Ecology. Michael Begon

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 with increasing temperature.

      The rate of photosynthesis of C₃ plants increases with the intensity of radiation, but reaches a plateau. In many species, particularly shade species, this plateau occurs at radiation intensities far below that of full solar radiation (see Figure 3.4). Plants with C₃ metabolism have low water‐use efficiency compared with C₄ and CAM plants (see later), mainly because in a C₃ plant, CO₂ diffuses rather slowly into the leaf and so allows time for a lot of water vapour to diffuse out of it through the open stomata.

      The rate of photosynthesis of C₃ plants also increases with the concentration of CO₂ within the plant, and because of the slow rate of diffusion, with the concentration of CO₂ in the atmosphere (see later). However, this rate is limited by the ability of C₃ plants to regenerate RuBP with which CO₂ can be combined, and therefore levels off as CO₂ concentrations increase.

      the C₄ pathway

      In the C₄ pathway, the Hatch–Slack cycle, the C₃ pathway is present but it is confined to cells deep in the body of the leaf. CO₂ that diffuses into the leaves via the stomata meets mesophyll cells containing the enzyme phosphoenolpyruvate (PEP) carboxylase. This enzyme combines atmospheric CO₂ with PEP to produce a four‐carbon acid. This diffuses, and releases CO₂ to the inner cells where it enters the traditional C₃ pathway. PEP carboxylase has a much greater affinity than RuBisCO for CO₂. There are profound consequences.

First, C₄ plants can absorb atmospheric CO₂ much more effectively than C₃ plants and the rate of photosynthesis is therefore much less dependent on CO₂ concentrations (but see later). Also, because of the reduced need to keep stomata open, C₄ plants may lose much less water per unit of carbon fixed. Furthermore, the wasteful release of CO₂ by photorespiration is almost wholly