Showing papers on "Photosynthesis published in 1979"

Biochemistry of the violaxanthin cycle in higher plants

Abstract: The biochemistry of the violaxanthin cycle in relationship to photosynthesis is reviewed. The cycle is a component of the thylakoid and consists of a reaction sequence in which violaxanthin is converted to zeaxanthin (de-epoxidation) and then regenerated (epoxidation) through separate reaction mechanisms. The arrangement of the cycle in the thylakoid is transmembranous with the de-epoxidation system situated on the loculus side and epoxidation on the outer side of the membrane. Photosynthetic activities affect turnover of the cycle but the cycle itself consists entirely of dark reactions. Light has at least two roles in de-epoxidation. It establishes through the proton pump the acidic pH in the loculus that is required for de-epoxidase activity and it induces a presumed conformational change in the inner membrane surface which determines the fraction of violaxanthin in the membrane that enters the cycle. De-epoxidation, which requires ascorbate, is presumed to proceed by a reductive-dehydration mechanism. Non-cyclic electron transport can provide the required reducing potential through the dehydroascorbate-ascorbate couple. Whether ascorbate reduces the de-epoxidase system directly or through an intermediate has not been settled. Epoxidation requires NADPH and O2 which suggests a reductive mechanism. In contrast with de-epoxidation, it has a pH optimum near neutrality. The coupling of photosynthetically generated NADPH to epoxidation has been shown. Turnover of the cycle under optimal conditions is estimated to be about two orders of magnitude below optimal electron transport rates. This low rate appears to exclude a direct role of the cycle in photosynthesis or a role in significantly affecting photosynthate levels in a back reaction. The fact that the cycle is sensitive to events both before and after Photosystem I suggests a regulatory role, possibly through effects on membrane properties. A model showing the various relationships of the cycle to photosynthesis is presented. The contrasting view that the cycle can participate directly in photosynthesis, such as in oxygen evolution, is discussed. Violaxanthin de-epoxidase has been purified. It is a lipoprotein which contains monogalactosyldiglyceride (MG) exclusively. The enzyme is a mono-de-epoxidase which is specific for 3-OH, 5–6-epoxy carotenoids that are in a 3R , 5S , 6R configuration. In addition, the polyene chain must be all- trans . A model has been presented which depicts enzymic MG in a receptor role and the stereospecific active center situated in a narrow well-like depression that can accommodate only the all- trans structure.

Contribution to reproductive effort by photosynthesis of flowers and fruits

Abstract: REPRODUCTION is often a lethal or semi-lethal activity, and for iteroparous plants it is often possible to show that reproduction has costs that are expressed in a reduced growth rate and/or an increased death rate1. Attempts have been made to compare life history patterns in flowering plants by measuring the fraction of a plant's annual dry matter production (or calorific value) that is allocated to reproduction (for example refs 2–4). The assumption is that the reproductive parts represent a cost in energy or materials. Clearly mineral nutrients and water must be gained by the reproductive structures from the remainder of the plant, but this is not necessarily true for the energetic and carbon economy of the reproductive structures. Many flowers and fruits are green and a fraction of the energy and carbon might be obtained by direct photosynthesis within these structures. This might be especially important during embryo growth if carpels and other organs that remain attached after flowering are green. In such cases the conventional estimation of reproductive effort (dividing seed weight by plant weight) would be incorrect and comparison of life history patterns and their evolutionary meaning would be invalid. There are reports of significant contributions of in situ photosynthesis in flower and seeds to their growth. Biscoe et al. have estimated that 47% of the carbon required for seed production in barley is provided by photosynthesis of reproductive and immediately adjacent plant structures5. Bazzaz and Carlson have shown that in the annual weed Ambrosia trifida L., net photosynthesis by reproductive structures amounts to 41 and 51%, respectively, of the carbon required for male and female inflorescences6. Here we report an analysis of the carbon budget of reproduction for 15 temperate deciduous trees. The budget was determined by measuring the weight, photosynthesis and dark respiration of flowers or inflorescences from bud break until seed maturity.

Microbial Transformations of Sulfur-Compounds in a Stratified Lake (Solar Lake, Sinai)

Abstract: Microbiological and chemical aspects of the sulfur cycle were studied in the chemocline of a tropical salt lake. Oxygen and sulfide coexisted in a O-lo-cm layer which migrated up and down during a diurnal cycle. Sulfide was rapidly oxidized by oxygen, with a half-life of 5-10 min, to produce mainly sulfate and thiosulfate. Thiosulfate and elemental sulfur had concentration maxima in the chemocline while polysulfide was abundant throughout the sulfide zone. Radiotracer experiments showed that the elemental sulfur was produced by anoxygenic photosynthesis in cyanobacteria. The elemental sulfur was further oxidized or again reduced, depending on the presence or absence of oxygen. Cyanobacteria in the che- mocline shifted between anoxygenic photosynthesis in the morning and oxygenic photosyn- thesis in the afternoon. A high dark CO, fixation was found in the chemocline which could be stimulated by sulfide, elemental sulfur, and thiosulfate. The oxidation rate of sulfide in the chemocline was dependent on the presence of bacteria.

Nutrient dependence of primary productivity in lakes1

Abstract: An analysis of growing season measurements of daily primary productivity, chlorophyll, water chemistry, and transparency from 58 north temperate lakes shows a strong correlation between volumetric rates of photosynthesis, chlorophyll, and nutrients. Mean daily rates of photosynthesis per unit volume euphotic zone, v, are correlated with mean chlorophyll concentration (r2 = 0.80). The mean daily rate of photosynthesis at optimal depth, Āopt, is highly correlated with mean total P(r2 = 0.95), and with mean total N(r2 = 0.91). In contrast, integral rates of photosynthesis are linked less tightly to nutrient concentration because of their simultaneous dependence on transparency. The Dillon-Rigler phosphorus loading model is extended to predict volumetric rates of photosynthesis (v and Āopt) in lakes where the N:P balance indicates control by phosphorus (N:P ⩾ 13).

Occurrence of C3 and C4 photosynthetic pathways in North American Grasses.

Abstract: A literature survey was made for the occurrence of C, and C, photosynthetic pathways in the United States Gramineae. Distinctive characteristics of the two photosynthetic pathways are discussed. Leaf anatomy, CO, compensation point, net enhancement of photosynthesis in oxygen-deficient atmosphere, QC discrimination, and initial product labeling were criteria selected to evaluate data for 6 subfamilies including 25 tribes, 138 genera, and 632 species. The Arundinoideae, Bambusoideae, Oryzoideae, and Pooideae (Festucoideae) are composed of species with C, pathways. All tribes within the Eragrostoideae have C, pathways with the exception of Unioleae. Within the Panicoideae, the Andropogoneae and all of the Paniceae, excepting the genera Sacciolepus, Isachne, Oplismenus, Amphicatpum, and Panicum, have C, pathways. The subgenus Dichanthelium within Panicum is C, while the Bupanicum subgenus contains plants with both C, and C, photosynthetic pathways. Plant productivity is dependent on several environmental and biological factors. The most important single factor is photosynthesis. A pathway for carbon dioxide (CO;?) fixation was described by Calvin and Bassham (1962) in which CO2 was incorporated into a 6-carbon compound and rapidly converted to a 3-carbon compound, 3-phosphoglyceric acid (3PGA). Previous to discoveries of Kortschalk et al. (1965) and Hatch and Slack (1966), the Calvin cycle (C,, reductive pentose pathway) was considered the major photosynthetic mechanism for carbon (C) fixation. However, Hatch and Slack (1966) described CO;! fixation in which labeled CO;! was first incorporated in 4-carbon compounds (malic, aspartic , or oxaloacetic acid) prior to transfer to sugars by way of 3-phosphoglycerate. The proposed mechanism involved the operation of two interconnected metabolic cycles. Downton (1970) described carbon fixation into Cd-dicarboxylic acids in mesophyll cells and subsequent incorporation into the Calvin cycle located in the bundle sheath cells. Plants (Cd plants) possessing the 4-carbon pathway (also called Cd, dicarboxylic acid, Kranz type, low CO;! compensation, tropical, Hatch and Slack, or p carboxylation pathway) were of tropical ongm and more efficient. They produced two- to threefold more dry matter than plants possessing the 3-carbon pathway (C, plants), especially in relatively sunny, warm, dry climates (Black 197 1). Distinctive characteristics associated with the Ca pathway prompted intensive research in photosynthetic processes of flowering plants. The most important photosynthetic pathways

Photosynthesis, respiration and water content in bryophytes

Abstract: Summary Response curves of photosynthesis to water content of bryophytes of dry habitats (e.g. Tortula intermedia, Camptothecium lutescens) show an optimum, with photosynthesis declining again at high water contents. Respiration may be stimulated by water stress, but is unaffected by high water contents. The steep portions of the photosynthesis and respiration curves lie within a similar range at low water contents. Some species of constantly moist habitats (e.g. Pellia epiphylla, Hookeria lucens) show photosynthesis increasing progressively to water contents of 500 to 1000 % of dry weight, and affected at much lower water deficits than respiration as the plant dries out. The response of photosynthesis and respiration to water potential is broadly similar in the two groups. In the species investigated there was generally measurable photosynthesis at –60 to – 100 bar, but little or none at – 150 to –200 bar. Respiration continues to somewhat lower water potentials of which the limits were not determined. Field measurements of the water content of shoots of five species over a period of 12 months showed much greater variation in Tortula muralis than in the woodland species. Maximum water contents in the field generally lay close to the optima of the photosynthesis response curves. The lower water contents recorded in these and in published data are considered in relation to sorption isotherms for bryophytes and other plant materials. The water associated with bryophyte shoots can be divided into (1) water within the cell walls (apoplast water), (2) water within the cytoplasm (symplast water), and (3) external capillary water. Changes in water content below about –200 bar take place chiefly within (1), between c.– 200 and c. - 2 bar within (2), and at higher water potentials chiefly within (3). Water movement within the shoots is physiologically important; the distribution and movement of water are mediated by the geometry of the capillary spaces of the cell walls and the plant surface. In species with papillose leaves, rates of capillary conduction in the interstices between the papillae are more than sufficient to balance evaporation. Conduction within the cell walls is likely to be important in species with non-papillose leaves, hut other pathways may also be involved, and water movement in these species requires further investigation.

Role of Potassium in Carbon Dioxide Assimilation in Medicago sativa L.

Abstract: Alfalfa was grown hydroponically in 0, 0.6, and 4.8 millimolar K in order to determine the influence of tissue level of K on photosynthesis, dark respiration, photorespiration, stomatal and mesophyll resistance to CO(2), photosystem I and II activity, and synthesis and activity of ribulose 1,5-bisphosphate carboxylase (RuBPc).A severe (0.0 millimolar) and mild (0.6 millimolar) K deficiency, compared to plants grown at 4.8 millimolar K, produced a significant decrease in photosynthesis and photorespiration, but an increase in dark respiration. Both deficient K levels increased hydrophyllic resistance to CO(2), but only the severe deficiency increased stomatal resistance.Photosystem I and II activity of isolated chloroplasts was not affected by K deficiency. The apparent activity of a crude RuBPc preparation was significantly reduced in severely deficient plants. Activity of the enzyme could not be restored to normal rates by the addition of K to the reaction medium.The specific activity of RuBPc isolated from severely K-deficient and K-sufficient leaflets was not significantly different, suggesting that K does not function in RuBPc activity. Incorporation of [(14)C]leucine into RuBPc, as a measure of synthesis, by K-deficient leaflets was reduced to 15% of K-sufficient leaflets. The addition of K to the reaction medium stimulated [(14)C]leucine incorporation into RuBPc and 10 millimolar KNO(3) increased incorporation to 80% of K-sufficient leaflets. Actinomycin D and cycloheximide suppressed the K-stimulated incorporation of [(14)C]leucine into RuBPc, suggesting that the K-stimulated synthesis of RuBPc most likely represents de novo synthesis.

Higher plant chloroplasts: Evidence that all the chlorophyll exists as chlorophyll-protein complexes.

Abstract: By using the polyacrylamide gel electrophoresis system described in this report, it was possible to fractionate all the photosynthetic pigments of maize (Zea mays L.) thylakoids into chlorophyll—protein complexes with negligible formation of free or detergent-complexed chlorophyll. Identical sodium dodecyl sulfate extracts of thylakoids have previously resulted in up to 50% of the chlorophyll migrating as free chlorophyll after electrophoresis. The major difference from previous gel electrophoresis systems is the replacement of sodium dodecyl sulfate in the electrophoresis buffer by Deriphat 160 (disodium N-lauryl-β-iminodipropionate), a zwitterionic detergent. The results suggest that: (i) no significant amount of free chlorophyll exists in the chloroplast thylakoid membranes in vivo, and (ii) most of the free pigment seen previously on gels was generated during the electrophoresis and was not a result of the solubilization technique. Additionally, the new chlorophyll-protein complexes resolved appear to have different characteristics (pigment content and size) that those observed in former systems.

Influence of light and ambient carbon dioxide concentration on nitrate assimilation by intact barley seedlings.

Abstract: The influence of light, dark, and ambient CO 2 on nitrate assimilation in 8- to 9-day-old barley seedlings was studied. To develop the photosynthetic apparatus fully, the seedlings were grown in nitrogen-free Hoagland solution for 5 days in darkness followed by 3 days in continuous light. The seedlings reduced nitrate and nitrite in both light and dark, although more slowly in darkness. The slower nitrate reduction in darkness was not due to decreased uptake, since the steady-state internal concentration of nitrate was doubled. The faster nitrate reduction in light was attributed to recent products of photosynthetic CO 2 fixation supplying reducing energy, possibly by shuttle reactions between chloroplasts and cytoplasm. In carbohydrate-deficient tissue, it appeared that recently fixed photosynthate could supply all of the energy required for nitrate reduction. When sufficient metabolites were present in the green tissue, light was not obligatory for the reduction of nitrate and nitrite.

Short‐term responses of nutrient‐deficient algae to nutrient addition

Abstract: Changes in net photosynthesis, dark respiration, ATP content, and some other aspects of composition were measured following phosphate or ammonium addition to cultures of the green alga Scenedesmus quadricauda (Turp.) Breb. deficient in phosphorus or nitrogen. The deficient nutrient was rapidly taken up. Light-saturated net photosynthesis was depressed below the pre-addition rate during nutrient uptake and did not increase markedly above that rate until several hours after uptake was complete. Dark respiration, on the other hand, was markedly stimulated during uptake of the deficient nutrient and to a lesser extent after uptake was complete. Phosphate addition to P-deficient cells caused a large increase in the ATP content within 2–4 h of addition, whereas ammonium addition to N-deficient cells caused much less or no increase in ATP content. Short-term enrichment experiments to detect nutrient limitations are evaluated in the light of these and similar results taken from the literature. The available evidence shows that photosynthetic responses cannot reliably be used in short-term enrichments but changes in respiration or ATP content may be useful in certain circumstances.

Photosynthate Partitioning into Starch in Soybean Leaves: I. Effects of Photoperiod versus Photosynthetic Period Duration

Abstract: Photosynthesis, photosynthate partitioning into foliar starch, and translocation were investigated in soybean plants (Glycine max (L.) Merr. cv. Amsoy 71), grown under different photoperiods and photosynthetic periods to determine the controls of leaf starch accumulation. Starch accumulation rates in soybean leaves were inversely related to the length of the daily photosynthetic period under which the plants were grown. Photosynthetic period and not photoperiod per se appears to be the important factor. Plants grown in a 14-hour photosynthetic period partitioned approximately 60% of the daily foliar accumulation into starch whereas 7-hour plants partitioned about 90% of their daily foliar accumulation into starch. The difference in starch accumulation resulted from a change in photosynthate partitioning between starch and leaf residual dry weight. Residual dry weight is defined as leaf dry weight minus the weight of total nonstructural carbohydrates. Differences in photosynthate partitioning into starch were also associated with changes in photosynthetic and translocation rates, as well as with leaf and whole plant morphology. It is concluded that leaf starch accumulation is a programmed process and not simply the result of a limitation in translocation.

The Respiratory Costs of Nitrogen Fixation in Soyabean, Cowpea, and White Clover II. COMPARISONS OF THE COST OF NITROGEN FIXATION AND THE UTILIZATION OF COMBINED NITROGEN

Abstract: Plants of soyabean, cowpea, and white clover were grown singly in pots in Saxcil growth cabinets at 23/18 °C, 30/24 °C, and 20/15 °C, respectively, until seed maturation or for 85 d (white clover). Two populations were produced within each species: one population effectively nodulated and wholly dependent for nitrogen on fixation in the root nodules, and a second population completely lacking nodules but receiving abundant nitrate nitrogen. In each species, the two populations were compared in terms of rate of gross photosynthesis, rate of shoot respiration, and rate of root respiration. Source of nitrogen had little or no effect on rate of photosynthesis or shoot respiration. In contrast, the rate of respiration of the nodulated roots of plants fixing their own nitrogen was greater, sometimes two-fold greater, than that of equivalent plants lacking nodules and utilizing nitrate nitrogen. This superiority in terms of rate of root respiration was generally confined to the period of intense nitrogen fixation. An analysis of the magnitude of this respiratory burden in terms of daily photosynthesis indicates that, in all three legumes, plants fixing their own nitrogen respire 11-13% more of their fixed carbon each day than equivalent plants lacking nodules and utilizing nitrate nitrogen. INTRODUCTION Biochemical studies of rhizobial metabolism indicate a relatively large energy requirement in the form of ATP and reductant for the synthesis in the nodule of ammonia from the dinitrogen molecule (Bulen and LeComte, 1966; Winter and Burris, 1968; Hardy and Havelka, 1975), but it seems generally to be thought that these energy costs, plus the ancillary costs of synthesizing and maintaining nodule tissue, do not perceptively decrease plant growth rate in comparison with plants utilizing a source of combined nitrogen such as nitrate, where the energy cost of reducing nitrate to ammonia within the plant is also thought to be substantial (Bergersen, 1971; Gibson, 1966, 1976; Minchin and Pate, 1973; Pate, 1976). However, an investigation of the influence of source of nitrogen on the carbon economy of Fiskeby soyabean (Ryle, Powell, and Gordon, 1978) indicated that 1 The Grassland Research Institute is financed through the Agricultural Research Council. This content downloaded from 157.55.39.235 on Fri, 07 Oct 2016 05:57:12 UTC All use subject to http://about.jstor.org/terms 146 Ryle, Powell, and Gordon—Cost of Nitrogen Fixation plants fixing atmospheric nitrogen respired 10-15% more of their fixed carbon than equivalent plants lacking nodules and utilizing nitrate nitrogen. In view of the important implications of this result, similar analyses were carried out on two other legumes, cowpea and white clover, to establish the generality of this response to the two sources of nitrogen. This paper reports the performance of the legumes in terms of three important physiological attributes: (i) photosynthesis of the whole plant, (ii) respiration of the shoot, and (iii) respiration of the root. The legumes were grown either wholly dependent on nitrogen fixation in their own nodules, or lacking nodules but receiving abundant nitrate nitrogen. Although the data from the soyabean experiment have been published in detail elsewhere (Ryle et ai., 1978), relevant measurements are given in the appropriate sections to facilitate comparisons with the cowpea and white clover data. MATERIALS AND METHODS The methods and conditions used to grow soyabean (Glycine max (L.). Merr. var. Fiskeby V), cowpea (Vigna unguiculata (L.) Walp var. K2809), and white clover (Trifolium repens L. var. Blanca) and the techniques used to measure their rates of photosynthesis and respiration have been described in detail elsewhere (Ryle, Powell, and Gordon, 1978; 1979). Briefly, plants were grown singly in 9-5 cm pots of Perlite in Saxcil growth cabinets at 23/18 °C (soyabean), 30/24 °C (cowpea), and 20/15 °C (white clover). Seeds were sterilized with u.v. radiation and, where appropriate, inoculated with an effective Rhizobium. The objective was to produce two populations of plants within each species: one population effectively nodulated and receiving a complete nutrient solution lacking only nitrogen, and a second population completely lacking nodules and receiving a very similar nutrient solution containing abundant nitrate nitrogen. The two populations from each species were grown in separate cabinets to minimize bacterial contamination. Nodulated plants were generally provided with 20 parts 10~6 of nitrate nitrogen until the nodules developed leghaemoglobin. Details of the growth conditions are summarized in Table 1. Measurements of photosynthesis and respiration were made at intervals of a few days until seed maturation in soyabean and cowpea or until day 85 in white clover; white clover remained vegetative in the 12 h photoperiod used here. An open, infrared gas analyser system was used to measure the flux of C02 into or out of the plant fractions under study (Ryle et al., 1978). After measurements of photosynthesis and respiration, the plants were harvested and their leaf areas determined with a Paton Industries electronic planimeter. Subsequently the plant fractions were dried at 100 °C and weighed, and their nitrogen contents determined (Ryle et ai, 1978). Table 1. Experimental material and conditions Species Soyabean Cowpea White clover Glycine max (L.) Vigna unguiculata (L.) Trifolium repens L. Merr. var. Fiskeby V Walp. var. K2809 var. Blanca Rhizobium CB 1809 CB 756 Rothamsted No. 5 Photoperiod 12 h 12 h 12 h Light 810 ± 10 //Einsteins 810 ± 10//Einsteins m~2 s~' m-2 s_1 (fluorescent) (fluorescent + incandescent) Day/night temperature 23/18 °C 30/24 °C 20/15 °C Growth medium Single plants in 9-5 cm pots of Perlite Nutrient solution Macroelements (parts 10~6) in demineralized water Ca K N S P Mg Na Fe 'Nitrate plants' 167 146 221 36 36 27 118 12 'Nodulated plants' 167 146 0 192 36 27 4 12 plus microelements Mn, Cu, B, Zn, Mo, and Co; pH adjusted 5-8-6-2; 150-400 ml d_1/plant according to plant size This content downloaded from 157.55.39.235 on Fri, 07 Oct 2016 05:57:12 UTC All use subject to http://about.jstor.org/terms Ryle, Powell, and Gordon—Cost of Nitrogen Fixation 147 RESULTS Plant growth The patterns of growth and the weights achieved by the three legumes given the two sources of nitrogen are shown in Fig. 1. In plants fixing their own nitrogen, the onset and, in the two annual legumes, the cessation of nitrogen fixation clearly set the limits for dry matter accumulation. However, the most important feature of the

Ligated chlorophyll cation radicals: Their function in photosystem II of plant photosynthesis

Abstract: Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl) a and its cation radicals. Oxidation potentials for Chl → Chl+. as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl+. ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P-680 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment.

Algal chloroplasts in the protoplasm of three species of benthic foraminifera: taxonomic affinity, viability and persistence

Abstract: The ultrastructure and pigment content of algal chloroplasts (derived from Bacillariophyceae or Chrysophyceae) are described from 3 benthic species of brackish-water foraminiferans.Elphidium williamsoni Haynes contains 4×106 chloroplasts mg-1, whereas the contents ofNonion germanicum (Ehrenberg) andE. excavatum (Terquem) are about 10% of this value. The two former contain chlorophyllsa andc and fucoxanthin, but these pigments were not detectable in the latter.E. williamsoni andN. germanicum had a net uptake of14C−HCO3-, proportional to their content of chlorophyll and number of chloroplasts, increasing linearly up to approximately 10 Klux. At light saturation the former assimilates 2.3x10-3 mg C mg-1 h-1 and the latter only about 20% of this value. Dark uptake was insignificant in all cases. Uptake could not be demonstrated inE. excavatum. The photosynthesis effected by these species is trivial in terms of the total benthic carbon fixation effected by the microflora. The chloroplasts survived longer in forminiferans kept in the dark than in light/dark adapted individuals. To keep a steady state population of chloroplasts under light/dark conditions,E. williamsoni must eat at least 65 chloroplasts individual-1 h-1, whereas the minimum consuption rate inN. germanicum is 20.

Ultraviolet radiation and phytoplankton photosynthesis1

Abstract: The effect of present-day levels of ultraviolet radiation (290 to 320 nm) on /sup 14/C incorporation by natural phytoplankton populations was measured. There is a statistical relatonship between dosage and reduction of /sup 14/C uptake. This effect could be detected throughout a significant portion of the euphotic zone, but the major effect is restricted to the upper portions of the euphotic zone. Overall, its effect on present-day estimates of oceanic production is minimal. Similarly, potential effects of elevated uv-B levels striking the surface of the sea, resulting from perturbations of the ozone layer, are restricted to the upper few meters of the water column.

Photosynthesis-The Light Reactions

Abstract: During the past three or four years there has been a substantial advance in our knowledge of the light reactions in photosynthesis. This comes at a time of heightened interest in sunlight as a source of energy for our daily needs. The exploration of potential practical applications in this area will be helped by identifying the common features of the light reactions in higher plants and algae and in the simpler photosynthetic bacteria. These include the processes of light absorption by an antenna of chlorophyll (Chl) and other pigment molecules, the transfer of excitation to a photochemical reaction center, and the initiation of the electron transfer reactions - all within a few picoseconds of the arrival of the photon.

The effects of emergence and submergence on the photosynthesis and respiration of marine macrophytes

Abstract: Respiration and gross and net photosynthesis were measured for three species of intertidal algae growing at 10 and 18°C when they were submerged and emerged. Evidence that the high intertidal alga,...

Physiological and environmental constraints in the ecology of the planktonic dinoflagellate Ceratium hirundinella

Abstract: SUMMARY. Constraints upon the vertical distribution of a population of Ceratium hirundinella, in a productive English lake during the summer stratification of 1976, are considered. They are interpreted in relation to vertical gradients of temperature, dissolved oxygen and irradiance, and to rates of photosynthesis and respiration measured as oxygen exchange in long- and short-term exposures. The motile cells tended to aggregate at an intermediate depth in the epilimnion, associated with a relative irradiance level of c. 10% or c. 140 μ einsteins m−2s−1 as measured with a horizontal PAR sensor. Higher irradiances, and conditions below the oxycline, were apparently unfavourable, but the intervening layer was severely compressed at the height of summer stratification, when concentrations of inorganic nitrogen were also minimal. Thus the population apparently passed through a critical period, at which the cellular content of chlorophyll-α was much reduced. The preferred irradiance level in the lake corresponded to that at which measured rates of net photosynthesis were maximal. Increased rates of oxygen evolution were measured at higher irradiances in very short exposures; this behaviour may be of importance to cells experiencing vertical movement in nature.

Decomposition of blue-green algal (cyanobacterial) blooms in lake mendota, wisconsin.

Abstract: Decomposition of natural populations of Lake Mendota phytoplankton dominated by blue-green algae (cyanobacteria) was monitored by using oxygen uptake and disappearance of chlorophyll, algal volume (fluorescence microscopy), particulate protein, particulate organic carbon, and photosynthetic ability (14CO2 up-take). In some experiments, decomposition of 14C-labeled axenic cultures of Anabaena sp. was also measured. In addition to decomposition, mineralization of inorganic nitrogen and phosphorus were followed in some experiments. Decomposition could be described as a first-order process, and the rate of decomposition was similar to that found by others using pure cultures of eucaryotic algae. Nitrogen and phosphorus never limited the decomposition process, even when the lake water was severely limited in soluble forms of these nutrients. This suggests that the bacteria responsible for decomposition can obtain all of their key nutrients for growth from the blue-green algal cells. Filtration of lake water through plankton netting that removed up to 90% of the algal biomass usually did not cause a similar decrease in oxygen demand, suggesting that most of the particulate organic matter used for respiration of the decomposing bacteria was in a small-particle fraction. Short-term oxygen demand correlated well with the particulate chlorophyll concentration of the sample, and a relationship was derived that could be used to predict community respiration of the lake from chlorophyll concentration. Kinetic analysis showed that not all analyzed components disappeared at the same rate during the decomposition process. The relative rates of decrease of the measured parameters were as follows: photosynthetic ability > algal volume > particulate chlorophyll > particulate protein. Decomposition of 14C-labeled Anabaena occurred at similar rates with aerobic epilimnetic water and with anaerobic sediment, but was considerably slower with anaerobic hypolimnetic water. Of the various genera present in the lake, Aphanizomenon and Anabaena were more sensitive to decomposition than was Microcystis. In addition to providing a general picture of the decomposition process, the present work relates to other work on sedimentation to provide a detailed picture of the fate of blue-green algal biomass in a eutrophic lake ecosystem.

Modes of electron transfer from molecular hydrogen in Anabaena cylindrica

Abstract: Several natural and artificial electron donors were assayed in the C2H2-reduction of heterocysts isolated from the cyanobacterium Anabaena cylindrica. Among these, molecular hydrogen was the most effective one when the assays were performed in the light. The C2H2-reduction and the Knallgas reaction of intact Anabaena filaments as well as the H2-supported C2H2-reduction of isolated heterocysts were compared for their sensitivity towards several inhibitors known to affect the photosynthetic or respiratory electron flow. Among these, dibromothymoquinone (DBMIB) affected all three reactions equally indicating that plastoquinone is a common intermediate of the H2-consumptions by either the respiratory or the photosynthetic electron transport. Metronidazole inhibited the H2-utilization via photosynthesis but did not affect the consumption of this gas by respiration and therefore allows to differentiate between the two pathways of hydrogen utilization. The studies with the inhibitors are suggestive for a segment of electron carriers on the membranes common to both photosynthesis and respiration in heterocysts of Anabaena.

Photosynthetic carboxylating enzymes in marine phytoplankton

Abstract: Measurements of ribulose-1,5-bisphosphate carboxylase (RUBPCase) and phosphoenolpyruvate carboxylase (PEPCase) in several phytoplankton cultures and natural phytoplankton populations showed a positive correlation of photosynthesis with RUBPCase activity but not with PEPCase activity. Observed ratios of RUBPCase:PEPCase were high in log phase cultures when photosynthesis was maximal and low in stationary phase. There was no significant correlation between photosynthesis and the ratio of RUBPCase:PEPCase activity in natural populations. The rate of carbon assimilation by the intact algae was significantly greater than that by the isolated carboxylases. In both natural phytoplankton populations and laboratory cultures, the ratio of photosynthesis:carboxylase activity depended on the physiological state of the cells, being greatest when the photosynthetic rate was highest. In all autotrophic organisms RUBPCase is present and catalyzes the carboxylation of ribulose- 1,5-bisphosphate (a 5-carbon sugar phosphate to form two molecules of 3-phosphoglycerate. The Calvin cycle (Bassham and Calvin 1957) is a means of regenerating the CO, acceptor RUBP, while producing intermediates for the synthesis of cell products. It is this cyclic regeneration of the CO, acceptor that permits growth with CO, as the sole car

Attenuation of light and daily integral rates of photosynthesis attained by planktonic algae1

Abstract: The daily integral rate of photosynthesis attained by planktonic algae in Lake Minnetonka, Minnesota, conforms to an equation for a rectangular hyperbola proposed by Bannister. The rate depends on the concentration of chlorophyll a in the mixed layer and also upon two kinetic parameters that are analogous to the kinetic constants in equations for reactions catalyzed by enzymes. One parameter is an upper limit, $, that would be attained by very dense populations, and the other is a concentration of chlorophyll, c’, at which attenuation of photosynthetically active radiation (PhAR) by the algae equals background attenuation by the water. The parameters were evaluated with the same statistical procedures used to evaluate the kinetic constants for enzyme reactions. The limit, 9, for the daily integral rate in this lake would be about 0.5 mol 0,. rnp2* d-l. Observed daily rates are usually ~0.7 9, because chlorophyll in the water usually intercepts <70% of the radiant energy. Coefficients for the attenuation of PhAR by chlorophyll, E,, and by the water, Q, were estimated from the linear regression of the total attenuation coefficient, E, on chlorophyll concentration. The estimate of E, is 0.022 + 0.005. m-‘*(mg Chl* m-3)-1. Background attenuation, E,, affects integral photosynthesis in the same way as a competitive inhibitor affects enzyme reactions: it is a constituent of c’ = E,/+ which, together with chlorophyll concentration, determines the fraction of underwater PhAR intercepted by planktonic algae.

Investigations on heat resistance of spinach leaves.

Abstract: Exposure of spinach plants to high temperature (35° C) increased the heat resistance of the leaves by about 3° C. This hardening process occurred within 4 to 6 h, whereas dehardening at 20°/15° C required 1 to 2 days. At 5° C dehardening did not take place. Hardening and dehardening occurred in both the dark and the light. The hardiness was tested by exposure of the leaves to heat stress and subsequent measurements of chlorophyll fluorescence induction and light-induced absorbance changes at 535 nm on the leaves and of the photosynthetic electron transport in thylakoids isolated after heat treatment. Heat-induced damage to both heat-hardened and non-hardened leaves seemed to consist primarily in a breakdown of the membrane potential of the thylakoids accompanied by partial inactivation of electron transport through photosystem II. The increase in heat resistance was not due to temperature-induced changes in lipid content and fatty acid composition of the thylakoids, and no conspicuous changes in the polypeptide composition of the membranes were observed. Prolonged heat treatment at 35° C up to 3 days significantly decreased the total lipid content and the degree of unsaturation of the fatty acids of membrane lipids without further increase in the thermostability of the leaves. Intact chloroplasts isolated from heat-hardened leaves retained increased heat resistance. When the stroma of the chloroplasts was removed, the thermostability of the thylakoids was decreased and was comparable to the heat resistance of chloroplast membranes obtained from non-hardened control plants. Compartmentation studies demonstrated that the content of soluble sugars within the chloroplasts and the whole leaf tissue decreased as heat hardiness increased. This indicated that in spinach leaves, sugars play no protective role in heat hardiness. The results suggest that changes in the ultrastructure of thylakoids in connection with a stabilizing effect of soluble non-sugar stroma compounds are responsible for acclimatization of the photosynthetic apparatus to high temperature conditions. Changes in the chemical composition of the chloroplast membranes did not appear to play a role in the acclimatization.