Algal Carbon Dioxide Concentrating Mechanisms

Abstract

Photosynthetic microorganisms like cyanobacteria and many eukaryotic algae acclimate to a limited availability of carbon dioxide (CO2) in their environment by inducing a process called the carbon dioxide concentrating mechanism. This process uses an active inorganic carbon (Ci; CO2 and/or HCO3) uptake system that leads to the internal accumulation of Ci to levels significantly higher than extracellular levels. Carbonic anhydrase activity converts much of the accumulated hydrogen carbonate to CO2, concentrating this substrate around Rubisco and thereby optimising photosynthetic efficiency even under low CO2 conditions. The efficiency of the process is further improved by the sequestration of Rubisco into specialised structures like the cyanobacterial carboxysome or the pyrenoid in eukaryotic algae. The carbon dioxide concentrating mechanism enhances carbon dioxide fixation and growth in algae. With the increasing demands for sustainable energy sources, algae with efficient carbon dioxide concentrating mechanisms are attractive models for biotechnological and transgenic applications for biofuel and biomass production.

Key Concepts:

  • The carbon dioxide concentrating mechanism helps photosynthetic algae optimise photosynthesis under limiting CO2 conditions.

  • Rubisco uses both carbon dioxide and oxygen as substrates.

  • Rubisco is localised in the carboxysome in cyanobacteria and the pyrenoid in the green alga, Chlamydomonas reinhardtii.

  • The charged hydrogen carbonate needs transporters to enter the cell and cross organellar membranes.

  • Carbonic anhydrase is an efficient enzyme that carries out the reversible interconversion of carbon dioxide and hydrogen carbonate and this reaction proceeds at a much faster rate than the uncatalysed reaction.

  • At higher pH levels most of the inorganic carbon is in the form of hydrogen carbonate.

  • Most algae with carbon dioxide concentrating mechanisms can take up both carbon dioxide and hydrogen carbonate.

  • Carbon dioxide can diffuse out of the cell so algae trap carbon dioxide in the form of the charged hydrogen carbonate anion.

  • Algae are used for large‐scale biomass production and are currently being explored as sources of bio‐fuel.

Keywords: algae; carbon concentrating mechanism; cyanobacteria; Chlamydomonas; photosynthesis; rubisco; pyrenoid; carboxysome; carbonic anhydrase; hydrogen carbonate transporter

Figure 1.

Model of the carbon dioxide concentrating mechanism in cyanobacteria. The figure depicts a cyanobacterial cell with a carboxysome (not drawn to scale). The different transport proteins and carbonic anhydrases are indicated. PGA stands for 3‐phosphoglyceric acid and the green line represents the thylakoid membrane. PM, plasma membrane; TM, thylakoid membrane.

Figure 2.

Proposed model for the carbon dioxide concentrating mechanism of the eukaryotic alga, Chlamydomonas reinhardtii. The figure depicts a unicellular algal cell with a chloroplast and a single pyrenoid. The thylakoid tubule within the pyrenoid is depicted in green. As indicated by the size and boldness of the lettering, the concentrations of carbon dioxide within the chloroplast and pyrenoid are higher than in the external environment. The hydrogen carbonate transporters characterised so far on the plasma membrane and chloroplast envelope and a putative one on the thylakoid membrane have been depicted as red circles. The dashed line indicates proposed functions for the two carbonic anhydrases CAH7 and CAH9 in the cytosol. CAH1 to CAH9 stand for the specific carbonic anhydrase isoforms 1 through 9. PGA stands for 3‐phosphoglyceric acid. PM, plasma membrane; CE, chloroplast envelope; TM, thylakoid membrane.

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Further Reading

Espie GS and Kimber MS (2011) Carboxysomes‐cyanobacterial RubisCO comes in small packages. Photosynthesis Research. doi: 10.1007/s11120‐011‐9656‐y.

Hewett‐Emmett D and Tashian RE (1996) Functional diversity, conservation, and convergence in the evolution of the α‐, β‐, and γ‐carbonic anhydrase gene families. Molecular Phylogenetics and Evolution 5: 50–77.

Moroney JV and Chen Z‐Y (1998) The role of the chloroplast in inorganic carbon uptake by eukaryotic algae. Canadian Journal of Botany 76: 1025–1034.

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Price GD, Badger MR and Caemmerer SV (2011) The prospect of using cyanobacterial bicarbonate transporters to improve leaf photosynthesis in C3 crop plants. Plant Physiology 155: 20–26.

Price GD, Badger MR, Woodger FJ and Long BM (2008) Advances in understanding the cyanobacterial CO2‐concentrating‐mechanism (CCM): functional components, Ci transporters, diversity, genetic regulation and prospects for engineering into plants. Journal of Experimental Botany 59: 1441–1461.

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Raven JA, Cockell CS and De La Rocha CL (2008) The evolution of inorganic carbon concentrating mechanisms in photosynthesis. Philosophical Transactions of the Royal Society B 363: 2641–2650.

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Mukherjee, Bratati, and Moroney, James V(Oct 2011) Algal Carbon Dioxide Concentrating Mechanisms. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000314.pub3]