Catchment properties and the photosynthetic trait composition of freshwater plant communities.

Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO2) to compensate for the low diffusivity and potential depletion of CO2 in water. Concentrations of bicarbonate and CO2 vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO2 concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO2 concentrations may alter the species compositions of freshwater plant communities.

Comparative functional plant ecology: rationale and potentials.

The long tradition and early training in plant taxonomy and evolutionary theory have led to the preconceived notion among plant ecologists that plant diversity represents the sum of distinct classes, rather than a continuum of variation. Thus, broad-scale comparisons across different taxa are viewed with scepticism. Yet, the unarticulated reluctance to compare the functional properties of phototrophic organisms across broad ranges of plant types is not well-grounded. Broad comparisons are likely to capture most of the functional variance that these organisms display and lead to general conclusions, albeit sometimes imprecise for individual species. Ecological synthesis could be fostered if scientists working with the different plant types made efforts to Increase connectance between them by highlighting similarities, instead of formulating Independent theories to account for similar phenomena.

Microsensor Analysis of Oxygen in the Rhizosphere of the Aquatic Macrophyte Littorella uniflora (L.) Ascherson.

Oxygen released by the roots of submerged plants may oxidize organic compounds from the roots and reduced substances continuously supplied by diffusion from the surrounding anoxic hydrosoil. We provide here the first visualization of this gradient environment obtained by microsensor analysis of oxygen in the rhizosphere of the freshwater plant Littorella uniflora (L.) Ascherson. The plants were rooted in an agar medium, in which amorphous FeS provided the main oxygen sink. The oxygen concentration at the root surface ranged from 20 to 450 [mu]M (atmospheric saturation = 280 [mu]M) between darkness and saturating light, and the oxic shell surrounding the roots varied from about 0.5 to 5 mm in thickness. The oxygen flux from the roots was a saturating function of the incident light intensity on the leaves, and the oxygen released was consumed mainly at the fluctuating oxic/anoxic interface. The oxic zones around individual roots are under dynamic control by light, root morphology, root density, and sediment reducing capacity, and, therefore, oxygen concentrations should be subject to substantial diurnal fluctuations in dense Littorella populations in nutrient-poor sediments.

Patterns in decomposition rates among photosynthetic organisms: the importance of detritus C:N:P content.

The strength and generality of the relationship between decomposition rates and detritus carbon, nitrogen, and phosphorus concentrations was assessed by comparing published reports of decomposition rates of detritus of photosynthetic organisms, from unicellular algae to trees. The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations. Differences in the carbon, nitrogen, and phosphorus concentrations of plant detritus accounted for 89% of the variance in plant decomposition rates of detritus orginating from photosynthetic organisms ranging from unicellular microalgae to trees. The results also demonstrate that moist plant material decomposes substantially faster than dry material with similar nutrient concentrations. Consideration of lignin, instead of carbon, concentrations did not improve the relationships obtained. These results reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers, and stress the importance of this coupling for carbon and nutrient flow in ecosystems.

The quantum efficiency of photosynthesis in macroalgae and submerged angiosperms.

Photon absorption and photosynthesis under conditions of light limitation were determined in six temperate marine macroalgae and eight submerged angiosperms. Photon absorption and photosynthetic efficiency based on incident light increased in proportion to chlorophyll density per area and approached saturation at the highest densities (∼300 mg chlorophyll m-2) encountered. Absorption and photosynthetic efficiency were higher in brown and red algae than in green algae and angiosperms for the same chlorophyll density because of absorption by accessory pigments. Among thin macroalgae and submerged angiosperms chlorophyll variations directly influence light absorption and photosynthesis, whereas terrestrial leaves have chlorophyll in excess and thus there is only a minor influence of pigment variability on light-limited photosynthesis. The quantum efficiency of photosynthesis averaged 0.062±0.019 (±SD) mol O2 mol-1 photons absorbed for macroalgae and, significantly less, 0.049±0.016 mol O2 mol-1 photons for submerged angiosperms. Of the measurements 80% were between 0.037 and 0.079 mol O2 mol-1 photons. The results are lower than values given in the literature for unicellular algae and terrestrial C3 species at around 0.1 mol O2 mol-1 photons, but resemble values for other marine macroalgae and terrestrial C4 species. The reason for these differences remains unknown, but may be sought for in differential operation of cyclic photophosphorylation and photorespiration.

The carboxylase activity of Rubisco and the photosynthetic performance in aquatic plants.

Activated carboxylase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), as well as photosynthetic rates were measured for 42 species of freshwater and marine macrophytes. While the carboxylase activity varied greatly among the species investigated (0.2-12.5 µmol CO2 mg-1 chlorophyll min-1), the submersed freshwater plants showed significantly lower activities than emergent, floating leaved or secondary submersed forms. The variability in photosynthetic rates correlated with the carboxylase activity only for the marine macroalgae, and their photosynthesis to carboxylase activity ratios were close to 1. These plants also had a consistently high inorganic carbon transport capability, and it is suggested that ribulose-1,5-bisphosphate carboxylase/oxygenase activity is an important internal factor regulating the photosynthetic capacity within this plant group where, apparently, the internal CO2 concentration is high and photorespiration is suppressed. Among the freshwater forms, it appears that their much lower inorganic carbon transport ability, rather than their carboxylase activity, limits the photosynthetic process.