The most extensively studied Biophysical CO2 Concentrating Mechanisms CCM's have been in cyanobacteria which use a carboxsome-based CCM, small polyhedral-shaped protein bodies containing both Rubisco and the enzyme Carbonic anhydrase (CA) which catalyses the dehydration of HCO3- to CO2 (Badger & Andrews 1987; Bowes 1993).
CA
H+ + HCO3- CO2 + H2O
In micro algae and some hornworts, an equivalent structure to the carboxysome is the pyrenoid, a starch coated protinaceous structure present in the chloroplast, believed to play a similar role (Badger et al 1993). to microalage, with a pyrenoid-based CCM (Pronina & Semenko 1992; Badger 1998).
To summarize the makeup of an generalized CCM, 4 components are needed; (1) a mechanism whereby rapid interconversion between CO2 and HCO3- can take place, extracellularly and intracellularly, the latter occurring at typical stromal pH values within the Rubisco-containing compartment or more effectively, at low pH in the thylakoid lumen; (2) a Ci-transport mechanism at plasma membrane, chloroplast envelope or both; (3) ATP energy to power Ci transport; (4) a diffusion barrier to prevent CO2 from diffusing away from Rubisco (Smith & Griffiths 2000).
It has been established that the first two features of the above list have been shown to involve CA in a range of eukaryotic algae and cyanobacteria which utilise CCMs and it has recently been suggested that CA might function as a diffusion barrier (Raven 1997).The pyrenoid structure thought to be associated with the CCM found in many micro algae (Badger et al 1994; Amoroso et al 1998; Woods 1999) and lichenised algae (Palmqvist 1993), has also been observed in some species of byrophytes from the class Anthocerotae (REF).
Moreover, Vaughn (1992) established, using immuno-gold labelling, that the enzyme Rubisco was found within the pyrenoid structure.
Smith & Griffiths (1996b), further established that Anthoceros crispus and Phaeoceros laevis (Smith & Griffths 2000) exhibited low compensation points, low K0.5 and a CO2 uptake and release pool after photosynthesis had been inhibited, using the C-reduction cycle inhibitor, glycoladehyde, (a method term light-dark transients), revealing a CO2 pumping mechanism termed a Dissolved Inorganic Carbon pump (DIC pump).
All of the above have been used as physiological CCM diagnostics in cyanobacteria and micro algae. To date, Phaeoceros laevis and Anthoceros crispus have been shown to possess an operative CCM showing similar characteristics to micro algal CCMs. But why retain this ancient microalgal relic in the aerial environment? Considering approx 450-500 million years ago CO2 Concentrations were (5400 –7000ppm) or indeed independently evolve the same (or similar) mechanism, at a latter period, during a drop in atmospheric CO2?