Evolution of photosynthesis II: Evolution and expansion of cam and c₄ photosynthetic types

The evolutionary traits described in the previous chapter are common to all photosynthetic types, and evolved originally in C₂-like species. Under current atmospheric conditions, the O₂inhibition of photosynthesis occurs through oxygenation of RuBP and subsequent loss of CO₂ through the reactions of photorespiration in C3plants. Consequently, a very significant part of photosynthetic evolution in vascular plants has been the development of mechanisms for reducing photorespiration by concentrating CO₂ around Rubisco, thus returning this enzyme to an atmospheric condition that resembles the primitive earth. These CO₂-concentrating mechanisms are known as CAM and C₄ photosynthesis, while all other plants are referred to as C3. Other CO₂-concentrating mechanisms exist in some algae and cyanobacteria (Kaplan and Reinhold, 1999; Raven et al., 2008), but these will not be discussed here. The evolutionary origins of CAM and C₄ are presumably tied to changes in paleoclimates and atmospheres, particularly to historic variations in CO₂ and O₂, and locally warm climates (Fig. 24.2 in previous chapter). Under higher CO2 partial pressure and/or lower temperature the C₃ pathway fixation does not exhibit limitations that would put a premium on coupling it with a CAM or C4 pathway. The origins of vascular plants date to around the mid-Silurian (~440 MA, Fig. 24.1 in previous chapter) and these plants were likely C3, although the astomatous CAM plant Stylites andicola has been suggested as a possible model of early plant evolution (Keeley et al., 1984). Coupling the C₃ pathway with one of the CO₂-concentrating mechanisms, CAM or C₄, occurred subsequent to the emergence of vascular plants and possibly arose more than once over the past 400 million years.