aquatic environments, variations in CO2 concentration can be just as striking, especially when water mixing is limited, for example during the summer ‘stratification’ of lakes, with layers of warm water towards the surface and colder layers beneath. Some examples from a study in Estonia are shown in Figure 3.15. At one extreme was the shallow Lake Äntu Sinijärv, which is supersaturated with CO2 (CO2 concentration higher than would result from equilibration with atmospheric CO2) as a result of the high concentrations of bicarbonate ions in the water flowing into it. Here, there was usually virtually no vertical stratification of CO2. Lake Saadjärv was deeper and thermally stratified and also had very high CO2 concentrations, but in this case there was strong CO2 stratification in the deeper layers. And finally, Lake Peipsi was a very large lake compared with the others (3555 km2 compared with <10 km2 for the others), similar in depth to Lake Äntu Sinijärv, but with very much lower CO2 concentrations overall. In this case, vertical stratification of CO2 concentrations led consistently to levels in the upper layers where the concentrations were lower than saturation, such that the lake was a sink for atmospheric CO2, whereas the other two lakes were net CO2 emitters.
Figure 3.15 Concentrations of CO2vary, variably, with depth in Estonian lakes. The profiles with depth of CO2 concentration over a number of days (different in each case) in three lakes in Estonia, as indicated. Note that the colour‐coding varies between the lakes to reflect their different concentration ranges, and that their depths are different.
Source: After Laas et al. (2016).
In aquatic habitats, especially under alkaline (high pH) conditions, dissolved CO2 tends to react with water to form carbonic acid, which in turn ionises, such that 50% or more of inorganic carbon in water may be in the form of bicarbonate ions. Indeed, the Estonian lakes highlight how, in many cases, overall concentrations of CO2 may be highly supersaturated. This may seem to suggest that aquatic plants will only rarely be limited by the availability of CO2, but in fact they commonly are limited, due to the low rates of CO2 diffusion in water, and around half of aquatic plants are able to use bicarbonate ions as an alternative source of dissolved organic carbon. However, since bicarbonate must ultimately be reconverted to CO2 for photosynthesis, this is likely to be less useful as a source of inorganic carbon, and in practice, many plants will be limited in their photosynthetic rate by the availability of CO2. We see an illustration of this in Figure 3.16. Ten species of aquatic plants, all capable of using bicarbonate as a source of CO2, were grown in two culture conditions, both with the same overall concentration of dissolved organic carbon (0.85 mM). In one case (low‐C) the water was in equilibrium with the surrounding air (saturated) and so the contribution of CO2 itself to this was small (0.012 mM). But in the other case (high‐C) the initial concentration, largely from bicarbonate ions, was much lower (0.40 mM) but gaseous CO2 was continually added to the water, supersaturating it, and raising the overall concentration to the low‐C level. All 10 species grew faster under the high‐C conditions (Figure 3.16a), apparently as a result of elevated growth efficiencies at higher concentrations of CO2, since the low‐C plants were investing more in, for example, leaf nitrogen (Figure 3.16b), enabling them to make more of the limited CO2 resource available to them. Even for these bicarbonate users, bicarbonate is good but CO2 is better.
Figure 3.16 Aquatic plants may be limited in their photosynthetic ability by the availability of CO2. (a) The relative growth rate (RGR, rate of growth per unit weight) for 10 species of aquatic plants, as indicated, when water was at equilibrium with the surrounding air with respect to CO2 (low‐C) such that the contribution of CO2 to dissolved inorganic carbon (compared with bicarbonate) was relatively small, and when CO2 was continually passed into the water (high‐C) such that the contribution of CO2 was large. In a two‐way analysis of variance, the effects of species and treatment were both significant (respectively, F = 11.6, P < 0.0001 and F = 52.9, P < 0.0001). (b) The leaf nitrogen content of the same 10 species in the same treatments. Again, the effects of species and treatment were both significant (respectively, F = 9.1, P < 0.0001 and F = 101.4, P < 0.0001). Means and SEs are shown in both parts.
Source: After Hussner et al. (2016).
3.4.1 C3, C4 and CAM
These variations in CO2 availability, along with associated variations in, for example, the difficulties of capturing CO2 while avoiding the loss of water, have led to the widespread evolution of carbon concentrating mechanisms (CCMs) that increase the availability of CO2 at the metabolic sites where it is required. Hence, while one might expect a process as fundamental to life on earth as carbon fixation in photosynthesis to be underpinned by a single unique biochemical pathway, in fact, even in higher plants there are three such pathways (and variants within them): the C3 pathway (the most common), the C4 pathway and CAM. The ecological consequences of the different pathways are profound, especially as they affect the reconciliation of photosynthetic activity and controlled water loss (see Section 3.3.1). Even in aquatic plants, where water conservation is not normally an issue, and most plants use the C3 pathway, there are many CCMs that serve to enhance the effectiveness of CO2 utilisation (Griffiths et al., 2017). These CCM‐based pathways are of profound importance. The C4 and CAM pathways account for 18–30% of the 60 Pg (approximately) of carbon assimilated each year on land; while CCMs in cyanobacteria and algae account for more than half the 50 Pg of carbon assimilated each year in the oceans (Raven et al., 2008).
the C3 pathway
In the C3 pathway, the Calvin–Benson cycle, CO2 is fixed, through combination with ribulose 1,5‐biphosphate (RuBP), into a three‐carbon acid (phosphoglyceric acid) by the enzyme RuBisCO (ribulose‐1,5‐biphosphate carboxylase‐oxygenase), which is present in massive amounts in the leaves (25–30% of the total leaf nitrogen). This same enzyme can also act as an oxygenase, as its name indicates, and this activity (photorespiration) can result in a wasteful release of CO2 – reducing by about one‐third the net amounts of CO2 that are fixed. Photorespiration increases with temperature with the consequence that the overall efficiency of carbon fixation declines
