EFFECTS OF CO2 ENRICHMENT ON THE BLOOM‐FORMING CYANOBACTERIUM MICROCYSTIS AERUGINOSA (CYANOPHYCEAE): PHYSIOLOGICAL RESPONSES AND RELATIONSHIPS WITH THE AVAILABILITY OF DISSOLVED INORGANIC CARBON1

Microcystis aeruginosa Kütz. 7820 was cultured at 350 and 700 μL·L ^{? 1} CO₂ to assess the impacts of doubled atmospheric CO₂ concentration on this bloom‐forming cyanobacterium. Doubling of CO₂ concentration in the airflow enhanced its growth by 52%–77%, with pH values decreased and dissolved inorganic carbon (DIC) increased in the medium. Photosynthetic efficiencies and dark respiratory rates expressed per unit chl a tended to increase with the doubling of CO₂. However, saturating irradiances for photosynthesis and light‐saturated photosynthetic rates normalized to cell number tended to decrease with the increase of DIC in the medium. Doubling of CO₂ concentration in the airflow had less effect on DIC‐saturated photosynthetic rates and apparent photosynthetic affinities for DIC. In the exponential phase, CO₂ and HCO₃ ^? levels in the medium were higher than those required to saturate photosynthesis. Cultures with surface aeration were DIC limited in the stationary phase. The rate of CO₂ dissolution into the liquid increased proportionally when CO₂ in air was raised from 350 to 700 μL·L ^{? 1}, thus increasing the availability of DIC in the medium and enhancing the rate of photosynthesis. Doubled CO₂ could enhance CO₂ dissolution, lower pH values, and influence the ionization fractions of various DIC species even when the photosynthesis was not DIC limited. Consequently, HCO₃ ^? concentrations in cultures were significantly higher than in controls, and the photosynthetic energy cost for the operation of CO₂ concentrating mechanism might decrease.     

Acclimation of photosynthesis to supersaturated CO2 in aquatic plant bicarbonate users

相似文献   

CARBON‐USE STRATEGIES IN MACROALGAE: DIFFERENTIAL RESPONSES TO LOWERED PH AND IMPLICATIONS FOR OCEAN ACIDIFICATION1

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO₂. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO₂ and HCO₃^? increase; CO₃^2? decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO₂ gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO₃^?]. We measured the short‐term photosynthetic responses of five macroalgal species with various carbon‐use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO₂ gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO₂, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO₃^? increased their use of CO₂ in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO₂ did so exclusively in all treatments. Although CO₂ is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO₂ is less energetically expensive than active HCO₃^? uptake, and so HCO₃^?‐using macroalgae may benefit in future seawater with elevated CO₂.     

Diurnal variation in light and carbon limitation of photosynthesis by two species of submerged freshwater macrophyte with a differential ability to use bicarbonate

SUMMARY.

• 1 Rates of photosynthetic oxygen evolution by Callitriche cophocarpa and Ranunculus peltatus in stream were measured on live occasions during the light period on 2 days at ambient light and ambient inorganic carbon, ambient light and saturating inorganic carbon, saturating light and ambient inorganic carbon, saturating light and saturating inorganic carbon and air-equilibrium inorganic carbon and ambient light.
• 2 Despite an ambient CO₂ concentration of about 220 μm , which is about ten times air-equilibrium, the concentration of inorganic carbon was more limiting than light on all the occasions that rates were measured. On average, rates of photosynthesis at ambient concentrations of CO₂ were about 130 and 425 μmol O₂ g^?1 DW h^?1 for C. cophocarpa and R. peltatus, respectively. These rates as a percentage of carbon saturated rates were only about 35% for C. cophocarpa and about 60% for R. peltatus. Ambient rates as a percentage of light saturated rates were about 80% for C. cophocarpa and about 95% for R. peltatus. Only in early morning and late evening where the photon irradiance was below 160 μmol m^?2 s^?1 was there evidence for slight light limitation.
• 3 Based on results from pH-drift experiments and from rates of photosynthesis as a function of CO₂ concentration in the presence and absence of HCO₃^?, C. cophocarpa was unable, but R. peltatus able to use HCO₃^? at an ambient HCO₃^? concentration of about 0·84 mm . The greater rates of photosynthesis at ambient CO₂ concentration and the lesser limitation by inorganic carbon shown by R. peltatus compared to C. cophocarpa was the result of HCO₃^?-use as laboratory experiments showed that R. peltatus performed similarly to C. cophocarpa if the HCO₃^? concentration was reduced to 60 μm .

     

Responses of CAM species to increasing atmospheric CO2 concentrations

Crassulacean acid metabolism (CAM) species show an average increase in biomass productivity of 35% in response to a doubled atmospheric CO₂ concentration. Daily net CO₂ uptake is similarly enhanced, reflecting in part an increase in chlorenchyma thickness and accompanied by an even greater increase in water‐use efficiency. The responses of net CO₂ uptake in CAM species to increasing atmospheric CO₂ concentrations are similar to those for C₃ species and much greater than those for C₄ species. Increases in net daily CO₂ uptake by CAM plants under elevated atmospheric CO₂ concentrations reflect increases in both Rubisco‐mediated daytime CO₂ uptake and phosphoenolpyruvate carboxylase (PEPCase)‐mediated night‐time CO₂ uptake, the latter resulting in increased nocturnal malate accumulation. Chlorophyll contents and the activities of Rubisco and PEPCase decrease under elevated atmospheric CO₂, but the activated percentage for Rubisco increases and the K_M(HCO₃ ^? ) for PEPCase decreases, resulting in more efficient photosynthesis. Increases in root:shoot ratios and the formation of additional photosynthetic organs, together with increases in sucrose‐Pi synthase and starch synthase activity in these organs under elevated atmospheric CO₂ concentrations, decrease the potential feedback inhibition of photosynthesis. Longer‐term studies for several CAM species show no downward acclimatization of photosynthesis in response to elevated atmospheric CO₂ concentrations. With increasing temperature and drought duration, the percentage enhancement of daily net CO₂ uptake caused by elevated atmospheric CO₂ concentrations increases. Thus net CO₂ uptake, productivity, and the potential area for cultivation of CAM species will be enhanced by the increasing atmospheric CO₂ concentrations and the increasing temperatures associated with global climate change.     

Influx and efflux of inorganic carbon in Synechococcus UTEX625

The CO₂ and HCO₃^? fluxes in air-grown cells of Synechococcus UTEX 625 al pH 8-0 were measured during dark to light and light to dark transitions using a mass spectrometer and sampling of the reaction medium. The kinetic parameters for initial uptake of CO₂ and HCO₃^? were determined during the initial period of illumination. The development of the internal C_i pool was followed up to steady-state photosynthesis, which occurred when the size of the internal inorganic carbon pool remained apparently constant for a limited period. The experimental procedure confirmed that only CO₂ transport occurred with 100mmolm^?3 Na⁺ and that both CO₂ and HCO^?3 transport occurred with 25molm^?3 Na⁺. The K_1/2 values of initial CO₂ and HCO₃ uptake were 0.7 and 17.2 mmolm^?3respectively and agreed closely with the K_1/2 values of net CO₂ and HCO₃^? transport during steady-state photosynthesis, which were 0.66 and 17.1 mmolm^?3 respectively. Maximum rates of CO₂and HCO₃^? transport were 423 and 219mmolh^?1 g^?1 Chl. Maximum CO₂ efflux observed upon darkening was 118mmolh^?1 g^?1 Chl. A permeability coefficient of the cell for CO₂ of 3 × 10^?8 m s^?1 was determined from the dark CO₂ efflux assuming an internal pH of 7.2 in the dark. Following the initial CO₂ uptake in the light, the extracellular [CO₂] steadily declined when only CO₂ transport was allowed, but an increase in the extracellular [CO₂] when HCO₃^? transport was allowed to proceed suggested that an enhanced CO₂ efflux occurred as a result of the larger size of the intracellular C_i pool.     

Strong leaf surface basification and CO2 limitation of seagrass induced by epiphytic biofilm microenvironments

Coastal eutrophication is a growing problem worldwide, leading to increased epiphyte overgrowth of seagrass leaves. Yet little is known about how epiphytes affect key biogeochemical conditions and processes in the seagrass phyllosphere. We used electrochemical microsensors to measure microgradients of O₂, pH, and CO₂ at the bare and epiphyte-covered leaf surface of seagrass (Zostera marina L.) to determine effects of epiphytes on the leaf chemical microenvironment. Epiphytes result in extreme daily fluctuations in pH, O₂, and inorganic carbon concentrations at the seagrass leaf surface severely hampering the plant's performance. In light, leaf epiphyte biofilms and their diffusive boundary layer lead to strong basification, markedly reducing the CO₂ and HCO₃^- availability at the leaf surface, leading to reduced photosynthetic efficiency as a result of carbon limitation and enhanced photorespiration. With epiphytes, leaf surface pH increased to >10, thereby exceeding final pH levels (~9.62) and CO₂ compensation points for active photosynthesis. In darkness, epiphyte biofilms resulted in increased CO₂ and hypoxia at the leaf surface. Epiphytes can lead to severe carbon limitation in seagrasses owing to strong phyllosphere basification leading to CO₂ depletion and costly, yet limiting, HCO₃^- utilization, increasing the risk of plant starvation.     

The CO2‐concentrating mechanism in the bloom‐forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) and effects of UVB radiation on its operation1

The bloom‐forming cyanobacterium Microcystis aeruginosa (Kütz.) Kütz. 854 was cultured with 1.05 W · m^?2 ultraviolet‐B radiation (UVBR) for 3 h every day, and the CO₂‐concentrating mechanism (CCM) within this species as well as effects of UVBR on its operation were investigated. Microcystis aeruginosa 854 possessed at least three inorganic carbon transport systems and could utilize external HCO₃^? and CO₂ for its photosynthesis. The maximum photosynthetic rate was approximately the same, but the apparent affinity for dissolved inorganic carbon was significantly decreased from 74.7 μmol · L^?1 in the control to 34.7 μmol · L^?1 in UVBR‐treated cells. At 150 μmol · L^?1 KHCO₃ and pH 8.0, Na⁺‐dependent HCO₃^? transport contributed 43.4%–40.2% to the photosynthesis in the control and 34.5%–31.9% in UVBR‐treated cells. However, the contribution of Na⁺‐independent HCO₃^? transport increased from 8.7% in the control to 18.3% in UVBR‐treated cells. The contribution of CO₂‐uptake systems showed little difference: 47.9%–51.0% in the control and 49.8%–47.2% in UVBR‐treated cells. Thus, the rate of total inorganic carbon uptake was only marginally affected, although UVBR had a differential effect on various inorganic carbon transporters. However, the number of carboxysomes in UVBR‐treated cells was significantly decreased compared to that in the control.     

Photosynthetic inorganic carbon uptake and accumulation in two marine diatoms

Some physiological characteristics of photosynthetic inorganic carbon uptake have been examined in the marine diatoms Phaeodactylum tricornutum and Cyclotella sp. Both species demonstrated a high affinity for inorganic carbon in photosynthesis at pH7.5, having K_1/2(CO₂) in the range 1.0 to 4.0mmol m^?3 and O_2? and temperature-insensitive CO₂ compensation concentrations in the range 10.8 to 17.6 cm³ m^?3. Intracellular accumulation of inorganic carbon was found to occur in the light; at an external pH of 7.5 the concentration in P. tricornutum was twice, and that in Cyclotella 3.5 times, the concentration in the suspending medium. Carbonic anhydrase (CA) was detected in intact Cyclotella cells but not in P. tricornutum, although internal CA was detected in both species. The rates of photosynthesis at pH 8.0 of P. tricornutum cells and Cyclotella cells treated with 0.1 mol m^?3 acetazolamide, a CA inhibitor, were 1.5- to 5-fold the rate of CO₂ supply, indicating that both species have the capacity to take up HCO₃^? as a source of substrate for photosynthesis. No Na⁺ dependence for HCO₃^? could be detected in either species. These results indicate that these two marine diatoms have the capacity to accumulate inorganic carbon in the light as a consequence, in part, of the active uptake of bicarbonate.     

SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS BY THREE SPECIES OF MARINE DIATOM1

The utilization of inorganic carbon by three species of marine diatom, Skeletonema costatum (Grev.) Cleve. Ditylum brightwellii (West) Grun., and Chaetoceros calcitrans Paulsen was investigated using an inorganic carbon isotopic disequilibnum technique and inorganic carbon dose-response curves. Stable carbon isotope data of the diatoms are also presented. Observed rates of photosynthetic oxygen evolution were greater than could be accounted for by the theoretical rate of CO₂ supply from the uncatalyzed dehydration of HCO₃^? in the external medium, suggesting use of HCO₃^? as an inorganic carbon source. Data from the isotopic disequilibrium experiment demonstrate the use of both HCO₃^? and CO₂ for photosynthesis. Carbon isotope discrimination values support the use of HCO₃^? by the diatoms.     

ACQUISITION OF INORGANIC CARBON BY THE MARINE HAPTOPHYTE ISOCHRYSIS GALBANA (PRYMNESIOPHYCEAE)1

Inorganic carbon acquisition has been investigated in the marine haptophyte Isochrysis galbana. External carbonic anhydrase (CA) was present in air‐grown (0.034% CO₂) cells but completely repressed in high (3%) CO₂‐grown cells. External CA was not inhibited by 1.0 mM acetazolamide. The capacity of cells to take up bicarbonate was examined by comparing the rate of photosynthetic O₂ evolution with the calculated rate of spontaneous CO₂ supply; at pH 8.2 the rates of O₂ evolution exceeded the CO₂ supply rate 14‐fold, indicating that this alga was able to take up HCO₃ ^? . Monitoring CO₂ concentrations by mass spectrometry showed that suspensions of high CO₂‐grown cells caused a rapid drop in the extracellular CO₂ in the light and addition of bovine CA raised the CO₂ concentration by restoring the HCO₃ ^? ‐CO₂ equilibrium, indicating that cells were maintaining the CO₂ in the medium below its equilibrium value during photosynthesis. A rapid increase in extracellular CO₂ concentration occurred on darkening the cells, indicating that the cells had accumulated an internal pool of unfixed inorganic carbon. Active CO₂ uptake was blocked by the photosynthetic electron transport inhibitor 3‐(3′,4′‐dichlorphenyl)‐1,1‐dimethylurea, indicating that CO₂ transport was supported by photosynthetic reactions. These results demonstrate that this species has the capacity to take up HCO₃ ^? and CO₂ actively as sources of substrate for photosynthesis and that inorganic carbon transport is not repressed by growth on high CO₂, although external CA expression is regulated by CO₂ concentration.     

A dynamic model for photosynthesis by an aquatic plant, Egeria densa

Abstract This paper describes a dynamic model for photosynthesis by an aquatic plant, Egeria densa. The model takes into account an HCO^?₃ pump, high diffusion resistances and PEP carboxylase, and develops a set of differential equations to form the time-dependent solutions for photosynthesis. The predicted changes in pH, [CO₂]_aq and total inorganic carbon are compared with experimental data and the model is found to describe the data. The model is then used to examine the effect of O₂ on photosynthesis under these conditions, and shows that the increase in internal CO₂ concentration due to the recycling of photorespiratory CO₂ directly stimulates gross CO₂ fixation and can more than compensate for the O₂ inhibition of gross photosynthesis. The importance of the HCO^?₃ pump in O₂ inhibition is also examined. The CO₂ compensation point (where inorganic carbon influx and efflux are equal) is examined and the importance of the HCO^?₃ pump and PEP carboxylase in reducing the compensation concentration is discussed. The model was developed in order to study the photosynthesis of an aquatic weed, which will be reported in a later paper.     

Photosynthesis of Ulva sp. II. utilization of CO2 and HCO−3 when submerged

The photosynthetic capacity of submerged Ulva sp. when utilizing CO₂ and HCO^?₃ as exogenous carbon forms has been investigated and compared with ambient carbon concentrations in sea water. Saturating concentrations of HCO^{? ₃} and CO₂ were 1200 and 100 μM, respectively at saturating light, and photosynthetic rates under such conditions averaged 700 μmolO₂·gDW^?1 ·h^?1. The HCO^?₃ concentration of sea water (≈2500μM), was thus found to be saturating for photosynthesis of Ulva. At the CO₂ concentration of sea water (≈ 10 μM), the contribution of this carbon form to photosynthesis could be 27% at the most. Under conditions of slow water movement, the relative importance of CO₂ utilization would probably be minimized in favour of HCO^?₃ utilization. It is concluded that HCO^?₃ uptake is not limiting photosynthesis for Ulva under natural conditions.     

PHOTOSYNTHETIC CARBON METABOLISM IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)1

The intertidal macroalga Enteromorpha compressa showed the ability to use HCO₃^? as an exogenous inorganic carbon (Ci) source for photosynthesis. However, although the natural seawater concentration of this carbon form was saturating, additional CO₂ above ambient Ci levels doubled net photosynthetic rates. Therefore, the productivity of this alga, when submerged, is likely to be limited by Ci. When plants were exposed to air, photosynthetic rates saturated at air-levels of CO₂ during mild desiccation. Based on carbon fixing enzyme activities and Ci pulsechase incorporation patterns, it was found that Enteromorpha is a C₃ plant. However, this alga did not show O₂ inhibited photosynthetic rates at natural seawater Ci conditions. It is suggested that such a C₄- like gas exchange response is due to the HCO₃^? utilization system concentrating CO₂ intracellularly, thus alleviating apparent photorespiration.     

PHOTOSYNTHETIC UTILIZATION OF INORGANIC CARBON IN THE ECONOMIC BROWN ALGA,HIZIKIA FUSIFORME (SARGASSACEAE) FROM THE SOUTH CHINA SEA1

The mechanism of inorganic carbon (C_i) acquisition by the economic brown macroalga, Hizikia fusiforme (Harv.) Okamura (Sargassaceae), was investigated to characterize its photosynthetic physiology. Both intracellular and extracellular carbonic anhydrase (CA) were detected, with the external CA activity accounting for about 5% of the total. Hizikia fusiforme showed higher rates of photosynthetic oxygen evolution at alkaline pH than those theoretically derived from the rates of uncatalyzed CO₂ production from bicarbonate and exhibited a high pH compensation point (pH 9.66). The external CA inhibitor, acetazolamide, significantly depressed the photosynthetic oxygen evolution, whereas the anion‐exchanger inhibitor 4,4′‐diisothiocyano‐stilbene‐2,2′‐disulfonate had no inhibitory effect on it, implying the alga was capable of using HCO₃^? as a source of C_i for its photosynthesis via the mediation of the external CA. CO₂ concentrations in the culture media affected its photosynthetic properties. A high level of CO₂ (10,000 ppmv) resulted in a decrease in the external CA activity; however, a low CO₂ level (20 ppmv) led to no changes in the external CA activity but raised the intracellular CA activity. Parallel to the reduction in the external CA activity at the high CO₂ was a reduction in the photosynthetic CO₂ affinity. Decreased activity of the external CA in the high CO₂ grown samples led to reduced sensitiveness of photosynthesis to the addition of acetazolamide at alkaline pH. It was clearly indicated that H. fusiforme, which showed CO₂‐limited photosynthesis with the half‐saturating concentration of C_i exceeding that of seawater, did not operate active HCO₃^? uptake but used it via the extracellular CA for its photosynthetic carbon fixation.     

Bicarbonate uptake via an anion exchange protein is the main mechanism of inorganic carbon acquisition by the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae) under variable pH

Macrocystis pyrifera is a widely distributed, highly productive, seaweed. It is known to use bicarbonate (HCO₃^?) from seawater in photosynthesis and the main mechanism of utilization is attributed to the external catalyzed dehydration of HCO₃^? by the surface‐bound enzyme carbonic anhydrase (CA_ext). Here, we examined other putative HCO₃^? uptake mechanisms in M. pyrifera under pH_T 9.00 (HCO₃^?: CO₂ = 940:1) and pH_T 7.65 (HCO₃^?: CO₂ = 51:1). Rates of photosynthesis, and internal CA (CA_int) and CA_ext activity were measured following the application of AZ which inhibits CA_ext, and DIDS which inhibits a different HCO₃^? uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO₃^? uptake by M. pyrifera is via an AE protein, regardless of the HCO₃^?: CO₂ ratio, with CA_ext making little contribution. Inhibiting the AE protein led to a 55%–65% decrease in photosynthetic rates. Inhibiting both the AE protein and CA_ext at pH_T 9.00 led to 80%–100% inhibition of photosynthesis, whereas at pH_T 7.65, passive CO₂ diffusion supported 33% of photosynthesis. CA_int was active at pH_T 7.65 and 9.00, and activity was always higher than CA_ext, because of its role in dehydrating HCO₃^? to supply CO₂ to RuBisCO. Interestingly, the main mechanism of HCO₃^? uptake in M. pyrifera was different than that in other Laminariales studied (CA_ext‐catalyzed reaction) and we suggest that species‐specific knowledge of carbon uptake mechanisms is required in order to elucidate how seaweeds might respond to future changes in HCO₃^?:CO₂ due to ocean acidification.     

UPTAKE OF INORGANIC CARBON BY CLADOPHORA GLOMERATA (CHLOROPHYTA) FROM THE BALTIC SEA1

Carbon uptake in the green macroalga Cladophora glomerata (L.) Kütz. from the brackish Baltic Sea was studied by recording changes in pH, alkalinity, and inorganic carbon concentration of the seawater medium during photosynthesis. The use of specific inhibitors identified three uptake mechanisms: 1) dehydration of HCO₃ ^? into CO₂ by periplasmic carbonic anhydrase, followed by diffusion of CO₂ into the cell; 2) direct uptake of HCO₃ ^? via a 4,4′‐diisothiocyanato‐stilbene‐2,2′‐disulfonate‐sensitive mechanism; and 3) uptake of inorganic carbon by the involvement of a vanadate‐sensitive P‐type H ⁺ ‐ATPase (proton pump). A decrease in the alkalinity of the seawater medium during carbon uptake, except when treated with vanadate, indicated a net uptake of the ionic species contributing to alkalinity (i.e. HCO₃ ^? , CO₃^{2 ?} , and OH ^? ) from the medium, where OH ^? influx is equivalent to H ⁺ efflux. This would suggest that the proton pump is involved in HCO₃ ^? transport. We also show that the proton pump can be induced by carbon limitation. The inducibility of carbon uptake in C. glomerata may partly explain why this species is so successful in the upper littoral zone of the Baltic Sea. Usually, carbon limitation is not a problem in the upper littoral of the sea. However, it may occur frequently within dense Cladophora belts with high photosynthetic rates that create high pH and low carbon concentrations in the alga's microenvironment.     

Carbonate ions appear to neither inhibit nor stimulate use of bicarbonate ions in photosynthesis by Ulva lactuca

Rates of photosynthesis by the marine macroalga Ulva lactuca were measured in a factorial experiment at five concentrations of HCO₃^? and CO₃^2- between 0·20 and 1·26 mol m^?3, but very low concentrations of CO₂. The results demonstrated that HCO₃^? was available for use, but an analysis of variance showed that CO₃^2- had neither an inhibiting nor a stimulating effect on rates of photosynthesis over this concentration range. Over the experiment, pH varied from 8·46 to 10·06 and this also had no significant effect on rates of photosynthesis. The lack of a stimulatory effect of high concentrations of CO₃^2- on the rate of photosynthesis at low concentrations of HCO₃^? was taken as circumstantial evidence for direct uptake of HCO₃^? rather than proton extrusion and external production of CO₂. In the rockpools in which U. lactuca grows, pH values up to 10·35 have been recorded, and for much of the time, CO₃^2- was the major form of inorganic carbon available. The apparent lack of an ability to use CO₃^2- under these conditions suggests that direct use of CO₃^2- as a source of inorganic carbon for photosynthesis is unlikely to be widespread.     

Terrestrial carbon balance in a drier world: the effects of water availability in southwestern North America

Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO₂ exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO₂ exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site‐years of annual eddy covariance measurements of net and gross CO₂ exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 – 1000 mm in annual precipitation and records of 4–9 years each. In addition to evaluating spatial relationships among CO₂ and water fluxes across sites, we separately quantified site‐level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis–ET relationship was linear, suggesting ET was a better proxy for water available to drive CO₂ exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site‐level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long‐term mean CO₂ exchanges with climatic ET. Consequently, a hypothetical 100‐mm change in ET, whether short term or long term, was predicted to alter net ecosystem production (NEP) by 64 gCm^?2 yr^?1. Most of the unexplained NEP variability was related to persistent, site‐specific function, suggesting prioritization of research on slow‐changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site‐level responses to interannual weather can be extrapolated for prediction of CO₂ exchanges over decadal and longer timescales relevant to societal response to climate change.     

Enhanced growth of the red algaPorphyra yezoensis Ueda in high CO2 concentrations

Leafy thalli of the red algaPorphyra yezoensis Ueda, initiated from conchospores released from free-living conchocelis, were cultured using aeration with high CO₂. It was found that the higher the CO₂ concentration, the faster the growth of the thalli. Aeration with elevated CO₂ lowered pH in dark, but raised pH remarkably in light with the thalli, because the photosynthetic conversion of HCO ₃ ^? to OH^? and CO₂ proceeded much faster than the dissociation of hydrated CO₂ releasing H⁺. Photosynthesis of the alga was found to be enhanced in the seawater of elevated dissolved inorganic carbon (DIC, CO₂ + HCO ₃ ^? + CO ₃ ^? ). It is concluded that the increased pH in the light resulted in the increase of DIC in the culture media, thus enhancing photosynthesis and growth. The relevance of the results to removal of atmospheric CO₂ by marine algae is discussed.