes, specialized transport proteins provide precise and efficient control, particularly under conditions of high metabolic demand and physiological pH, making them key targets for both fundamental biological studies and applied environmental technologies.

Carbon-concentrating mechanism 

Carbon-concentrating mechanisms (CCMs) are cellular systems in many bacteria that actively transport and accumulate inorganic carbon (CO₂ and/or HCO₃⁻) to elevate its intracellular concentration around Rubisco, thereby enhancing carboxylation efficiency and minimizing oxygenation reactions. CO₂ transporters are essential components of CCMs, as they mediate the active uptake of inorganic carbon across cellular membranes, overcome diffusion limitations imposed by low environmental CO₂ availability, and generate a concentrated intracellular bicarbonate pool that can be converted to CO₂ in proximity to Rubisco, ensuring efficient carbon fixation and improved metabolic performance under carbon-limiting conditions.

Understanding and engineering carbon-concentrating mechanisms (CCMs) in microalgae, with a particular focus on the extremophilic red alga Cyanidioschyzon merolae has become interesting topic. CCMs are vital for enhancing photosynthetic efficiency because they elevate inorganic carbon around Rubisco, thereby boosting carbon fixation and reducing wasteful oxygenation. 

C. merolae represents a unique model system for CCM research: it thrives in highly acidic (pH 1- 2), high-temperature environments where inorganic carbon availability is limited, and evidence suggests it employs a non-canonical CCM that operates without typical structures like pyrenoids, taking up CO₂ directly and displaying gas-exchange traits consistent with carbon concentration. 

Optimizing inorganic carbon uptake of C. merolae by improving membrane transport components such as bicarbonate importers and exploring strategies for efficient export of metabolic products have been the topic of our research. By engineering these transport systems, we seek to enhance C. merolae’s carbon capture capability and integrate it with synthetic biology approaches to convert fixed carbon into valuable compounds, while also addressing broader environmental goals like CO₂ reduction and bioremediation.

DAB complex protein represent inorganic carbon pumps in prokaryotes

One of the recent protein complexes discovered for carbon-concentrating mechanisms (CCMs) is the DAB complex. In the γ-proteobacterium Halothiobacillus neapolitanus, two-gene operons called DAB1 and DAB2 encode proteins named DabA and DabB that assemble into a heterodimeric membrane-associated complex with a putative carbonic anhydrase-like active site and function as an energy-coupled inorganic carbon (Cᵢ) pump. Physiological and biochemical evidence shows that DabAB complexes actively accumulate bicarbonate in the cytosol by coupling the hydration of CO₂ to a transmembrane cation gradient, providing a unidirectional uptake mechanism that contributes to the CCM. These DAB operons are widespread across diverse bacteria and archaea, and functional homologs have been demonstrated in organisms such as Bacillus anthracis and Vibrio cholerae, suggesting that DAB complexes constitute a distinctive class of energized Cᵢ pumps that play a critical role in prokaryotic inorganic carbon metabolism (Desmarais (2019)).

References: 

Desmarais, John J., et al. "DABs are inorganic carbon pumps found throughout prokaryotic phyla." Nature Microbiology 4.12 (2019): 2204-2215.Miyagishima, Shin-Ya, and Kan Tanaka. "The unicellular red alga Cyanidioschyzon merolae—the simplest model of a photosynthetic eukaryote." Plant and Cell Physiology 62.6 (2021): 926-941.