Carboxysomes are specialized structures found within certain microorganisms, serving as miniature factories for converting carbon dioxide into organic matter. These tiny compartments play a foundational role in a widespread biological process, enabling efficient carbon fixation. Their presence allows these microbes to contribute significantly to how life sustains itself on Earth.
Understanding Carboxysome Structure
Carboxysomes are classified as bacterial microcompartments (BMCs), characterized by their polyhedral protein shells. These shells are composed of thousands of protein subunits [1, 2, 4.6C]. Hexameric shell proteins form the faces of the structure, while pentameric proteins are positioned at the 12 vertices, contributing to the icosahedral or quasi-icosahedral geometry. Small pores perforate these hexameric proteins, allowing for the passage of small molecules like bicarbonate and the products of carbon fixation.
Enclosed within this protein shell are two primary enzymes: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase. RuBisCO is the predominant enzyme in carbon fixation, binding carbon dioxide. Carbonic anhydrase facilitates the interconversion between carbon dioxide and bicarbonate, a reaction that supports the function of the carboxysome. The protein shell acts as a barrier, confining these enzymes and their substrates to a localized environment.
How Carboxysomes Enhance Carbon Fixation
The primary function of carboxysomes is to optimize carbon dioxide fixation through a carbon concentrating mechanism (CCM). This mechanism begins with the active transport of inorganic carbon, primarily in the form of bicarbonate, into the cell’s cytoplasm. Bicarbonate is preferred for transport because, unlike neutral carbon dioxide, it does not readily diffuse back across the cell membrane, stockpiling carbon within the cell.
Once inside the cytoplasm, bicarbonate diffuses into the carboxysome through pores in its protein shell. Within the carboxysome’s lumen, the enclosed carbonic anhydrase enzyme rapidly converts bicarbonate back into carbon dioxide. This localized conversion creates a high concentration of carbon dioxide directly around the RuBisCO enzyme. The elevated carbon dioxide levels improve RuBisCO’s efficiency by promoting its carboxylation reaction.
This high carbon dioxide environment minimizes photorespiration, a reaction where RuBisCO binds oxygen instead of carbon dioxide. By reducing oxygenation and enhancing carboxylation, carboxysomes allow RuBisCO to operate at a higher rate and with specificity for carbon dioxide. The protein shell also prevents the newly generated carbon dioxide from escaping the compartment, ensuring its efficient use by RuBisCO. This strategy allows organisms to thrive even in environments with low external carbon dioxide availability or high oxygen levels.
Organisms That Rely on Carboxysomes
Carboxysomes are found in various autotrophic microorganisms, those that perform photosynthesis or chemosynthesis. Cyanobacteria, also known as blue-green algae, universally possess carboxysomes and are significant global photosynthesizers [1, 3, 5, 4.6C]. These aquatic prokaryotes rely on carboxysomes to efficiently fix carbon dioxide, especially in environments where dissolved carbon dioxide levels are low or oxygen concentrations are high. The carbon concentrating mechanism provided by carboxysomes allows cyanobacteria to maintain high rates of carbon fixation despite these environmental challenges.
Beyond cyanobacteria, several groups of chemosynthetic bacteria also utilize carboxysomes for carbon fixation [1, 2, 4.6C]. Examples include Halothiobacillus, Acidithiobacillus, Nitrobacter, and Nitrococcus, which obtain energy from chemical reactions rather than sunlight [4.6C]. These bacteria often inhabit diverse environments where carbon dioxide might be scarce, making the carboxysome a valuable adaptation for efficient carbon uptake in these diverse autotrophic microbes.
Broader Ecological Importance
Carboxysomes facilitate efficient carbon fixation in cyanobacteria and other microbes, and are ecologically important. Cyanobacteria are major primary producers in aquatic ecosystems, accounting for a significant portion of global carbon dioxide fixation and atmospheric oxygen production. Their ability to capture carbon, even in challenging conditions, underpins the productivity of vast marine and freshwater food webs. Without carboxysomes, the ecological contribution of these organisms would be substantially reduced.
The activity of carboxysome-containing microbes directly influences the global carbon cycle by converting inorganic carbon dioxide into organic compounds, which then become available to other organisms in the food web. Over geological timescales, the continuous operation of these structures has contributed to shaping Earth’s atmosphere and climate. The oxygen released as a byproduct of photosynthesis by these organisms, enabled by carboxysomes, has been instrumental in the evolution of aerobic life forms. These microscopic compartments have a significant impact on planetary biochemistry and the sustainability of diverse ecosystems.