Carbon concentration mechanisms CCMs

The best known carbon concentrating mechanism (CCM), is that of freshwater cyanobacteria... 

The ambient concentration of CO2 in seawater ( 10 pM) is low compared to the half saturation constant of RubisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) the enzyme that fixes the inorganic carbon into PG3 (3-phos-phoglycerate) in the first step of the Calvin cycle (the dark reaction of photosynthesis). Microalgae have thus evolved various carbon concentrating mechanisms (CCM) to augment the CO2 concentration at the site of fixation [55]. These mechanisms all involve interconversion between CO2 and HCOJ at some point. But at neutral pH the hydration/dehydration reaction of CO2/ HCOj" has a half life of 30 s, much too slow for a cellular process (for example, diffusion from one end of the cell to the other takes on the order of 10 ms), and requires catalysis. The enzyme CA is an extraordinary effective catalyst some CA catalyze the CO2/HCO3 reaction at a rate that nearly reaches the limit imposed by the diffusion of molecules [56]. 

Cadmium carbonic anhydrase (CDC A) is the first member of a new class of carbonic anhydrases, the class. CDCAl, which uses Cd as its metal cofactor when Zn is limiting, was isolated from the marine diatom T. weissflogii. The amino acid sequence of CDCAl contains a triple repeat with 85% identity between repeats [10]. CDCAl is a key enzyme in the carbon concentrating mechanism (CCM) through which T. weissflogii increases the concentration of CO2 at the site of fixation byRuBisCO [92]. 

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