Coupling biochemical and biophysical processes at the leaf level: an equilibrium photosynthesis model for leaves of C3 plants

The paper presents a generic computer model for estimating short-term steady-state fluxes of CO₂, water vapor, and heat from broad leaves and needle-leaved coniferous shoots of C₃ plant species. The model explicitly couples all major processes and feedbacks known to impact leaf biochemistry and biophysics including biochemical reactions, stomatal function, and leaf-boundary layer heat- and mass-transport mechanisms. The ability of the model to successfully predict measured photosynthesis and stomatal-conductance data as well as to simulate a variety of observed leaf responses is demonstrated. A model application investigating physiological and environmental regulation of leaf water-use efficiency (WUE) under steady-state conditions is discussed. Simulation results suggest that leaf physiology has a significant control over the environmental sensitivity of leaf WUE. The implementation of a highly efficient solution technique allows the model to be directly incorporated into plant-canopy and terrestrial ecosystem models.     

O3对水稻叶片氮代谢、脯氨酸和谷胱甘肽含量的影响

臭氧(O3)被认为是重要的气污染物之一,水稻又是主要的粮食作物,因而准确地评估O3浓度升高对水稻生长发育的影响具有十分重要的意义。采用开顶式气室法模拟研究了O3对水稻叶片可见伤害症状、氮代谢、脯氨酸和谷胱甘肽含量的影响。结果显示,O3污染胁迫会导致水稻叶片产生明显的伤害症状,具体表现为:老叶叶鞘褪绿,有褐斑,直至完全干枯;稻穗小且黄化,籽粒不饱满;水稻成熟期提前等。O3浓度升高对水稻叶片的硝酸还原酶活性有显著影响。当O3浓度为40、80和120nL.L-1时,水稻叶片硝酸还原酶活性与对照组相比均降低,其中,分蘖期分别降低了25.3%、67.4%和86.3%;拔节期分别降低了57.4%、75.7%和97.8%;抽穗期分别降低了91.0%、97.2%和99.3%;乳熟期分别降低了89.5%、89.5%和96.7%。水稻叶片铵态氮和硝态氮含量随着O3浓度的升高而显著地降低,例如当O3浓度为40、80和120nL.L-1时,与对照相比,水稻叶片硝态氮含量分别降低46.3%、52.7%和65.7%,铵态氮含量分别降低6.5%、12.9%和43.4%。O3污染胁迫下水稻叶片脯氨酸含量在不同生长期变化不同,分蘖期、拔节期和抽穗期脯氨酸含量在40nL.L-1浓度O3熏蒸下急剧地提高,但是随着O3浓度的增加,脯氨酸含量又不断地降低。在水稻乳熟期,脯氨酸含量均随着O3浓度的增加而显著地下降。O3污染胁迫导致水稻叶片还原型谷胱甘肽(GSH)含量显著低于对照组,而氧化型谷胱甘肽(GSSG)含量显著高于对照组。当O3浓度为40、80和120nL.L-1时,乳熟期水稻叶片GSH含量分别比对照组降低68.7%、80.2%和78.2%,GSSG含量分别比对照提高494.4%、527.2%和439.8%。研究表明,O3污染胁迫对水稻叶片氮代谢和抗氧化系统产生了极显著的影响。     

Integrating effects of leaf nitrogen,age, rank,and growth temperature into the photosynthesis-stomatal conductance model LEAFC3-N parameterised for barley (Hordeum vulgare L.)

A crucial challenge for including biophysical photosynthesis–transpiration models into complex crop growth models is to integrate the plasticity of photosynthetic processes that is related to factors like nitrogen (N) content, age, and rank of leaves, or to the adaptation of plants to growth temperature (T_g). Here we present a new version of the combined photosynthesis-stomatal conductance model LEAFC3-N [Müller, J., Wernecke, P., Diepenbrock, W., 2005. LEAFC3-N: a nitrogen sensitive extension of the CO₂ and H₂O gas exchange model LEAFC3 parameterised and tested for winter wheat (Triticum aestivum L.). Ecological Modelling 183, 183–210.] that was revised, extended and completely re-parameterised for barley (Hordeum vulgare L.) with special regard for these factors to facilitate the use of the model in ecophysiological studies and in crop modelling. The analysis is based on novel comprehensive data on photosynthetic CO₂ and light response curves measured at two oxygen concentrations and different temperatures on leaves of barley (H. vulgare L.) differing in leaf N and chlorophyll content. Plants were grown in climatic chambers or in the field at different N and T_g.We thoroughly revised the existing and introduced new nitrogen relations for key model parameters that account for a linear increase with leaf N of V_max, J_max, T_p, and R_dmax (maximum rates of carboxylation, electron transport, triose phosphate export, and mitochondrial respiration), a saturation-type increase of φ (quantum yield of electron transport), and a non-linear decrease of θ and m (curvature of the light dependence of electron transport rate, scaling factor of the stomata model). The adaptation of photosynthetic characteristics to T_g was included into the model by linear relations that were observed between T_g and the activation energy ΔH_a of the temperature response characteristics of V_max, J_max, and T_p as well as of the nitrogen dependency of these characteristics. Based on an analysis of diurnal time courses of gas exchange rates it was found necessary including not only the relation between leaf water potential (Ψ) and stomatal conductance as used originally in LEAFC3, but additional effects on V_max and J_max. With the above-listed extensions, the model was capable to reproduce the observed plasticity and the recorded diurnal time courses of gas exchange rates fairly well. Thus, we conclude that the new model version can be used under a broad range of conditions, both for ecophysiological studies and as a submodel of crop growth models. The results presented here for barley will facilitate adapting photosynthesis models like LEAFC3-N to other C₃-species as well. The modelling of the effects of drought stress should be further elaborated in future based on more specific experiments.     

Beurteilung des Ozonrisikos für die Waldregionen Bayerns am Beispiel des Jahres 2002 und des Extremtrockenjahres 2003 auf der Basis der externen Ozonexposition und der internen Ozonaufnahme

Background, aim, and scope Increasing background concentrations of ground-level tropospheric ozone and more frequent and prolonged summer drought incidences due to climate change are supposed to increase the stress on Bavarian forests. For such scenarios growth reduction and yield losses are predicted. Sustainable forest management in Bavaria aims to significantly increase the proportion of beech (Fagus sylvatica L.) because of its broad ecological amplitude. In our regional study different approaches for calculating ozone impact were used to estimate the risks for Bavarian forests in the average climatic, rather moist year 2002 and the extremely dry year 2003.Materials and methods Measurements were conducted for eleven forest ecosystem sites and two forest research sites representing typical Bavarian forest stands under different climatic conditions and situated in different altitudes. For risk assessment currently used approaches were applied either based on the calculation of the cumulative ozone exposure (external dose; MPOC maximal permitted ozone concentration; critical level AOT40_phen? accumulated ozone exposure over a threshold of 40 nl [O₃] l^–1, for the effective phenolgy of beech) or based on the calculation of the phytomedically relevant ozone flux into the stomata (internal dose, critical level AF_st>1,6, accumulated stomatal flux above a flux threshold of 1.6 nmol O₃?m^–2 PLA; PLA = projected leaf area). For calculations continuously recorded ozone concentrations and meteorological and phenological data from nearby rural open field background measuring stations from the national air pollution control and from forested sites were used. Additionally ozone induced leaf symptoms were assessed.Results The exposure-based indices AOT40_phen and MPOC as well as the flux-based index AF_st>1.6suggest that Bavarian forests are at risk from O₃ during a rather moist average year concerning climate conditions (2002) as well as in an extreme dry year (2003). Thus, growth reductions of 5?% are predicted when thresholds are exceeded. Threshold exceedance occurred in both years at all plots, mostly already at the beginning of the growing season and often even many times over. Ozone induced leaf symptoms could be detected only on a few plots in a very slight occurrence.Discussion The results for the applied critical level indices differed depending on climatic conditions during the growing seasons: Regarding exposure-based indices, the highest degree of threshold exceedance occurred in the dry year of 2003 at all plots; the flux-based approach indicated the highest stomatal ozone uptake and thus an increased risk at moist sites or during humid years, whereas the risk was decreasing at dry sites with prolonged water limitation. Hence, soil and accordingly plant water availability was the decisive factor for the flux-modelled internal ozone uptake via stomata. Drought and increased ozone impact can generate synergistic, but also antagonistic effects for forest trees. At water limited rather dry forest sites restricted transpiration and thus production, but concurrently lower ozone uptake and reduced risk for damage can be expected.Conclusions, recommendations, and perspectives For realistic site-specific risk assessment in forest stands the determination of the internal ozone dose via modeling flux based internal stomatal ozone uptake is more appropriate than the calculation of the external ozone dose. The predicted 5?% growth reductions are in discrepancy with the frequently observed increment increase during the last decades in forest stands. Comprehensive and significant statistical verification for ozone induced forest growth reduction as well as the systematic validation of thresholds for ozone in the field is still lacking. However, a multiplicity of different specific new and retrospective growth analysis data should allow closing the gap. Moreover, the determination of canopy transpiration with sap flow measurements is a novel approach to provide cause-effect related, site specific results for the effective internal ozone dose as well as for canopy water supply and consecutively for regional risk estimation. A further future objective is the refinement of O₃ flux modelling by further consideration of soil/water budget characteristics and the above mentioned improved estimations of crown and canopy transpiration. Further, the introduction of threshold ranges for forest trees in view of their specific regional climatic conditions and their validation in real forest stands is necessary for developing meaningful ozone risk predictions for forests.     

Photosynthesis and carbon storage between tides in a brown alga, Fucus vesiculosus

Intertidal macroalgae may spend a significant part of their lives in air. During photosynthesis in air, they encounter much lower concentrations of inorganic carbon than in seawater. Because they accumulate inorganic carbon from seawater, we investigated whether they similarly accumulate it from air. We measured photosynthesis in the intertidal species Fucus vesiculosus L. during 1990 and 1991 with a gas-phase O₂ electrode or CO₂-exchange apparatus in air and with a liquid-phase O₂ electrode in seawater. Maximum rates were rapid and similar in air and seawater regardless of the method. Tissue from seawater could carry on photosynthesis in CO₂-free air, indicating that carbon was stored in the tissue. After 2 h, this store was depleted and photosynthesis ceased. Supplying CO₂ in air replenished the store. Under identical conditions, terrestrial C₃ and C₄ species showed no evidence of this store, but a CAM (crassulacean acid metabolism) species did. However, in contrast to the CAM behavior, F. vesiculosus did not store CO₂ significantly in the dark. We found a small acid-releasable pool of carbon in the tissue that disappeared as photosynthesis depleted the carbon store. However, the pool was too small to account for the total carbon stored. While CO₂ was being acquired or released from the store in the light, photosynthesis was not inhibited by 21% O₂. These results indicate that there are two parallel paths for the supply of CO₂ to photosynthesis. The first depends on inorganic carbon in seawater or in air and supports rapid photosynthesis. The second involves CO₂ slowly released from an organic intermediate. The release protects CO₂ fixation from the inhibitory effects of 21% O₂. Photosynthesis in F. vesiculosus thus appears to be C₃-like in its rapid fixation of CO₂ from a small inorganic pool into phosphoglycerate. However, it is C₄-like in its pre-fixation of carbon in an organic pool in the light, and is CAM-like in its ability to slowly use this pool as a sole source of CO₂. The organic pool may serve to protect photosynthetic CO₂ fixation against the inhibitory effects of O₂ in air and in the boundary layer in seawater. Received: 6 March 1998 / Accepted: 16 October 1998     

Effects of O2, pH and DIC on photosynthetic net-O2 evolution by marine macroalgae

Net photosynthetic O₂ evolution by five marine macroalgae:Ulva lactuca L.,Enteromorpha sp.,Ceramium strictum Harvey,Fucus serratus L., andF. vesiculosus L., collected from Danish waters in the summer of 1983 was followed at increasing O₂ and with pH either fixed close to pH 7, 8 or 9, or drifting upwards during photosynthesis in a closed chamber to determine the effects of changing O₂, pH and DIC (dissolved inorganic carbon) on photosynthesis. Increasing O₂, increasing pH and decreasing DIC together limited O₂ evolution. Raising the O₂ concentration with pH and DIC held constant resulted in less inhibition of net-O₂ evolution than when all three factors acted together. The O₂ inhibition of photosynthesis was similar to the reported O₂ inhibition of ribulose 1,5-bisphosphate carboxylase isolated from lower and higher plants. Net-O₂ evolution as a function of the molar ratio of O₂ to HCO^– ₃ + CO₂ in solution provided a general, linear relationship (r ² = 0.72 to 0.84), predicting inhibition of photosynthesis based on O₂ pH and DIC changing together. Slopes of this relationship, representing competition between O₂ and carbon based on external concentrations, were similar for the five taxonomically different algae, suggesting that similar processes act to reduce net-O₂ evolution.     

Modeling compensated root water and nutrient uptake

Plant root water and nutrient uptake is one of the most important processes in subsurface unsaturated flow and transport modeling, as root uptake controls actual plant evapotranspiration, water recharge and nutrient leaching to the groundwater, and exerts a major influence on predictions of global climate models. In general, unsaturated models describe root uptake relatively simple. For example, root water uptake is mostly uncompensated and nutrient uptake is simulated assuming that all uptake is passive, through the water uptake pathway only. We present a new compensated root water and nutrient uptake model, implemented in HYDRUS. The so-called root adaptability factor represents a threshold value above which reduced root water or nutrient uptake in water- or nutrient-stressed parts of the root zone is fully compensated for by increased uptake in other soil regions that are less stressed. Using a critical value of the water stress index, water uptake compensation is proportional to the water stress response function. Total root nutrient uptake is determined from the total of active and passive nutrient uptake. The partitioning between passive and active uptake is controlled by the a priori defined concentration value c_max. Passive nutrient uptake is simulated by multiplying root water uptake with the dissolved nutrient concentration, for soil solution concentration values below c_max. Passive nutrient uptake is thus zero when c_max is equal to zero. As the active nutrient uptake is obtained from the difference between plant nutrient demand and passive nutrient uptake (using Michaelis–Menten kinetics), the presented model thus implies that reduced passive nutrient uptake is compensated for by active nutrient uptake. In addition, the proposed root uptake model includes compensation for active nutrient uptake, in a similar way as used for root water uptake. The proposed root water and nutrient uptake model is demonstrated by several hypothetical examples, for plants supplied by water due to capillary rise from groundwater and surface drip irrigation.     

Nitrate uptake and assimilation in Thalassiosira weissflogii and Phaeodactylum tricornutum: interactions with photosynthesis and with the uptake of other ions

Interactions of the nitrate, phosphate, and ammonium uptake systems and the interactions of these systems with photosynthesis were investigated for Thalassiosira weissflogii and Phaeodactylum tricornutum preconditioned in continuous culture. The cultures were supplied with NO _- ³ and PO ₌ ⁴ in an N:P atomic ratio of 15:1, and residual concentrations of both nutrients in the growth chamber were very low. The rate of NO _- ³ uptake was reduced by the addition of NH ₊ ⁴ or PO ₌ ⁴ . The rate of PO ₌ ⁴ uptake by T. weissflogii was reduced by the addition of NH ₊ ⁴ . The rate of carbon fixation was reduced by NO _- ³ additions and slightly reduced by the addition of PO ₌ ⁴ . There were two components of NO _- ³ uptake, one light-dependent and one light-independent. Uptake inhibition by added PO ₌ ⁴ acted on the light-independent component. The change in the C fixation rate due to added NO _- ³ was equal to the rate of NO _- ³ uptake by the light-dependent component on a molar basis. Nitrate assimilation (reduction) rates were calculated from the time course of extracellular and intracellular NO _- ³ concentrations. The light-dark change in the assimilation rate was similar to the light-dark change in the uptake rate, suggesting close coupling between the light-dependent components of uptake and assimilation. The assimilation rate dropped upon exhaustion of extracellular NO _- ³ , implying that an uptake-coupled component of assimilation is unavailable for the assimilation of intracellular NO _- ³ . The reduction in the C fixation rate due to NO _- ³ was temporally associated with uptake rather than assimilation, but may reflect interaction with either the light-dependent uptake step or the closely coupled assimilation. Phosphate additions reduced the rate of NO _- ³ uptake, while the rate of assimilation was unaffected.     

The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral <Emphasis Type="Italic"> Galaxea fascicularis</Emphasis>

The mechanism of calcification and its relation to photosynthesis and respiration were studied with Ca²⁺, pH and O₂ microsensors using the scleractinian coral Galaxea fascicularis. Gross photosynthesis (Pg), net photosynthesis (Pn) and dark respiration (DR) were measured on the surface of the coral. Light respiration (LR) was calculated from the difference between Pg and Pn. Pg was about seven times higher than Pn; thus, respiration consumes most of the O₂ produced by the algal symbiont's photosynthesis. The respiration rate in light was ca. 12 times higher than in the dark. The coupled Pg and LR caused an intense internal carbon and O₂ cycling. The resultant product of this cycle is metabolic energy (ATP). The measured ATP content was about 35% higher in light-incubated colonies than in dark-incubated ones. Direct measurements of Ca²⁺ and pH were made on the outer surface of the polyp, inside its coelenteron and under the calicoblastic layer. The effects on Ca²⁺ and pH dynamics of switching on and off the light were followed in these three compartments. Ca²⁺ concentrations decreased in light on the surface of the polyp and in the coelenteron. They increased when the light was switched off. The opposite effect was observed under the calicoblastic layer. In light, the level of Ca²⁺ was lower on the polyp surface than in the surrounding seawater, and even lower inside the coelenteron. The concentration of calcium under the calicoblastic layer was about 0.6 mM higher than in the surrounding seawater. Thus Ca²⁺ can diffuse from seawater to the coelenteron, but metabolic energy is needed for its transport across the calicoblastic layer to the skeleton. The pH under the calicoblastic layer was more alkaline compared with the polyp surface and inside the coelenteron. This rise in pH increased the supersaturation of aragonite from 3.2 in the dark to 25 in the light, and brought about more rapid precipitation of CaCO₃. When ruthenium red was added, Ca²⁺ and pH dynamics were inhibited under the calicoblastic layer. Ruthenium red is a specific inhibitor of Ca-ATPase. The results indicated that Ca-ATPase transports Ca²⁺ across the calicoblastic layer to the skeleton in exchange for H⁺. Addition of dichlorophenyldimethylurea completely inhibited photosynthesis. The calcium dynamics under the calicoblastic layer continued; however, the process was less regular. Initial rates were maintained. We conclude that light and not energy generation triggers calcium uptake; however, energy is also needed.     

Mechanisms for the uptake of inorganic carbon by two species of symbiont-bearing foraminifera

The mechanisms for uptake of inorganic carbon (Ci) for photosynthesis and calcification of a perforate foraminifer, Amphistegina lobifera Larsen, and an imperforate species, Amphisorus hemprichii Ehrenberg, from the Gulf of Eilat, Red Sea were studied in 1986–1987 using ¹⁴C tracer techniques. Total Ci uptake of A. lobifera and photosynthetic carbon uptake of A. hemprichii fit the Hill-Whittingham equation that describes the overall rate of enzymatic reactions that are provided with their substrate through a diffusion barrier. This suggests that diffusion is the rate limiting step for total Ci uptake in A. lobifera. Photosynthesis by the isolated symbionts and uptake of CO₃ ^2- for calcification obey Michaelis-Menten kinetics indicating that enzymatic reactions determine the rate of the separate processes. Both photosynthesis and calcification can be inhibited without affecting each other. Calcification rates in A. lobifera were optimal at Ca levels around normal seawater concentration and were sensitive to inhibitors of respiratory adenosine triphosphate (ATP) generation and Ca-ATP-ase. This indicates that Ca uptake is also active. Calcification rates of A. hemprichii increased linearly as a function of external Ci concentration over the entire experimental range (0 to 4 mM Ci). In contrast, photosynthetic rates showed Hill-Whittingham type kinetics. The dependence of calcification on the CO₃ ^2- concentration was also linear, suggesting that its diffusion is the rate limiting step for calcification in A. hemprichii. Increasing Ca concentrations yielded higher calcification rates over the entire range measured (0 to 40 mM Ca). Calcification in A. hemprichii was less sensitive to inhibitors of ATP generation than in A. lobifera, suggesting that in A. hemprichii energy supply is less important for this process.     

Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis

We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O₂ microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabled parallel measurements of O₂ concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O₂ levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETR_max) (84±15 and 41±10, respectively) than sun-adapted coenosarc and polyp tissues (136±14 and 77±13, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O₂ concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 μmol photons m⁻² s⁻¹. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII.     

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by ¹⁵N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K _I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K _I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K _I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K _Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Impact of photosynthesis and transpiration on nitrogen removal in constructed wetlands

To determine the impact of photosynthesis and transpiration on nitrogen removal in wetlands, an artificial wetland planted with reeds was constructed to treat highly concentrated domestic wastewater. Under different meteorological and hydraulic conditions, the daily changes of photosynthesis and transpiration of reeds, as well as nitrogen removal efficiency were measured. It was found that net photosynthesis rate per unit leaf area was maintained on a high level (average 19.0 μmol CO₂/(m²·s)) from 10:00 to 14:00 in July 2004 and reached a peak of 21.1 μmol CO₂/(m²·s) when Photon Flux Density was high during the day. Meanwhile, TN and NH₄ ⁺-N removal efficiency rose to 79.6% and 89.6%, respectively—the maximum values observed in the test. Correlation coefficient analysis demonstrated a positive correlation among photon flux density, net photosynthetic rate, transpiration rate, and TN and NH₄ ⁺-N removal efficiency. In contrast, there was a negative correlation between stomatal conductance and TN and NH₄ ⁺-N removal efficiency. Results suggest that the photosynthesis and transpiration of wetland plants have a great impact on nitrogen removal efficiency of wetlands, which can be enhanced by an increase in the photosynthesis and transpiration rate. In addition, the efficiency of water usage by reeds and nitrogen removal efficiency could be affected by the water level in wetlands; a higher level boosts nitrogen removal efficiency.     

Diffusive boundary layers and photosynthesis of the epilithic algal community of coral reefs

The effects of mass transfer resistance due to the presence of a diffusive boundary layer on the photosynthesis of the epilithic algal community (EAC) of a coral reef were studied. Photosynthesis and respiration of the EAC of dead coral surfaces were investigated for samples from two locations: the Gulf of Aqaba, Eilat (Israel), and One Tree Reef on the Great Barrier Reef (Australia). Microsensors were used to measure O₂ and pH at the EAC surface and above. Oxygen profiles in the light and dark indicated a diffusive boundary layer (DBL) thickness of 180–590 μm under moderate flow (~0.08 m s^?1) and >2,000 μm under quasi-stagnant conditions. Under light saturation the oxygen concentration at the EAC surface rose within a few minutes to 200–550% air saturation levels under moderate flow and to 600–700% under quasi-stagnant conditions. High maximal rates of net photosynthesis of 8–25 mmol O₂ m^?2 h^?1 were calculated from measured O₂ concentration gradients, and dark respiration was 1.3–3.3 mmol O₂ m^?2 h^?1. From light–dark shifts, the maximal rates of gross photosynthesis at the EAC surface were calculated to be 16.5 nmol O₂ cm^?3 s^?1. Irradiance at the onset of saturation of photosynthesis, E _k, was <100 µmol photons m^?2 s^?1, indicating that the EAC is a shade-adapted community. The pH increased from 8.2 in the bulk seawater to 8.9 at the EAC surface, suggesting that very little carbon in the form of CO₂ occurs at the EAC surface. Thus the major source of dissolved inorganic carbon (DIC) must be in the form of HCO₃ ^?. Estimates of DIC fluxes across the DBL indicate that, throughout most of the daytime under in situ conditions, DIC is likely to be a major limiting factor for photosynthesis and therefore also for primary production and growth of the EAC.     

Mercury accumulation in soils and plants in the Almadén mining district, Spain: one of the most contaminated sites on Earth

Although mercury (Hg) mining in the Almadén district ceased in May 2002, the consequences of 2000 years of mining in the district has resulted in the dissemination of Hg into the surrounding environment where it poses an evident risk to biota and human health. This risk needs to be properly evaluated. The uptake of Hg has been found to be plant-specific. To establish the different manners in which plants absorb Hg, we carried out a survey of Hg levels in the soils and plants in the most representative habitats of this Mediterranean area and found that the Hg concentrations varied greatly and were dependent on the sample being tested (0.13–2,695 μg g⁻¹ Hg). For example, the root samples had concentrations ranging from 0.06 (Oenanthe crocata, Rumex induratus) to 1095 (Polypogon monspeliensis) μg g⁻¹ Hg, while in the leaf samples, the range was from 0.16 (Cyperus longus) to 1278 (Polypogon monspeliensis) μg g⁻¹ Hg. There are four well-differentiated patterns of Hg uptake: (1) the rate of uptake is constant, independent of Hg concentration in the soil (e.g., Pistacia lentiscus, Quercus rotundifolia); (2) after an initial linear relationship between uptake and soil concentration, no further increase in Hg_plant is observed (e.g., Asparagus acutifolius, Cistus ladanifer); (3) no increase in uptake is recorded until a threshold is surpassed, and thereafter a linear relationship between Hg_plant and Hg_soil is established (e.g., Rumex bucephalophorus, Cistus crispus); (4) there is no relationship between Hg_plant and Hg_soil (e.g., Oenanthe crocata and Cistus monspeliensis). Overall, the Hg concentrations found in plants from the Almadén district clearly reflect the importance of contamination processes throughout the study region.     

Inhibition of diatom photosynthesis by germanic acid: Separation of diatom productivity from total marine primary productivity

Germanic acid was shown to inhibit photosynthetic ¹⁴CO₂ uptake in marine diatoms. Inhibition was not complete even at concentrations of 20 mg Ge/1 nor in cultures incubated for extended periods of time (up to 24 h). The decrease in photosynthesis due to Ge(OH)₄ was independent of the stage of growth of the diatom culture. At 0.5 and 1.0 mg Ge/1, the degree of inhibition was dependent on the concentration of Si(OH)₄ in the medium. At 5 and 10 mg Ge/1, inhibition was not affected by Si(OH)₄ concentrations as high as those found in the sea-120 g-at Si/1. The effect of Ge(OH)₄ on photosynthesis is specific for diatoms; other marine phytoplankton were not inhibited. In mixed cultures of diatoms and marine flagellates, the reduction in ¹⁴CO₂ fixation upon addition of Ge(OH)₄ was used to calculated the proportion of diatom photosynthesis to total photosynthesis, and calculated proportions agreed well with actual proportions. Inhibition by Ge(OH)₄ was also used to estimate the percent of diatom photosynthesis in a natural marine community, and this was compared with the diatom portion of the crop. Diatom photosynthesis was higher than one might expect from crop figures, although both diatom photosynthesis and diatom numbers in the crop were low.     

Experimental ecology of the temperate scleractinian coral Astrangia danae I. Partition of respiration,photosynthesis and calcification between host and symbionts

Calcification, photosynthesis and respiration of the scleractinian coral Astrangia danae were calculated from the changes in total alkalinity, pH, calculated total CO₂, and oxygen concentration produced by colonies incubated in glass jars. A correction for changes in ammonia, nitrate and nitrite was taken into account and the method evaluated. The fluxes of oxygen and CO₂ were highly correlated (r=0.99). The statistical error of alkalinity determinations was less than 10% of the changes observed in the slowest calcifying samples. Metabolism of polyparium alone was estimated by difference after removal of tissue and reincubation of bare corallum. Zooxanthellae concentration in the polyps was obtained from cell counts made on homogenates of polyp tissue. The calculated photosynthetic rate of the zooxanthellae in vivo was 25 mol O₂ (10⁸ cell)^-1 h^-1 at a light intensity of 120 Ein m^-2 s^-1. In corals having 0.5x10⁹ zooxanthellae/dm² of colony area up to 8% of the total photosynthesis was attributed to the corallum microcosm. Polyp respiration, photosynthesis, and CaCO₃ uptake rates were all much higher than rates previously reported from A. danae, apparently because in these experiments the organisms were better fed. This increased photosynthesis in turn enhanced calcification still further. The symbiosis therefore appears to provide a growth advantage even to fed corals, under the conditions of these experiments.     

Some observations on the influence of salinity on growth and photosynthesis inPorphyra umbilicalis

Notwithstanding the great importance of the salinity factor in the marine environment, the knowledge of influence of salinity on growth of marine benthic algae is very limited. Rate of growth (mg, cm²) and O₂ output of the intertidal red algaPorphyra umbilicalis from Helgoland, North Sea, were measured during a 3 week culture in 3 different salinities (1/2-, 1- and 2-concentrated artificial sea water; Table 1). Under hypertonic conditions (2-concentrated sea water) growth rate and photosynthesis rate were depressed, compared to values obtained in normal concentrated sea water. Under hypotonic conditions (1/2-concentrated sea water), growth expressed in mg was the same as in normal concentrated sea water, or higher when expressed in cm³. Rate of O₂ output was almost unaltered in one of the two experiments, lowered in the other. Cell size increased at higher salinity, while swelling of cell walls and intercellular substances as well as the intensity of colouring decreased with salinity. The discrepancies between growth and photosynthesis under hypotonic conditions cannot be completely explained by the observed influences of salinity on morphological structures (cell size, swelling of cell substances). Detailed studies on the time course of photosynthesis and respiration rates, and preparation of a metabolic balance for the algae are necessary.     

Modeling seasonal course of carbon fluxes and evapotranspiration in response to low temperature and moisture in a boreal Scots pine ecosystem

Environmental conditions act above and below ground, and regulate carbon fluxes and evapotranspiration. The productivity of boreal forest ecosystems is strongly governed by low temperature and moisture conditions, but the understanding of various feedbacks between vegetation and environmental conditions is still unclear. In order to quantify the seasonal responses of vegetation to environmental factors, the seasonality of carbon and heat fluxes and the corresponding responses for temperature and moisture in air and soil were simulated by merging a process-based model (CoupModel) with detailed measurements representing various components of a forest ecosystem in Hyytiälä, southern Finland. The uncertainties in parameters, model assumptions, and measurements were identified by generalized likelihood uncertainty estimation (GLUE). Seasonal and diurnal courses of sensible and latent heat fluxes and net ecosystem exchange (NEE) of CO₂ were successfully simulated for two contrasting years. Moreover, systematic increases in efficiency of photosynthesis, water uptake, and decomposition occurred from spring to summer, demonstrating the strong coupling between processes. Evapotranspiration and NEE flux both showed a strong response to soil temperature conditions via different direct and indirect ecosystem mechanisms. The rate of photosynthesis was strongly correlated with the corresponding water uptake response and the light use efficiency. With the present data and model assumptions, it was not possible to precisely distinguish the various regulating ecosystem mechanisms. Our approach proved robust for modeling the seasonal course of carbon fluxes and evapotranspiration by combining different independent measurements. It will be highly interesting to continue using long-term series data and to make additional tests of optional stomatal conductance models in order to improve our understanding of the boreal forest ecosystem in response to climate variability and environmental conditions.