The effect of the land invasion by early land plants probably had a profound effect on the carboxylating enzyme ribulose 1,5 bisphosphate carboxylase / oxygenase, Rubisco, a photosynthetic enzyme, that had evolved in an aquatic media for some 3,800 million years into an aerial CO2 concentration during the late Ordovician early Silurian of approx. 5400 – 7000ppm CO2 (Berner 1998), without the diffusive barriers placed on CO2 by water.
Rubisco catalyses the first step of the dark reaction side of the photosynthetic reaction termed the Calvin cycle (REF). The first step of this reaction involves Rubisco covalently attaching CO2 to a 5 carbon sugar, RuBP, and the simultaneous hydrolysis of the six carbon intermediate to form 2 molecules of PGA, of which one bears the carbon introduced from CO2.
However, Rubisco has a poor ability to distinguish between CO2 from the O2 molecule, perhaps because there is no formal binding site for CO2. Consequently, when RuBP is bound to an active site of Rubisco, it can be attacked by O2, producing the two products; 2 – phosphoglycolate (P-glycolate) and PGA, a process termed an oxygenase reaction. To salvage lost C as a result of oxygenation of RuBP, the photorespiratory cycle (evolved earlier) recycles P-glycolate to release C at an energetic cost (REF).
The poor kinetic properties of Rubisco, along with diffusive limitation to the passage of CO2 through water and cell boundary layers were all limiting factors to photosynthesis in the aquatic medium placing a strong selection pressure towards the development of mechanisms for increasing CO2 concentrating around the Rubisco molecule, mechanisms termed, Carbon Concentrating Mechanisms (CCMs), possibly evolving 3400 MYA. The effect of raising the internal CO2 environment using CCMs (sometimes by 50 – 100 fold would and does counteract the above constraints). Whether the early land plants retained the CCM is uncertain, however, an inference may be made to suggest a pyrenoid-based CCM was still present in some, if not all, early land plants (before c3 orientation), as the pyrenoid (only found in microalage) is found also in a group of byrophytes from the Class Anthocerotae.