Crassulacean acid metabolism in the context of other carbon-concentrating mechanisms in freshwater plants: a review.

Inorganic carbon can be in short supply in freshwater relative to that needed by freshwater plants for photosynthesis because of a large external transport limitation coupled with frequent depleted concentrations of CO(2) and elevated concentrations of O(2). Freshwater plants have evolved a host of avoidance, exploitation and amelioration strategies to cope with the low and variable supply of inorganic carbon in water. Avoidance strategies rely on the spatial variation in CO(2) concentrations within and among lakes. Exploitation strategies involve anatomical and morphological features that take advantage of sources of CO(2) outside of the water column such as the atmosphere or sediment. Amelioration strategies involve carbon-concentrating mechanisms based on uptake of bicarbonate, which is widespread, C(4)-fixation, which is infrequent, and crassulacean acid metabolism (CAM), which is of intermediate frequency. CAM enables aquatic plants to take up inorganic carbon in the night. Furthermore, daytime inorganic carbon uptake is generally not inhibited and therefore CAM is considered to be a carbon-conserving mechanism. CAM in aquatic plants is a plastic mechanism regulated by environmental variables and is generally downregulated when inorganic carbon does not limit photosynthesis. CAM is regulated in the long term (acclimation during growth), but is also affected by environmental conditions in the short term (response on a daily basis). In aquatic plants, CAM appears to be an ecologically important mechanism for increasing inorganic carbon uptake, because the in situ contribution from CAM to the C-budget generally is high (18-55%).

Light regulation of root and leaf NO3 - uptake and reduction in the floating macrophyte Lemna minor.

• The regulation of NO3 - uptake kinetics and reduction in relation to long- and short-term changes in irradiance was explored in roots and photosynthetic tissues of Lemna minor. • The NO3 - uptake kinetics, nitrate reductase activity, plant morphology, chlorophyll and tissue NO3 - , organic-N, starch and sugars were measured on roots and fronds of L. minor grown at four combinations of irradiance- and NO3 - availability. • Long-term acclimatizations paralleled those of terrestrial plants. Short-term changes in irradiance, however, changed frond NO3 - uptake proportionally with frond chlorophyll and N content, indicating a relationship between responsiveness and the metabolic potential of the plants. Root uptake changed to balance the change in frond uptake keeping whole plant uptake varying by < 40%. Nitrate reductase activity was primarily located in the roots and was correlated with frond uptake, indicating a transport of NO3 - from shoot to root before reduction. • This study shows that irradiance can affect the contribution of root and leaf uptake by aquatic plants and that roots play a major role in NO3 - reduction despite large foliar uptake.

High internal resistance to CO(2) uptake by submerged macrophytes that use HCO(3) (-): measurements in air, nitrogen and helium.

Previous studies have shown that in water the affinity of submerged macrophytes for CO(2) is higher for species restricted to CO(2) than for species with an additional ability to use bicarbonate. We measured slopes of CO(2) uptake versus CO(2) concentration in the gas phase in air, nitrogen and helium for pairs of species, having or lacking the ability to use bicarbonate, but with similar leaf morphology. For species restricted to CO(2), the slope in nitrogen and helium was 1.5 times and 3.2 times greater than in air. The increased slope in nitrogen results from a suppression of photorespiration. The further increase in helium reflects the increased rate of diffusion of CO(2) and shows that, even in gas, external diffusion through the boundary layer is a significant hindrance to CO(2) uptake. In contrast, in species able to use bicarbonate, the uptake slope was not affected by gas composition, suggesting that photorespiration is absent or photorespired CO(2) is efficiently trapped and that internal resistance is high relative to external resistance. Elodea canadensis specimens grown under high concentrations of CO(2) de-regulated their ability to use bicarbonate, and slopes of CO(2) uptake in helium were significantly greater than in air or nitrogen. Overall, these results are consistent with the notion that while a high affinity for CO(2) will maximise carbon uptake in species restricted to CO(2), for species able to use bicarbonate, a high internal resistance would reduce loss of CO(2) and help maintain high concentrations of CO(2) at the site of fixation.

Freshwater angiosperm carbon concentrating mechanisms: processes and patterns.

Aquatic angiosperms are derived from terrestrial ancestors and appear to have re-invaded water on many occasions. While removing problems of water supply and reducing the need for supporting tissue, freshwaters have a potentially low and fluctuating supply of CO2 for photosynthesis, as well as generally low light. This paper reviews the structural, morphological, physiological, and biochemical features of freshwater macrophytes in the context of maximising net carbon uptake underwater, and discusses how inorganic carbon may influence macrophyte ecology. Submerged leaves tend to have a low photosynthetic capacity on an area basis, matching the low rates of supply of CO2 and light. Morphological and structural strategies to overcome potential carbon limitation include possession of aerial or floating leaves, and lacunal connexions to high concentrations of sedimentary CO2 via the roots. Physiological and biochemical strategies include crassulacean acid metabolism, C4-like metabolism in Hydrilla and Egeria, and the ability to use HCO3-. The activity of all these can be regulated by environmental conditions to maximize growth rate. Use of HCO3-. is the most widespread carbon acquisition strategy, present in about half of the tested submerged angiosperms. It is more common in lakes of high alkalinity and in the elodeid growth form.