Apart from a possible advantage a CCM might provide a solid pore-less thallus over CO2 conductance, (Beardall et al. 1982; Raven et al 1985) suggest that the CCM may confer another advantage, namely improved nitrogen use efficiency of growth. This is hypothesised to result from the idea that less nitrogen has to be diverted to the synthesis of Rubsico, and possibly photrespiratory enzymes. In an early Silurian aerial environment, nitrogen availability and acquisition may have been limiting compared to a water column as the case may still be today in certain environments (Crittenden, Katucka & Oliver 1994). As the Rubisco molecule has a low turnover rate then retention of a CCM that conveys Nitrogen efficiency would be advantageous (Raven and Lucus 1985). However, this has not been conclusively demonstrated.
