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In addition to photosynthetic reduction of CO2 to organic compounds, many cyanobacteria can take up CO2 and mineralize it to recalcitrant carbonates, such as calcium carbonate (CaCO3). Thus cyanobacteria present two different modes of CO2 capture; through assimilation into organic biomass via photosynthesis and the Calvin-Benson-Bassham cycle, and through incorporation into inorganic structures via biomineralization (Fig. 1).
Carbonate biomineralization, e.g., mineralization of CO2 by calcium carbonate (CaCO3) precipitation (calcification) is a common phenomenon in marine, freshwater and terrestrial ecosystems and of vast ecological and geological importance . Spectacular manifestations of cyanobacterial calcification are presented by stromatolites, which were abundant in the Precambrian era and that represent one of the earliest records of life on Earth. Another magnificent illustration of microbial calcification is the White Cliffs of Dover, which are mainly eukaryotic microalgal in origin.
In cyanobacteria, calcification is intimately associated with the carbon concentrating mechanism (CCM), a biochemical system that allows the cells to raise the concentration of CO2 at the site of the carboxylating enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) up to 1000-fold over that in the surrounding medium [1, 2]. The salient features of the CCM in cyanobacteria are shown in Fig. 2. Details differ between cyanobacteria and the mechanisms are incompletely understood but the general arrangement consists of transport of HCO3-, the major uptake form of inorganic carbon (Ci) in cyanobacteria, across the outer membrane and the plasma membrane, through HCO3-/Na+ symports or ATP-driven uniports, as well as diffusion of CO2, into the cytosol. Conversion of cytosolic CO2 is carried out by NADPH dehydrogenase (NDH) complexes on the thylakoid and plasma membranes. HCO3- then enters the carboxysome, the protein-enclosed compartment that houses most of the Rubisco population, where it is converted to CO2 in a reaction catalyzed by carbonic anhydrase (CA). At non-limiting Ci concentrations the CCM recedes to a basic, constitutive level, characterized by mainly CO2 uptake.
The conversion of CO2 to HCO3- via the NDH complexes relies on CA-like activities in associated proteins. The active transport of HCO3- is dependent on extra ATP generated by cyclic electron transport around Photosystem I (PSI) in the photosynthetic electron transport chain (PET). The Ci transporters and the NDH complexes together constitute the combination of constitutive and inducible HCO3- uptake systems of the cyanobacterial CCM. When cells are exposed to CO2/HCO3- limitation (<50 ppm CO2), the inducible transport systems are activated, accompanied with increases in Rubisco activity and carboxysome content. Interestingly, the explanation to why many cyanobacteria and eukaryotic microalgae have the ability to tolerate very high CO2 concentrations, in some cases well above 50% CO2 might be found in the CCM. Inhibition of Rubisco through acidification under high CO2 conditions is prevented by the CA reaction and by state II transition of PET (rearrangement of the phycobilisomes to favor light absorption by PSI).
We are studying cyanobacterial biomineralization, particularly calcification and formation of magnesium carbonates, in cyanobacteria for a number of reasons.
1. A comprehensive understanding of carbonate biomineralization in cyanobacteria is critical for our appreciation of this process in the global carbon cycle, and how it may be affected by climate change. In this context it should be recognized that calcification as a natural phenomenon by marine or freshwater phytoplankton serves as a CO2 source rather than a sink, i.e., calcification releases CO2 to the atmosphere while consuming alkalinity.
2. An attractive scenario for cultivation of cyanobacteria is to use flue gas from coal-fired power plants as a CO2 source. This allows us to combine production of biofuels in cyanobacteria with mitigation of greenhouse gas emissions by beneficial recycling of CO2. Therefore, we are interested in learning if calcification positively or negatively impacts biomass production, how this differs between ambient and high (5 – 15%) CO2 levels, and how the extent and nature of carbonate mineralization in cyanobacteria is affected by high CO2 levels.
3. We are exploring if carbonate mineralization in cyanobacteria can be exploited for biological carbon capture and storage (CCS) . For example, can the released CO2 be efficiently recaptured and used for photosynthesis? Can the biomineralization process be combined with accelerated weathering of silicates?
4. Information on how the calcification process in cyanobacteria is affected by different CO2 levels is important also for our ability to interpret paleological data.
 Jansson, C. (2012) Employing cyanobacteria for biofuel synthesis and CCS. In: Solar Energy (Radu, R. ed.), InTech, pp 367-378.
 Jansson, C., and Northen, T. (2010) Calcifying cyanobacteria - the potential of biomineralization for carbon capture and storage. Curr. Opin. Biotechnol. 21, 365-371.
 Jansson, C., Wullschleger, S., Kalluri, U., and Tuskan, G.T. (2010) Phytosequestration: Carbon biosequestration by plants and the prospects for genetic engineering. BioScience. 60, 685-696.