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0 0.2 0.4 0.6 0.8 1.0 1.2 Distance from base of leaf (index)

Figure 52. The stomatal density along mature leaves of Littorella uniflora (shoreweed) from the base to the tip (Nielsen et al. 1991).

11.5 Crassulacean Acid Metabolism (CAM) in Aquatic Plants

Though aquatic plants by no means face the same problems connected with water shortage as desert plants, some of them [Isoetes (quillwort) species] have a similar photosynthetic metabolism: Crassula-cean Acid Metabolism (CAM) (Keeley 1990). They accumulate malic acid during the night and have rates of CO2 fixation during the night that are similar in magnitude as those during the day, when the CO2 supply from the water is very low (Fig. 53). The aerial leaves of Isoetes howellii, in contrast to the submerged leaves of the same plants, do not show a diurnal fluctuation in the concentration of malic acid.

Why would an aquatic plant have a similar photosynthetic pathway as is common in species from arid habitats? CAM in Isoetes is considered an adaptation to very low levels of CO2 in the water, especially during the day (Fig. 53), and allows the plants to assimilate additional CO2 at night. This nocturnal CO2 fixation gives them access to a carbon source that is unavailable to other species. Though some of the carbon fixed in malic acid comes from the surrounding water, where it accumulates due to the respiration of aquatic organisms, some is also derived from the plant's own respiration during the night. A CAM pathway has also been discovered in other genera of aquatic vascular plants (Maberly & Madsen 2002).

11.6 Carbon-Isotope Composition of Aquatic Plants

There is a wide variation in carbon-isotope composition among different aquatic plants, as well as a large difference between aquatic and terrestrial plants (Fig. 54). A low carbon-isotope fractionation might reflect the employment of the C4 pathway of photosynthesis, although the typical Kranz anatomy is usually lacking. Only about a dozen aquatic C4 species have been identified, and very few have submersed leaves with a well developed Kranz anatomy (Bowes et al. 2002). A low carbon-isotope fractionation in aquatic plants might also reflect the CAM pathway of photosynthesis. Isoetids often have rather negative ¿13C values, due to the isotope composition of the substrate (Table 15). Four factors account for the observed variation in isotope composition of freshwater aquatics (Keeley & Sandquist 1992):

1. The isotope composition of the carbon source varies substantially. It ranges from a ¿13C value of +1%, for HCO33 derived from limestone, to -30%, for CO2 derived from respiration. The average ¿13C value of CO2 in air is -8%. The isotope composition also changes with the water depth (Table 16).

2. The species of inorganic carbon fixed by the plant; HCO33 has a <513C that is 7-11% less negative than that of CO2.

3. Resistance for diffusion across the unstirred boundary layer is generally important (except in rapidly streaming water), thus decreasing carbon-isotope fractionation (Box 2).

4. The photosynthetic pathway (C3, C4, and CAM) that represent different degrees of fractionation.

The isotope composition of plant carbon is dominated by that of the source (see 1 and 2 above), because diffusional barriers are strong (see 3). This accounts for most of the variation as described in Fig. 54, rather than biochemical differences in the photosynthetic pathway (Osmond et al. 1982).

11.7 The Role of Aquatic Macrophytes in Carbonate Sedimentation

The capacity of photosynthetic organisms [e.g., Chara (musk-grass), Potamogeton (pondweed), and Elodea (waterweed)] to acidify part of the apoplast and use HCO33 (Sect. 11.3) plays a major role in the formation of calcium precipitates in fresh water, on both an annual and a geological time scale. Many calcium-rich lake sediments contain plant-induced carbonates, according to:

0 0.2 0.4 0.6 0.8 1.0 1.2 Distance from base of leaf (index)

Figure 52. The stomatal density along mature leaves of Littorella uniflora (shoreweed) from the base to the tip (Nielsen et al. 1991).

Figure 53. CAM photosynthesis in submerged leaves give the pH values. Open and filled symbols refer to of Isoetes howellii(quillwort) in a pool. (A) Malic acid the light and dark period, respectively (after Keeley &

levels, (B) rates of CO2 uptake, and (C) irradiance at Busch 1984). Copyright American Society of Plant the water surface, water temperatures, and concen- Biologists. trations of CO2 and O2; the numbers near the symbols

Figure 53. CAM photosynthesis in submerged leaves give the pH values. Open and filled symbols refer to of Isoetes howellii(quillwort) in a pool. (A) Malic acid the light and dark period, respectively (after Keeley &

levels, (B) rates of CO2 uptake, and (C) irradiance at Busch 1984). Copyright American Society of Plant the water surface, water temperatures, and concen- Biologists. trations of CO2 and O2; the numbers near the symbols

This reaction occurs in the alkaline compartment that is provided at the upper side of the polar leaves of aquatic macrophyytes (Sect. 11.3). Similar amounts of carbon are assimilated in photosynthesis and precipitated as carbonate. If only part of the CO2 released in this process is assimilated by the macrophyte, as may occur under nutrient-deficient conditions, CO2 is released to the atmosphere. On the other hand, if the alkalinity of the compartment is relatively low, there is a net transfer of atmospheric CO2 to the water (McConnaughey et al. 1994).

Figure 54. Variation in the carbon-isotope composition (<S13C) of freshwater and marine aquatic species. The observed variation is due to variation in S13C values of the substrate and in the extent of diffusional limitation (Osmond et al. 1982).

Figure 54. Variation in the carbon-isotope composition (<S13C) of freshwater and marine aquatic species. The observed variation is due to variation in S13C values of the substrate and in the extent of diffusional limitation (Osmond et al. 1982).

Table 15. Carbon-isotope composition (ô13C in %o) of submerged and emergent Isoetes howellii plants.

Pondwater carbonate Submerged Leaves Roots Emergent Leaves Roots

Source: Keeley & Busch (1984).

* Values are given for both leaves and roots and also for the pondwater carbonate.

Table 16. Changes in the dissolved carbon isotope composition with depth as reflected in the composition of the organic matter at that depth.

Water depth (m)

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