Light-Induced Polar pH Changes in Leaves of Elodea canadensis: II. Effects of Ferricyanide: Evidence for Modulation by the Redox State of the Cytoplasm

The effect of an extracellular electron acceptor, ferricyanide, on the light-induced polar leaf pH changes of the submerged angiosperm Elodea canadensis in light and in darkness was determined. The rate of transmembrane ferricyanide reduction was stimulated by increased light intensity and was inhibited by inorganic carbon, indicating that changes in the redox state of the chloroplast were reflected at the plasma membrane. The addition of ferricyanide inhibited the light-induced polar leaf pH reaction. This effect could be balanced by increasing the light intensity. In the dark, the acidification induced by ferricyanide was not influenced by diethylstilbestrol at concentrations that completely inhibited the polar leaf pH changes. This indicates that the ferricyanide-induced H⁺ extrusion and the H⁺ transport during the polar reaction were mediated by different mechanisms.     

Characteristics of light-dependent inorganic carbon uptake by isolated spinach chloroplasts

The light-dependent accumulation of radioactively labeled inorganic carbon in isolated spinach (Spinacia oleracea L.) chloroplasts was determined by silicone oil filtering centrifugation. Intact chloroplasts, dark-incubated 60 seconds at pH 7.6 and 23°C with 0.5 millimolar sodium bicarbonate, contained 0.5 to 1.0 millimolar internal inorganic carbon. The stromal pool of inorganic carbon increased 5- to 7-fold after 2 to 3 minutes of light. The saturated internal bicarbonate concentration of illuminated spinach chloroplasts was 10- to 20-fold greater than that of the external medium. This ratio decreased at lower temperatures and with increasing external bicarbonate. Over one-half the inorganic carbon found in intact spinach chloroplasts after 2 minutes of light was retained during a subsequent 3-minute dark incubation at 5°C. Calculations of light-induced stromal alkalization based on the uptake of radioactively labeled bicarbonate were 0.4 to 0.5 pH units less than measurements performed with [¹⁴C]dimethyloxazolidine-dione. About one-third of the binding sites on the enzyme ribulose 1,5-bisphosphate carboxylase were radiolabeled when the enzyme was activated in situ and ¹⁴CO₂ bound to the activator site was trapped in the presence of carboxypentitol bisphosphates. Deleting orthophosphate from the incubation medium eliminated inorganic carbon accumulation in the stroma. Thus, bicarbonate ion distribution across the chloroplast envelope was not strictly pH dependent as predicted by the Henderson-Hasselbach formula. This finding is potentially explained by the presence of bound CO₂ in the chloroplast.     

Interactions of blue light and inorganic carbon supply in the control of light-saturated photosynthesis in brown algae

Photosynthetic capacities of five species of brown algae in red light were found to be strongly limited by the inorganic carbon supply of natural sea water. Under these conditions, pH 8·2 and dissolved inorganic carbon concentration (DIG) of 2·1 mol m^?3, a short pulse of blue light was found to increase the subsequent rate of photosynthesis in saturating red light. The degree of blue light stimulation varied between species, ranging from an increase of over 200% of the original rate in Colpomenia peregrins to only 10% in Dictyota dichotoma. Increasing the DIG concentration of sea water by bicarbonate addition resulted in carbon saturation of photosynthesis in all five species. Blue light stimulation was greatly reduced at these higher DIG concentrations. The response in Laminaria digitata was examined in more detail by manipulation of pH and DIG to produce solutions with different concentrations of dissolved CO₂. At a CO₂ concentration typical of normal sea water (12·4 mmol m^?3), blue light treatment increased photosynthetic rate by approximately 50%. Blue light stimulation was increased to over 150% at CO₂ concentrations below that of sea water, whereas at concentrations above that of sea water, the effect was diminished. Therefore, the effect of blue light on photosynthetic capacity appears to involve an increase in the rate of supply of carbon dioxide to the plant.     

Inorganic Carbon Accumulation and Photosynthesis in a Blue-green Alga as a Function of External pH

The blue-green alga Coccochloris peniocystis photosynthesizes optimally over the pH range of 7.0 to 10.0, but the O₂-evolution rate is inhibited below pH 7.0 and ceases below pH 5.25. Measurement of the inorganic carbon pool in this alga in the light, using the silicone-fluid filtration technique demonstrated that the rate of accumulation of dissolved inorganic carbon remained relatively constant over a wide pH range. At external dissolved inorganic carbon concentrations of 0.56 to 0.89 millimolar the internal concentration after 30 seconds illumination was greater than 3.5 millimolar over the entire pH range. Intracellular pH measured in the light using [¹⁴C]5,5-dimethyloxazolidine-2,4-dione and [¹⁴C]methylamine dropped from pH 7.6 at an external pH of 7.0 to pH 6.6 at an external pH of 5.25. Above an external pH of 7.0 the intracellular pH rose gradually to pH 7.9 at an external pH 10.0. Ribulose-1,5-bisphosphate carboxylase activity of cell-free algal extracts exhibited optimal activity at pH 7.5 to 7.8 but was inactive below pH 6.5. It is suggested that the inability of Coccochloris to maintain its intracellular pH when in an acidic environment restricts its photosynthetic capacity by a direct pH effect on the principal CO₂ fixing enzyme.     

INORGANIC CARBON TRANSPORT IN RELATION TO CULTURE AGE AND INORGANIC CARBON CONCENTRATION IN A HIGH-CALCIFYING STRAIN OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

The relationships among inorganic carbon transport, bicarbonate availability, intracellular pH, and culture age were investigated in high-calcifying cultures of Emiliania huxleyi (Lohmann) Hay & Mohler. Measurement of inorganic carbon transport by the silicone-oil centrifugation technique demonstrated that gadolinium, a potential Ca²⁺ channel inhibitor, blocked intracellular inorganic carbon uptake and photosynthetic ¹⁴CO₂₊ fixation in exponential-phase cells. In stationary-phase cells, the intracellular inorganic carbon concentration was unaffected by gadolinium. Gadolinium was also used to investigate the link between bicarbonate and Ca²⁺ transport in high-calcifying cells of E. huxleyi. Bicarbonate availability had significant and rapid effects on pH_i in exponential- but not in stationary-phase cells. 4′, 4′-Diisothiocyanostilbene-2,2′-disulfonic acid did not block bicarbonate uptake from the external medium by exponential-phase cells. Inorganic carbon utilization by exponential- and stationary-phase cells of Emiliania huxleyi was investigated using a pH drift technique in a closed system. Light-dependent alkalization of the medium by stationary-phase cells resulted in a final pH of 10.1 and was inhibited by dextran-bound sulphonamide, an inhibitor of external carbonic anhydrase. Exponential-phase cells did not generate a pH drift. Overall, the results suggest that for high-calcifying cultures of E. huxleyi the predominant pathway of inorganic carbon utilization differs in exponential and stationary phase cells of the same culture.     

Forms of Dissolved Carbon Dioxide Required for Photosystem II Activity in Chloroplast Membranes

High concentrations of both bicarbonate and formate inhibit photosynthetic O₂ evolution at pH 8.0. At this pH, only 2.4% of the total dissolved carbon dioxide exists as CO₂. At pH 7.3, where 11% of the total dissolved carbon dioxide exists as CO₂, HCO₃⁻ no longer inhibits. While formate still inhibits O₂ evolution at pH 7.3, its effect can be partially overcome if CO₂ is also present. The rate of binding of added ¹⁴C-labeled inorganic carbon is nearly 10-fold more rapid when the internal pH of thylakoid membranes is at 6.0 than when it is at 7.8. These observations suggest that CO₂, not HCO₃⁻, is initially bound to the photosystem II reaction center and that the location of the binding site is on the inside thylakoid surface. However, additional data presented here suggest that, after binding, CO₂ is hydrated to HCO₃⁻ + H⁺ in a pH-dependent reaction. Two possible explanations of the “bicarbonate effect” are presented.     

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus

Scenedesmus cells grown on high CO₂, when adapted to air levels of CO₂ for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO₂ from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O₂ evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO₂ pump with K_0.5 for O₂ evolution of less than 5 micromolar CO₂ was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO₃⁻ concentrating process between pH 8 to 10 lowered the K_0.5 (DIC) from 7300 micromolar HCO₃⁻ in high CO₂ grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO₃⁻ pump was inhibited by vanadate (K_i of 150 micromolar), as if it involved an ATPase linked HCO₃⁻ transporter. The CO₂ pump was formed on low CO₂ by high-CO₂ grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO₃⁻ pump was partially activated on low CO₂ within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO₃⁻ pump at pH 9 had requirements similar to the activation of the CO₂ pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO₂-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO₂ pump and in some algae an additional vanadate-sensitive plasmalemma HCO₃⁻ transporter for a HCO₃⁻ pump.     

Assessment of <Emphasis Type="Italic">Nannochloropsis gaditana</Emphasis> growth and lipid accumulation with increased inorganic carbon delivery

Algal biomass refineries for sustainable transportation fuels, in particular biodiesel, will benefit from algal strain enhancements to improve biomass and lipid productivity. Specifically, the supply of inorganic carbon to microalgal cultures represents an area of great interest due to the potential for improved growth of microalgae and the possibility for incorporation with CO₂ mitigation processes. Combinations of bicarbonate (HCO₃^?) salt addition and application of CO₂ to control pH have shown compelling increases in growth rate and lipid productivity of fresh water algae. Here, focus was placed on the marine organism, Nannochloropsis gaditana, to investigate growth and lipid accumulation under various strategies of enhanced inorganic carbon supply. Three gas application strategies were investigated: continuous sparging of atmospheric air, continuous sparging of 5% CO₂ during light hours until nitrogen depletion, and continuous sparging of atmospheric air supplemented with 5% CO₂ for pH control between 8.0 and 8.3. These gas sparging schemes were combined with addition of low concentrations (5 mM) of sodium bicarbonate at inoculation and high concentration (50 mM) of sodium bicarbonate amendments just prior to nitrogen depletion. The optimum scenario observed for growth of N. gaditana under these inorganic carbon conditions was controlling pH with 5% CO₂ on demand, which increased both growth rate and lipid accumulation. Fatty acid methyl esters were primarily comprised of C16:0 (palmitic) and C16:1 (palmitoleic) aliphatic chains. Additionally, the use of high concentration (50 mM) of bicarbonate amendments further improved lipid content (up to 48.6%) under nitrogen deplete conditions when paired with pH-controlled strategies.     

Salicylhydroxamic Acid (SHAM) Inhibition of the Dissolved Inorganic Carbon Concentrating Process in Unicellular Green Algae

Rates of photosynthetic O₂ evolution, for measuring K_0.5(CO₂ + HCO₃⁻) at pH 7, upon addition of 50 micromolar HCO₃⁻ to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K₁(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO₂ uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O₂ evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO₃), and the rate of ¹⁴CO₂ fixation with 100 micromolar [¹⁴C] HCO₃⁻. Salicylhydroxamic acid inhibition of O₂ evolution and ¹⁴CO₂-fixation was reversed by higher levels of NaHCO₃. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO₂ accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.     

The Role of External Carbonic Anhydrase in Inorganic Carbon Acquisition by Chlamydomonas reinhardii at Alkaline pH

The role of external carbonic anhydrase in inorganic carbon acquisition and photosynthesis by Chlamydomonas reinhardii at alkaline pH (8.0) was studied. Acetazolamide (50 micromolar) completely inhibited external carbonic anhydrase (CA) activity as determined from isotopic disequilibrium experiments. Under these conditions, photosynthetic rates at low dissolved inorganic carbon (DIC) were far greater than could be maintained by CO₂ supplied from the spontaneous dehydration of HCO₃⁻ thereby showing that C. reinhardii has the ability to utilize exogenous HCO₃⁻. Acetazolamide increased the concentration of DIC required to half-saturate photosynthesis from 38 to 80 micromolar, while it did not affect the maximum photosynthetic rate. External CA activity was also removed from the cell-wall-less mutant (CW-15) by washing. This had no effect on the photosynthetic kinetics of the algae while the addition of acetazolamide to washed cells (CW-15) increased the K_½^DIC from 38 to 80 micromolar. Acetazolamide also caused a buildup of the inorganic carbon pool upon NaHCO₃ addition, indicating that this compound partially inhibited internal CA activity. The effects of acetazolamide on the photosynthetic kinetics of C. reinhardii are likely due to the inhibition of internal rather than a consequence of the inhibition of external CA. Further analysis of the isotopic disequilibrium experiments at saturating concentration of DIC provided evidence consistent with active CO₂ transport by C. reinhardii. The observation that C. reinhardii has the ability to take up both CO₂ and bicarbonate throws into question the role of external CA in the accumulation of DIC in this alga.     

Effect of light supply and carbon source on cell growth and cellular composition of a newly isolated microalga Chlorella vulgaris ESP‐31

Light supply is one of the most important factors affecting autotrophic growth of microalgae. This study investigated the effect of the type and light intensity of artificial light sources on the cell growth of an indigenous microalga Chlorella vulgaris ESP‐31 obtained from southern Taiwan. In addition, a dissolved inorganic carbon source (i.e. sodium bicarbonate) was used to improve the biomass production of strain ESP‐31. The results show that a new fluorescent light source (TL5) was effective in indoor cultivation of microalgae. Better overall productivity of 0.029 g dry cell weight/L‐d was obtained when using TL5 lamps as the light source with a light intensity of 9 W/m². A carbon source (sodium bicarbonate) concentration of 1000 mg/L was found to be optimal for the growth of strain ESP‐31 in terms of both biomass production and carbon source utilization. Under the optimal growth conditions, the resulting microalgal biomass consisted of 25–30% protein, 6–10% carbohydrate, and 30–40% lipid.     

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.     

Modulation and adaptation of carbonic anhydrase activity in Microcystis spp. under different environmental factors

The carbonic anhydrase (CA) activities were determined in three cyanobacterial species, namely Microcystis aeruginosa Kütz., Microcystis viridis (A.Br.) Lemm, and Microcystis wesenbergii (Kom.) Kom, which were dominant in a lake (Dianchi Lake) subject to major blooms. In more detailed experiments on M. aeruginosa, the effects of inorganic carbon, pH, temperature, nitrogen/phosphorus ratio, glucose, and light intensity on CA activity were also investigated. Because of the relatively alkaline pH value of the culture media for the optimum growth of algal cells, bicarbonate ions were the main form of exogenous inorganic carbon. The results showed that the CA activity of M. aeruginosa was influenced dramatically by the concentration of bicarbonate. Consequently, it was suggested that bicarbonate ions were the main form of exogenous inorganic carbon that M. aeruginosa could utilize. Cultures grown in the dark exhibited CA activity six times higher than that of cells cultured mixotrophically with the addition of glucose. Features of eutrophic water bodies promoted an increase in CA activity, and the resulting higher CA activity would accelerate the utilization of inorganic carbon and favor the growth and blooming of Microcystis spp. in eutrophic lakes. Although the experiments were carried out under controlled experimental conditions, they could provide some basic data that would prove useful for the control of cyanobacterial blooms in nature.     

Measurement of carbon dioxide compensation points of freshwater algae

A technique is described for the measurement of total dissolved inorganic carbon by acid release as CO₂ followed by its conversion to methane and detection by flame ionization in a modified gas chromatograph. This method was used to determine the dissolved inorganic carbon concentration reached at compensation point when algae were allowed to photosynthesize in a closed system in a buffer at known pH, and the CO₂ compensation point was calculated from this concentration. The CO₂ compensation points of 16 freshwater algae were measured at acid and alkaline pH in air-saturated medium: at acid pH the CO₂ compensation points ranged from 4.8 to 41.5 microliters per liter while at alkaline pH they ranged from 0.2 to 7.2 microliters per liter. Removal of O₂ from the medium caused a slight lowering of compensation point at acid pH but had little effect at alkaline pH. These low, O₂-insensitive compensation points are characteristic of C₄ plants. It is suggested that these low CO₂ compensation points are maintained by an active bicarbonate uptake by algae especially at alkaline pH.     

Seasonal and tidal variability of environmental carbon related physico-chemical variables and inorganic C acquisition in Gracilariopsis longissima and Enteromorpha intestinalis from Los Toruños salt marsh (Cádiz Bay, Spain)

The seasonal and tidal variability of inorganic C acquisition mechanisms, photosynthesis, internal composition and growth were studied in two co-occurring macroalgae in Los Toruños salt marsh (Cádiz Bay), Gracilariopsis longissima and Enteromorpha intestinalis. This variability was monitored together with physico-chemical variables affecting carbon availability, photosynthesis, and growth. The environmental variables, such as light, temperature, pH, salinity, oxygen, alkalinity, dissolved inorganic carbon (DIC) and CO₂, displayed not only an expected seasonal cycle but also a daily (tidal) variability, with abrupt and rapid changes influenced by biological activities, physical variables, tidal state and tidal timing. In contrast to environmental variables, photosynthesis, pigments and C:N composition were affected by seasonal changes but not by tidal regimes, as organisms integrated these short-term fluctuations in physico-chemical variables. Photosynthesis, pigments and internal N composition were maximal in autumn and minimal in summer for both species. Growth showed a seasonal trend, displaying a summer drop with negative values. This response can be the result of extreme values of environmental variables (temperature, light, pH, nutrients, and the shortage of DIC) in summer, in comparison with higher growth rates in September onwards. The use of inhibitors of carbon acquisition in situ at normal DIC concentrations (2.2. mM) revealed species-dependent differences. While the external carbonic anhydrase (CA) activity showed a constitutive character in G. longissima, it showed little effect in E. intestinalis, which relies on internal CA activity. The 4, 4′-diisothiocyanatostilbene-2,2′-disulfonate (DIDS)-sensitive bicarbonate transport in G. longissima was effective in winter. In contrast, DIDS stimulated photosynthesis in summer, and relieved AZ inhibition. This response could suggest a stimulation of a H⁺ extrusion mediated-CO₂ transport in periods of low CO₂ availability.     

The utilization of bicarbonate ions by the macroalga Ascophyllum nodosum (L.) Le Jolis

Abstract A comparison of some of the methods used to determine whether aquatic plants have the ability to utilize bicarbonate ions as a source of inorganic carbon for photosynthesis has been applied to the intertidal macroalga Ascophyllum nodosum. These include: observing photosynthesis at a high pH (below the alga's CO₂ compensation point), pH compensation point determinations, comparing the photosynthetic characteristics at low pH (5.20) and at high pH (7.95), estimating the maximal rates at which CO₂ can diffuse through the unstirred layer and the rate at which CO₂ can be produced from bicarbonate dehydration in the unstirred layer. All indicated that Ascophyllum nodosum can use bicarbonate ions for photosynthesis, though some were not always consistent. Calculating the total inorganic carbon concentration from pH measurements and acidification CO₂ determinations revealed that the assumption that the alkalinity remains constant during pH drift experiments is not always valid.     

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum: II. Effects of NO(3) and NH(4) Addition to CO(2) Efflux in the Light

The effects of nitrate and ammonium addition on net and gross photosynthesis, CO₂ efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO₂ efflux in the light. The nitrate and ammmonium induced rates of CO₂ efflux were 31.0 and 33.8 micromoles CO₂ per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO₂ was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279).     

The effects of pH and dissolved inorganic carbon on external carbonic anhydrase activity in Chlorella saccharophila

External carbonic anhydrase (CA) activity in Chlorella saccharophila is suppressed by growth at high dissolved inorganic carbon and at acid pH. External CA activity was shown to be suppressed by growth at pHs below 7.0, with total repression at pH5.0. Growth in the presence of the buffer 3-[N-Morpholino]propane-sulphonic acid (MOPS) between pH 7 and 8 suppressed CA activity. Cells grown at pH8.0 aerated at 6 dm³ h^?1 exhibited external CA activity of 5 units mg^?1 Chl once the dissolved inorganic carbon (DIC) was reduced to 300 mmol m^?3, and this increased to 30 units mg^?1 Chl over a period of 3d while the DIC dropped to 30mmol m^?3. Cells aerated at 180 dm³ h^?1 showed a similar trend in CA activity, although the onset was delayed by 1 d and the DIC did not drop below 300 mmol m^?3. Cells grown at pH 7.8 near an air equilibrium DIC of 300 mmol m^?3had no detectable external CA activity. It is probable that it is the CO² supply to the cell, and not total DIC or HCO^?₃ which controls external CA activity. Cells grown at pH 5.0 had no detectable activity, although they reduced the CO² concentration to 0.6 mmol m^?3. The loss of CA upon transfer of air-grown cells to 10 mmol mol^?1 CO² took place over 48 h and was light dependent, while the loss upon transfer from alkaline pH to acid pH look place over 12 h and was independent of light. The effects of pH are independent of the response to CO₂.     

Seasonal variation in dissolved carbon concentrations and fluxes in the upper Purus River, southwestern Amazon

One of the less studied components of carbon cycling that could improve our understanding of how and how strongly Amazonian ecosystems act as sinks or sources of carbon is the amount that is carried downstream by rivers. In this paper, we show that a headwater river can carry from 25 to 130 % of the reported sink for Amazonian forests, therefore not being negligible for ecosystem-level carbon budgets. Based on monthly measurements from May 2004 to April 2005 of the upper Purus River, southwestern Amazonia, we found that: water pH, dissolved oxygen, specific electrical conductivity, and dissolved inorganic carbon (DIC) were inversely related to water discharge and precipitation; pCO₂ was directly and strongly related to discharge and precipitation, and to a lesser extent to pH and dissolved oxygen; and dissolved organic carbon (DOC) was not related to any measured variable. Annual flux of dissolved carbon (DIC + DOC) at the sampling site was estimated as 604 ± 55 Gg C a^?1. More than 75 % was in the form of bicarbonate, with the remainder as CO₂ and DOC. This amount is equivalent to 0.15 ± 0.01 Mg C ha^?1 a^?1 in the upstream drainage basin, which is on the same order of magnitude as terrestrial carbon fixation.     

DARK CARBON FIXATION STUDIES ON THE INTERTIDAL MACROALGA ASCOPHYLLUM NODOSUM (PHAEOPHYTA) 1

The characteristics of dark carbon fixation by Ascophyllum nodosum were investigated. In longitudinal profile the maximum rates of dark and light dependent fixation are found at the apex. The use of Michaelis-Menten kinetics did not suitably describe the relationship between the uptake rate in the dark and the total inorganic carbon concentration. Dark fixation was saturated at a total inorganic carbon concentration [TIC] of 2.5 mM. The use of the Hill-Whittingham equation to describe the uptake curve indicates that the process is diffusion limited. Comparisons of dark fixation at high (8.0) and low (5.2) pH suggest that bicarbonate ions are used as a source of inorganic carbon. The transfer of ¹⁴C, fixed in the dark, from the ethanol soluble to the insoluble fraction was relatively slow irrespective of the light treatment during the chase period. Ascophyllum nodosum displays a small diel fluctuation in the pH of aqueous extracts and titratable acidity similar to that displayed by CAM plants. The significance of dark fixation to the overall carbon budget is discussed.