Carbon concentration mechanisms in photosynthetic microorganisms.

Unicellular green algae and cyanobacteria have mechanism to actively concentrate dissolved inorganic carbon into the cells, only if they are grown with air levels of CO2. The carbon concentration mechanisms are commonly known as "CCM" or "DIC-pumps". The DIC-pumps are environmental adaptation that function to actively transport and accumulate inorganic carbon (HCO3- and CO2; Ci) within the cell and then uses this Ci pool to actively increase the concentration of CO2 at the site of ribulose bisphosphate carboxylase-oxygenase (Rubisco), the primary CO2-fixing enzyme. The current working model for dissolved inorganic carbon concentration mechanism in unicellular green algae includes several isoforms of carbonic anhydrase (CA), and ATPase driven active transporters at the plasmalemma and at the inner chloroplast envelopes. In the past fifteen years, significant progress has been made in isolating and characterizing the various isoforms of carbonic anhydrase at the biochemical and molecular level. However, we have an inadequate understanding of active transporters that are located on the plasmalemma and at the chloroplast envelopes. In this mini-review we focus on certain aspects of the induction, function and significance of the dissolved inorganic carbon concentration mechanisms in aquatic photosynthetic microorganisms.