Physiological reasons for the retention of a CCM on land, or indeed its recent evolution in response to falling atmospheric CO2, is still unclear, as the diffusive limitations of water are no longer limiting and unlike algae they are not moving up and down a water column, facing sudden shifts in HCO3- and light. However, diffusive limitations might have still been acting on the early liverwort-like plants due to heavily cuticlised solid thalli (prior to air chamber evolution), to reduce water loss. Furthermore without pores or stomata (as seen in modern day C3 liverworts) the conductance of CO2 within the solid thallus might have been low enough to require the retention of a CCM.
Robe, Richardson & Griffiths, (unpublished) conducted a detailed comparative study to evaluate the relative costs and benefits of a biophysical CCM in Phaeoceros laevis (one chloroplast per cell with an unventilated thalli) as opposed to photosynthesis being based on the diffusive supply of CO2 in a number of liverworts ranging from Pellia spp. representing the unventilated thalloid structure similar to Phaeoceros, together with liverworts showing increasing levels of complexity and ventilation (viz Conocephalum, Marchantia and Lunularia). The results showed that the limitations to gas exchange in an unventilated thallus such as Pellia, were so great as to render minimal rates of CO2 assimilation, with a high internal conductance to CO2 (gi); meanwhile, the advantages of the CCM in Phaeoceros were to restore the rates of CO2 assimilation and electron transport to those equivalent to the highly ventilated bryophyte thallus of Lunularia.
In conclusion, the operation of the CCM certainly does ensure that most of the Anthocerotae can compare with other more” advanced” bryophytes.