Algal Carbon Dioxide Concentrating Mechanisms


Photosynthetic microorganisms like cyanobacteria and most 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 proceed 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 biofuel.

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 (no 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 light green. The hydrogen carbonate transporters characterised so far are on the plasma membrane (HLA3 and LCI1), the chloroplast envelope (LCIA) and the thylakoid membrane (BST). Hydrogen carbonate inside the thylakoid lumen is converted to CO2 by the carbonic anhydrase, CAH3, while CO2 leaking from the pyrenoid is converted back to hydrogen carbonate by the θ‐carbonic anhydrase LCIB/C. PGA, 3‐phosphoglyceric acid; PM, plasma membrane; CYT, cytoplasm; CE, chloroplast envelope.


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Further Reading

Espie GS and Kimber MS (2011) Carboxysomes‐cyanobacterial RubisCO comes in small packages. Photosynthesis Research 109: 7–20. DOI: 10.1007/s11120‐011‐9656‐y.

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Moroney JV and Ynalvez RA (2007) Proposed carbon dioxide concentrating mechanism in Chlamydomonas reinhardtii. Eukaryotic Cell 6: 1251–1259.

Mackinder LC, Chen C, Leib RD, et al. (2017) A spatial interactome reveals the protein organization of the algal CO2‐concentrating mechanism. Cell 171: 133–147.

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.

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.

Raven JA (2010) Inorganic carbon acquisition by eukaryotic algae: four current questions. Photosynthesis Research 106: 123–134.

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Raven JA and Beardall J (2016) The ins and outs of CO2. Journal of Experimental Botany 67: 1–3.

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Mukherjee, Bratati, and Moroney, James V(Feb 2020) Algal Carbon Dioxide Concentrating Mechanisms. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000314.pub4]