Vascular plants often inhabit regions subject to tidal submergence while others carry out their entire life cycle under water. Examples of common submerged aquatic macrophytes are pond weeds and seagrasses. Once again, an evolutionary selective pressure for these plants has been the availability of CO₂. Low levels of dissolved CO₂ are common in both inland and marine waters, particularly at more alkaline pH. In more productive inland lakes, CO₂ content can vary enormously, requiring considerable flexibility in the actual mode of carbon acquisition. At high pH, HCO₃^– becomes the more abundant form of inorganic carbon, whereas dissolved CO₂ will predominate at low pH (Section 18.2). Consequently, when SAM plants evolved from their C₃ progenitors on land, there was some adaptive advantage in devices for CO₂ accumulation because CO₂ rather than HCO₃^– is substrate for Rubisco. The nature of this ‘CO₂ pump’ and the energetics of carbon assimilation are not fully characterised in SAM plants but considerable CO₂ concentrations do build up within leaves, enhancing assimilation and suppressing photorespiration.

In summary:

Regardless of photosynthetic mode, and despite catalytic limitations, Rubisco is ubiquitous and remains pivotal to carbon gain in our biosphere. As a corollary, carbon loss via photorespiration is an equally universal feature of C₃ leaves, and the evolution of devices that overcome such losses have conferred significant adaptive advantages to C₄, CAM and SAM plants.