Glycolate metabolism in cyanobacteria

A comparative analysis of glycolate excretion in 11 cyanobacteria showed that 8 strains, although grown and assayed in air, excreted glycolate. The largest quantities were excreted by the filamentous strains Plectonema boryanum 73110 and Anabaena cylindrica (Lemm). The carbon lost by excretion was at most 9% of the net fixed carbon in air for heterocystous cyanobacteria but increased (up to 60%) in some strains under a high pO₂ (0.03 kPa CO₂ in pure O₂). A. cylindrica excreted glycolate at a maximum level of 2 and 10 μmol (mg chl a )⁻¹ h⁻¹ in air and at high pO₂, respectively. The excretion continued for several hours. Increases in light intensity and pO₂ and a shift in pH from 7 to 9 increased the amount of glycolate excreted. A. cylindrica also showed the most O₂-sensitive fixation of CO₂. In vitro activity of phosphoglycolate phosphatase (EC 3.1.3.18) was found in all strains tested, with the highest activities noted for Gloeobacter violaceus 7.82 and Gloeothece 6909 and for young cultures of A. cylindrica . The lowest activities were found in Anabaena 7120 and Anacystis nidulans 625, strains excreting no or only minor quantities of glycolate.     

Consequence of restricted mitochondrial oxidative metabolism on photosynthetic carbon assimilation in mesophyll protoplasts: Decrease in light activation of four chloroplastic enzymes

The patterns of light activation of 4 chloroplastic enzymes were examined in mesophyll protoplasts of pea ( Pisum sativum ) in the absence or presence of oligomycin (inhibitor of oxidative phosphorylation) or antimycin A (inhibitor of cytochrome pathway) or salicylhydroxamic acid (SHAM, inhibitor of alternative pathway). The results were compared with those of DCMU (inhibitor of photosynthetic electron transport). The light activation of NADP glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH), fructose-1,6-bisphosphatase (FBPase), phosphoribulokinase (PRK) (enzymes of the Calvin cycle) and NADP malate dehydrogenase (NADP-MDH) (reflects chloroplast redox state) was more pronounced at limiting CO₂ (0.1 m M NaHCO₃) than that at optimal CO₂ (1.0 m M NaHCO₃). SHAM decreased markedly (up to 33%) the light activation of all 4 enzymes, while antimycin A or oligomycin exerted only a limited effect (<10% decrease). Antimycin A or oligomycin or SHAM had no significant effect on light activation of these 4 enzymes in isolated chloroplasts. However, DCMU caused a remarkable decrease in light activation of enzymes in both protoplasts (up to 78%) and chloroplasts (up to 69%). These results suggest that the restriction of alternative pathway of mitochondrial metabolism results in a marked decrease in the light activation of key chloroplastic enzymes in mesophyll protoplasts but not in isolated chloroplasts. Such a decrease in the light activation of enzymes could be also a secondary feedback effect because of the restriction on carbon assimilation.     

Glycolate metabolism in cyanobacteria. III. Nitrogen controls excretion and metabolism of glycolate in Anabaena cylindrica

The effect of nitrogen on excretion and metabolism of glycolate in Anabaena cylindrica (CCAP 1403/2a) was studied. Glycidate, an inhibitor of glutamate:glyoxylate aminotransferase (EC 2.6.1.4), reduced the L-methionine-DL-sulfoximine-induced NH₄⁺ release by ca 40%, while net CO₂ fixation and C₂H₂ reduction were not lowered. This indicates that at least a part of the glyoxylate synthesized in A. cylindrica is metabolized via glycine to serine. Addition of NH₄Cl or glutamate to the medium reduced the excretion of glycolate. At pH 9, under air, NH₄Cl reduced the excretion by 10–30% and under high pO2 (0.03 kPa CO₂ in O₂) by about 80–90%. At pH 7.5, under high pO₂, NH₄Cl and glulamate reduced the excretion by about 40 and 80%, respectively. Also, the presence of NH₄Cl stimulated the animation of glyoxylate under such conditions as shown by an increased glycine pool and a decreased glutamate pool. We suggest that nitrogen regulates the capacity of A. cylindrica to retain and recycle glycolate intracellularly and that glutamate serves as an amino donor in the conversion of glyoxylate to glycine.     

Involvement of photorespiration and glycolate pathway in carbonic anhydrase induction and inorganic carbon concentration in Chlamydomonas reinhardtii

Interaction between induction of carbonic anhydrase (CA) activity, induction of inorganic carbon (C_i) concentrating mechanisms and the photorespiratory glycolate pathway has been studied in wild type 6145c and photorespiratory mutant 18–7F (low in phosphoglycolate phosphatase activity) cells of C. reinhardtii . Cell transfer from high CO₂ (5%, v/v) to low CO₂ (0.03%) provoked an increase of extracellular and total (extracellular plus intracellular) CA in both wild type and mutant cells. During adaptation to low CO₂ conditions, both strains excreted ammonium to the medium at a similar rate in the presence of l -methionine- d-l -sulfoximine (MSX), an inhibitor of glutamine synthetase (GS). MSX also provoked ammonium excretion by air adapted wild type and mutant cells, even though both strains had high levels of CA activity and of C_i concentrating activities.
GS increased in both strains after transfer from high to low CO₂ conditions. However, this increase was abolished by aminooxyacetate, an inhibitor of the glyoxylate-serine aminotransferase, and by glycolaldehyde, an inhibitor of triose phosphate to ribulose 1,5-bisphosphate conversion. CA synthesis did not occur in the presence of either aminooxyacetate or glycolaldehyde. Algae grown in high CO₂ in the presence of aminooxyacetate did not induce C_i concentrating mechanisms. Integration of these three processes, i.e., CA synthesis, C_i-concentration, and photorespiratory glycolate pathway is proposed in the framework of carbon metabolism of the alga.     

Gas exchange in intact isolated chloroplasts from Chlamydomonas reinhardtii during starch degradation in the dark

During starch degradation in intact isolated chloroplasts from Chlamydomonas reinhardtii gas exchange was studied with a mass spectrometer. Oxygen uptake by intact chloroplasts in the dark never exceeded 1.5% of the starch degradation rate [maximum 15 nmol O₂ (mg Chl)⁻¹ h⁻¹ consumed. 1 000 nmol glucose (mg Chl)⁻¹h⁻¹ degraded]. Evolution of CO₂ under aerobic conditions [9.8–28 nmol (mg Chl)⁻¹ h⁻¹] was stimulated by addition of 0.1–0.5 m M oxaloacetate [393–425 nmol CO₂ (mg Chl)⁻¹ h⁻¹]. Pyridoxal phosphate (5 m M ) inhibited starch degradation by more than 80%, but had no effect on O₂ uptake. Starch degradation rates and CO₂ evolution did not differ under acrobic and anaerobic conditions. Increasing P_i in the reaction medium from 0.5 m M to 5.0 m M stimulated starch degradation by 230 and 260% under aerobic and anaerobic conditions, respectively. A rapid autooxidation of reduced ferredoxin was observed in a reconstituted system consisting of purified Chlamydomonas ferredoxin, purified Chlamydomonas NADP-ferredoxin oxidoreductase (EC 1.6.7.1) and NADPH. Addition of isolated thylakoids from C. reinhardtii did not affect the rate of O₂ uptake. Our results clearly indicate the absence of any oxygen requirement during starch degradation in isolated chloroplasts.     

CO2, light and temperature influence senescence and protein degradation in wheat leaf segments

Effects of environmental conditions influencing photosynthesis and photorespiration on senescence and net protein degradation were investigated in segments from the first leaf of young wheat ( Triticum aestivum L. cv. Arina) plants. The segments were floated on H₂O at 25, 30 or 35°C in continuous light (PAR: 50 or 150 µmol m⁻² s⁻¹) in ambient air and in CO₂‐depleted air. Stromal enzymes, including phosphoglycolate phosphatase, glutamine synthetase, ferredoxin‐dependent glutamate synthase, phosphoribulokinase, and the peroxisomal enzyme, glycolate oxidase, were detected by SDS‐PAGE followed by immunoblotting with specific antibodies. In general, the net degradation of proteins and chlorophylls was delayed in CO₂‐depleted air. However, little effect of CO₂ on protein degradation was observed at 25°C under the lower level of irradiance. The senescence retardation by the removal of CO₂ was most pronounced at 30°C and at the higher irradiance. The stromal enzymes declined in a coordinated manner. Immunoreactive fragments from the degraded polypeptides were in most cases not detectable. However, an insolubilized fragment of glycolate oxidase accumulated in vivo, especially at 25°C in the presence of CO₂. Detection of this fragment was minimal after incubation at 30°C and completely absent on blots from segments kept at 35°C. In CO₂‐depleted air, the fragment was only weakly detectable after incubation at 25°C. The results from these investigations indicate that environmental conditions that influence photosynthesis may interfere with senescence and protein catabolism in wheat leaves.     

Stimulation of respiration by Pb2+ in detached leaves and mitochondria of C3 and C4 plants

The exposure of detached leaves of C₃ plants (pea, barley) and C₄ plant (maize) to 5 m M Pb (NO₃)₂ for 24 h caused a reduction of their photosynthetic activity by 40–60%, whereas the respiratory rate was stimulated by 20–50%. Mitochondria isolated from Pb²⁺-treated pea leaves oxidized substrates (glycine, succinate, malate) at higher rates than mitochondria from control leaves. The respiratory control (RCR) and the ADP/O ratio were not affected. Pb²⁺ caused an increase in ATP content and the ATP/ADP ratio in pea and maize leaves. Rapid fractionation of barley protoplasts incubated at low and high CO₂ conditions, indicated that the increased ATP/ADP ratio in Pb²⁺-treated leaves resulted mainly from the production of mitochondrial ATP. The measurements of membrane potential of mitochondria with a TPP⁺-sensitive electrode further showed that mitochondria isolated from Pb²⁺-treated leaves had at least as high membrane potential as mitochondria from control leaves. The activity of NAD-malate dehydrogenase in the protoplasts from barley leaves treated with Pb²⁺ was 3-fold higher than in protoplasts from control leaves. The activities of photorespiratory enzymes NADH-hydroxypyruvate reductase and glycolate oxidase as well as of NAD-malic enzyme were not affected. The presented data indicate that stimulation of respiration in leaves treated by lead is in a close relationship with activation of malate dehydrogenase and stimulation of the mitochondrial ATP production. Thus, respiration might fulfil a protective role during heavy metal exposure.     

Glycolate metabolism in green algae

Using ¹⁴C-labelled substrates, the succession of the single steps in the glycolate metabolism was investigated in Mougeotia scalaris and Eremosphaera viridis , which, within the group of green algae, are representatives of the evolutionary lines of Charophyta and Chlorophyta , respectively. In both algae the same metabolites are formed as in higher plants, although in Eremosphaera , which in contrast to Mougeotia does not possess leaf peroxisomes, all reactions are exclusively mitochondrial. Concomitant with the oxidation of glycolate, the synthesis of ATP was demonstrated in Eremosphaera . Formation of tartronic semi-aldehyde or other products different from those in land plants could not be demonstrated in either of these algae. Excretion of glycolate by Mougeotia and Eremosphaera is enhanced by decreasing the CO₂ concentration as well as by increasing the light intensity, but is completely stopped about 14 h later. Whereas increasing enzyme activities of the glycolate pathway apparently reduces glycolate excretion in Mougeotia , activation of CO₂ pumps seems to be the dominant reaction to prevent glycolate excretion in Eremosphaera . Mesostigma viride is one of the phylogenetically oldest algae in the group of Charophyceae . As this alga has already been demonstrated to contain microbodies with enzymes of leaf peroxisomes, the peroxisomal glycolate pathway must have originated at a very early stage. Surprisingly, the organelles from Mesostigma contain also the glyoxysomal marker enzyme isocitrate lyase suggesting these microbodies to be prototypes from which both glyoxysomes and leaf peroxisomes evolved.     

In vitro carbon dioxide excretion from erythrocytes of two species of Antarctic fishes and its inhibition by catecholamines

This study was designed to investigate whether the blood of Pagothenia borchgrevinki , exhibits a Haldane effect, and whether activation of a Na⁺/H⁺ antiporter increases transport of intracellular protons and Bohr protons out of the erythrocytes resulting in inhibition of CO₂ excretion in both P. borchgrevinki , and Dissostichus mawsoni. When carbon dioxide dissociation curves were determined from blood samples pooled from three fish under oxygenated and deoxygenated conditions a Haldane effect was observed. Using an in vitro , CO₂ excretion assay, the rate of HCO₃⁻ dehydration was determined on blood and plasma equilibrated under an N₂atmosphere then rapidly oxygenated with air in the presence of 10⁻⁵ M noradrenaline or acetazolamide (10⁰⁴M). Whole blood and plasma from P. borchgrevinki , and D. mawsoni , were equilibrated with 0·5% CO₂ in air and assayed in the presence of 10⁻⁵ M noradrenaline. Erythrocyte CO₂ excretion rates were depressed significantly by noradrenaline in both species. The whole blood HCO₃⁻ dehydration rate was depressed significantly following rapid oxygenation in the presence of acetazolamide indicating that the pathway of CO₂ excretion included activation of intracellular carbonic anhydrase and an adrenergic receptor.     

Mitigation of adverse effects of rising CO2 on a planktonic herbivore by mixed algal diets

Putative future increase in atmospheric CO₂ is expected to adversely affect herbivore growth due to decrease in contents of key nutrients such as nitrogen and phosphorus (P) relative to carbon in primary producers including plant and algal species. However, as many herbivores are polyphagous and as the response of primary producers to elevated CO₂ is highly species-specific, effects of elevated CO₂ on herbivore growth may differ between feeding conditions with monospecific and multiproducer diets. To examine this possibility, we performed CO₂ manipulation experiments under a P-limited condition with a planktonic herbivore, Daphnia , and three algal species, Scenedesmus obliquus (green algae), Cyclotella sp. (diatoms) and Synechococcus sp. (cyanobacteria). Semibatch cultures with single algal species (monocultures) and multiple algal species (mixed cultures) were grown at ambient (360 ppm) and high CO₂ levels (2000 ppm) that were within the natural range in lakes. Both in the mono- and mixed cultures, algal steady state abundance increased but algal P : C and N : C ratios decreased when they were grown at high CO₂. As expected, Daphnia fed monospecific algae cultured at high CO₂ had decreased growth rates despite increased algal abundance. However, when fed mixed algae cultured at high CO₂, especially consisting of diatoms and cyanobacteria or the three algal species, Daphnia maintained high growth rates despite lowered P and N contents relative to C in the algal diets. These results imply that algal diets composed of multiple species can mitigate the adverse effects of elevated CO₂ on herbivore performance, although the magnitude of this mitigation depends on the composition of algal species involved in the diets.     

Importance of oxidative electron transport over oxidative phosphorylation in optimizing photosynthesis in mesophyll protoplasts of pea (Pisum sativum L.)

The role of mitochondrial respiration in optimizing photosynthesis was assessed in mesophyll protoplasts of pea ( Pisum sativum L., cv. Arkel) by using low concentrations of oligomycin (an inhibitor of oxidative phosphorylation), antimycin A (inhibits cytochrome pathway of electron transport) and salicylhydroxamic acid (SHAM, an inhibitor of alternative oxidase). All three compounds decreased the rate of photosynthetic O₂ evolution in mesophyll protoplasts, but did not affect chloroplast photosynthesis. The inhibition of photosynthesis by these mitochondrial inhibitors was stronger at optimal CO₂ (1.0 m M NaHCO₃) than that at limiting CO₂ (0.1 m M NaHCO₃). We conclude that mitochondrial metabolism through both cytochrome and alternative pathways is essential for optimizing photosynthesis at limiting as well as at optimal CO₂. The ratios of ATP to ADP in whole protoplast extracts were hardly affected, despite the marked decrease in their photosynthetic rates by SHAM. Similarly, the decrease in the ATP/ADP ratio by oligomycin or antimycin A was more pronounced at limiting CO₂ than at optimal CO₂. The mitochondrial oxidative electron transport, through both cytochrome and alternative pathways, therefore akppears to be more important than oxidative phosphorylation in optimizing photosynthesis, particularly at limiting CO₂ (when ATP demand is expected to be low). Our results also confirm that the alternative pathway has a significant role in contributing to the cellular ATP, when the cytochrome pathway is limited.     

Subcellular distribution of pyruvate-degrading enzymes in Chlamydomonas reinhardtii studied by an improved protoplast fractionation procedure

The subcellular distribution of pyruvate-degrading enzymes has been determined in Chlamydomonas reinhardtii (Dangeard) by protoplast induction with autolysine, dig-itonin lysis and further fractionation by differential centrifugation using a Percoll cushion. Mitochondrial and plastidic fractions contained intact and physiologically competent organelles - RC 1.7, ADP/O 2.7 and rate of malate oxidation 76 nmol O, (mg protein)^-1min^-1 for mitochondria, CO_2; fixation 46.8 μmol (mg Chi)^-1 h^-1 for chloroplasts.
Results from protoplast fractionation were further confirmed by the determination of enzyme activities within trypsin-treated organelles. Mitochondria (formate fermentation) and chloroplasts (chlorofermentation) were shown to possess the capacity for anaerobic pyruvate degradation. Pyruvate dehydrogenase (NAD⁺, EC 1.2.4.1), pyruvate formate-lyase (EC 2.3.1.54) and lactate dehydrogenase (NADH, EC 1.1.1.27) showed equal distribution between mitochondria and chloroplasts, whereas activities of phosphotransacetylase (EC 2.3.1.8) and acetate kinase (EC 2.7.2.1) were only detectable in the mitochondrial fraction. NADH- and NADPH-dependent activities of both alcohol dehydrogenase (EC 1.1.1.1) and aldehyde dehydrogenase (acylating, EC 1.2.1.10) were localized in the mitochondrial and cytoplasmic or the plastidic and cytoplasmic fractions, respectively, whereas pyruvate decarboxylase (EC 4.1.1.1) was only detected in the cytoplasmic fraction.     

Mode of interference of chlorosis-inducing herbicides with peroxisomal enzyme activities

In rye leaves ( Secale cereale L. cv. Petkus "Kustro") bleached in the presence of the chlorosis-inducing herbicides aminotriazole, haloxidine, San 6706 or difunone in white light of 54.2 W m^-2 (5000 lx), catalase activity was very low. In addition, the activities of glycolate oxidase and hydroxypyruvate reductase were strongly diminished in treatments with San 6706 and difunone. The lowering of the peroxisomal enzyme activities was observed in red, but not in blue light and did not occur after treatment with the non-bleaching pyridazinone derivative San 9785. The deficiencies of the peroxisomal enzymes did not appear to be involved in the initiation of the chlorosis. Instead they are probably produced as secondary consequences of the bleaching. Low peroxisomal enzyme activities were also obtained without herbicide treatment by growing the leaves in an atmosphere of 2% O₂ and 3% CO₂, but in this case were not accompanied by an increased sensitivity of the Chl to photooxidative bleaching. The peroxisomal enzymes reached as high activities as in untreated controls when the herbicide-treated leaves were grown at a low light intensity of 0.106 W m^-2 (10 lx). After transfer of herbicide-treated leaves grown under 0.106 W m^-2 to 306 W m^-2 (30 000 lx), catalase was strongly inactivated, even at 0°C. In treatments with San 6706 and difunone the increase of the activities of glycolate oxidase and hydroxypyruvate reductase was either stopped, remaining unchanged, or the enzymes were slightly inactivated after exposure to 306 W m^-2 (30 000 lx). The observations suggest that the inactivation of peroxisomal enzymes results from photooxidative events in the chloroplasts.     

Energetics of syntrophic fatty acid oxidation

Abstract: Fatty acids are key intermediates in methanogenic degradation of organic matter in sediments as well as in anaerobic reactors. Conversion of butyrate or propionate to acetate, (CO₂), and hydrogen is endergonic under standard conditions, and becomes possible only at low hydrogen concentrations (10^-4-10^-5 bar). A model of energy sharing between fermenting and methanogenic bacteria attributes a maximum amount of about 20 kJ per mol reaction to each partner in this syntrophic cooperation system. This amount corresponds to synthesis of only a fraction (one-third) of an ATP to be synthesized per reaction. Recent studies on the biochemistry of syntrophic fatty acid-oxidizing bacteria have revealed that hydrogen release from butyrate by these bacteria is inhibited by a protonophore or the ATPase inhibitor DCCD ( N , N '-dicyclohexyl carbodiimide), indicating that a reversed electron transport step is involved in butyrate or propionate oxidation. Hydrogenase, butyryl-CoA dehydrogenase, and succinate dehydrogenase acitivities were found to be partially associated with the cytoplasmic membrane fraction. Also glycolic acid is degraded to methane and CO₂ by a defined syntrophic coculture. Here the most difficult step for hydrogen release is the glycolate dehydrogenase reaction ( E '₀=−92 mV). Glycolate dehydrogenase, hydrogenase, and ATPase were found to be membrane-bound enzymes. Membrane vesicles produced hydrogen from glycolate only in the presence of ATP; protonophores and DCCD inhibited this hydrogen release. This system provides a suitable model to study reversed electron transport in interspecies hydrogen transfer between fermenting and methanogenic bacteria in methanogenic biomass degradation.     

The induction of the CO2 concentrating mechanism in a starch-less mutant of Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii the formation of a starch sheath surrounding the pyrenoid is observed when cells grown under high CO₂ (5% CO₂ in air) are transferred to low CO₂ (0.03%) conditions. Formation of the starch sheath occurs coincidentally with induction of the CO₂ concentrating mechanism and with de novo synthesis of 5 polypeptides with molecular masses of 21, 36, 37, 42–44 kDa. We studied the effect of CO₂ concentrations on photosynthesis, ultrastructure and protein synthesis in Chlamydomonas reinhardtii cw-15 (wild phenotype for photosynthesis) and in the starch-less mutant BAFJ -6, with the aim to clarify the role of the pyrenoid starch sheath in the operation of the CO₂ concentrating mechanism and whether these low CO₂-inducible polypeptides are involved in the formation of starch sheath. When wild type and starch-less mutant cells were transferred from high to low CO₂, the CO₂ requirement for half-maximal rates of photosynthesis decreased from 40 μM to 2 μM CO₂. ³⁵SO₄^2- labeling analyses showed that the starch-less mutant induced the 5 low CO₂-inducible polypeptides. These observations suggest that the starch-less mutant was able to induce a fully active CO₂ concentrating mechanism. Since the starch-less mutant did not form a pyrenoid starch sheath, we suggest that the starch sheath is not involved in the operation of the CO₂ concentrating mechanism and that none of these 5 low CO₂-inducible proteins is involved in the formation of the starch sheath in Chlamydomonas .     

The inhibition of photosynthesis and photorespiration in isolated mesophyll cells of Phaseolus and Lycopersicon by reduced osmotic potentials

Mesophyll cells isolated from Phaseolus vulgaris and Lycopersicon esculentum show decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44 M (−1.08 MPa) to 0.77 M (−1.88 MPa). Photorespiration assayed by ¹⁴CO₂ release in CO₂-free air and by ¹⁴CO₂ release from the oxidation of [1–¹⁴C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO₂ compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I⁻¹1 at −1.08 MPa to 130 μl 1⁻¹ for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non- stomatal inhibition of CO₂ assimilation and glycolate pathway metabolism occurs simultaneously with stomatal closure.     

Effect of sink size on photosynthesis and carbohydrate content of leaves of three spring wheat varieties

Effects of CO₂ on stomatal movements of Commelina communis L. were studied with plants, epidermal strips and guard cell protoplasts. With plants, the stomatal response induced by a blue light pulse was studied for different ambient CO₂ concentration ranging from CO₂-deprived air to 100 Pa in darkness or under red light. It was observed that the blue light response could be obtained not only under a red light background but also in darkness and CO₂-free air, the two responses being quite similar.
With epidermal strips, the effect of CO₂ on ferricyanide reductase activity at the guard cell plasmalemma was studied by transmission electron microscopy. In the presence of ferric ions, reduced ferricyanide gives an electron dense precipitate of Prussian Blue. In darkness and air, no precipitate was observed. In darkness and CO₂-free air as well as under light and normal air, a precipitate was found along the plasmalemma of the guard cells, indicating a ferricyanide reductase activity. With guard cell protoplasts suspended in a medium either in equilibrium with air or in a CO₂-free medium the H⁺ extrusion induced by a blue light pulse added to a red light background was measured. A low CO₂ content was obtained by adding photosynthetic algae to the suspension of guard cell protoplasts. In a CO₂-free medium the rate of H⁺ extrusion was enhanced.
The results are discussed on the basis of a possible competition for reducing power between CO₂ fixation and a putative blue light dependent redox chain located on the plasma membrane.     

Starch fermentation via a formate producing pathway in Chlamydomonas reinhardii, Chlorogonium elongatum and Chlorella fusca

Starch degradation was investigated during anaerobic dark incubation in the algae Chlamydomonas reinhardii, Chlorogonium elongatum and Chlorella fusca . The pathway of algal formate fermentation was elucidated by determination of the relationship between substrate consumption and product accumulation. The fate of reducing equivalents was also determined. Investigations were done on dependence of pH, fermentation time, cell cycle, and after addition of H₂, hypophosphite and inhibitors of protein synthesis.
A mixed acid fermentation that produced formate, acetate and ethanol (2:1:1) with only small amounts of H₂ and CO₂ was shown for the algal strains used. The failure of inhibition with cycloheximide and chloramphenicol indicated the constitutive presence of all fermenting enzymes. Nevertheless, glycerol, D(–)lactate and stoichiometrical amounts of ethanol and CO₂ were found additionally at extreme pH (pH 4.6 and 7.9), and after addition of H₂ and hypophosphite (7 m M ). During long-term incubation (28 h) fermentation changed from mixed acid to ethanol production. The pathways of algal fermentation did not depend on cell cycle, and fermentation rate corresponded directly to the actual starch content of algal cells. The results gave evidence for synthesis of formate during anaerobic metabolism in algae by a thioclastic cleavage of pyruvate via the enzyme pyruvate formate lyase. This indicated an algal fermentation pathway thought to be present only in procaryotic organisms.     

Metabolic interactions between algal symbionts and invertebrate hosts

Some invertebrates have enlisted autotrophic unicellular algae to provide a competitive metabolic advantage in nutritionally demanding habitats. These symbioses exist primarily but not exclusively in shallow tropical oceanic waters where clear water and low nutrient levels provide maximal advantage to the association. Mostly, the endosymbiotic algae are localized in host cells surrounded by a host-derived membrane (symbiosome). This anatomy has required adaptation of the host biochemistry to allow transport of the normally excreted inorganic nutrients (CO₂, NH₃ and PO₄³⁻) to the alga. In return, the symbiont supplies photosynthetic products to the host to meet its energy demands. Most attention has focused on the metabolism of CO₂ and nitrogen sources. Carbon-concentrating mechanisms are a feature of all algae, but the products exported to the host following photosynthetic CO₂ fixation vary. Identification of the stimulus for release of algal photosynthate in hospite remains elusive. Nitrogen assimilation within the symbiosis is an essential element in the host's control over the alga. Recent studies have concentrated on cnidarians because of the impact of global climate change resulting in coral bleaching. The loss of the algal symbiont and its metabolic contribution to the host has the potential to result in the transition from a coral-dominated to an algal-dominated ecosystem.     

Direct and indirect effects of Cd2+ on photosynthesis in sugar beet (Beta vulgaris)

Sugar-beet plants ( Beta vulgaris L. cv. Monohill) were cultivated for 4 weeks in a complete nutrient solution. Indirect effects of cadmium were studied by adding 5, 10 or 20 μ M CdCl₂ to the culture medium while direct effects were determined by adding 1, 5, 20, 50 or 2 000 μ M CdCl₂ to the assay media. The photosynthetic properties were characterized by measurement of CO₂ fixation in intact plants, fluorescence emission by intact leaves and isolated chloroplasts, photosystem (PS) I and PSII mediated electron transport of isolated chloroplasts, and CO₂-dependent O₂ evolution by protoplasts. When directly applied to isolated leaves, protoplasts and chloroplasts. Cd²⁺ impeded CO₂ fixation without affecting the rates of electron transport of PSI or PSII or the rate of dark respiration. When Cd²⁺ was applied through the culture medium the capacity for, and the maximal quantum yield of CO₂ assimilation by intact plants both decreased. This was associated with: (1) decreased total as well as effective chlorophyll content (PSII antennae size), (2) decreased coupling of electron transport in isolated chloroplasts, (3) perturbed carbon reduction cycle as indicated by fluorescence measurements. Also, protoplasts isolated from leaves of Cd²⁺-cultivated plants showed an increased rate of dark respiration.