Physiological and Biochemical Characteristics of Sugar Beet Plants Grown at an Increased Carbon Dioxide Concentration and at Various Nitrate Doses

Three-week-old sugar beet (Beta vulgaris L.) seedlings were grown for an additional four weeks under controlled conditions: in river sand watered with a modified Knop mixture containing one half-fold (0.5N), standard (1N), and or threefold (3N) nitrate amount, at the irradiance of 90 W/m² PAR, and at the carbon dioxide concentrations of 0.035% (1C treatment) or 0.07% (2C treatment). The increase in the carbon dioxide concentration and in the nitrogen dose resulted in an increase in the leaf area and the leaf and root dry weight per plant. With the increase in the nitrogen dose, morphological indices characterizing leaf growth increased more noticeably in 1C plants than in 2C plants. And vice versa, the effects of increased CO₂ concentration were reduced with the increase in the nitrogen dose. Roots responded to the changes in the CO₂ and nitrate concentrations otherwise than leaves. At a standard nitrate dose (1N), the contents of proteins and nonstructural carbohydrates (sucrose and starch) in leaves depended little on the CO₂ concentration. At a double CO₂ concentration, the content of chlorophyll somewhat decreased, and the net photosynthesis rate (P _n) calculated per leaf area unit increased. An increase in the nitrogen dose did not affect the leaf carbohydrate content of the 1C and 2C plants except the leaves of the 2C-3N plants, where the carbohydrate content decreased. In 1C and 2C plants, an increase in the nitrogen dose caused an increase in the protein and chlorophyll content. Specific P _n values somewhat decreased in 1C-0.5N plants and had hardly any dependence on the nitrate dose in the 2C plants. The carbohydrate content in roots did not depend on the CO₂ concentration, and the content was the highest at 0.5N. Characteristic nitrogen dose-independent acclimation of photosynthesis to an increased carbon dioxide concentration, which was postulated previously [1], was not observed in our experiments with sugar beet grown at doubled carbon dioxide concentration.     

Effects of growth conditions on exopolysaccharide production by Cyanospira capsulata

Batch cultures of Cyanospira capsulata, a heterocystous cyanobacterium possessing a thick polysaccharidic capsule, were characterized by increasing viscosity owing to the continuous release of a soluble polysaccharide (EPS) into the culture medium. Both capsulated trichomes and solubilized EPS contributed to the flow properties of whole cultures. A typical pseudoplastic behaviour, the more marked the more aged were the cultures, was evidenced.The production of EPS was investigated under different growth conditions by changing some nutritional and physical parameters known to affect the synthesis of exopolysaccharides in algae and cyanobacteria. Among the factors tested (Ca²⁺, Mg²⁺ or PO₄⁻⁴ deficiencies, salinity and pH) only Mg²⁺ shortage caused a significant enhancement of the EPS production. Under continuous illumination in open ponds, the EPS productivity of batch cultures on standard mineral medium was about 5·8 g m⁻² day⁻¹, whereas under Mg²⁺ deficiency with a consequent increase of the cultures' viscosity     

Acclimation conditions modify physiological response of the diatom Thalassiosira pseudonana to elevated CO2 concentrations in a nitrate‐limited chemostat

Diatoms are responsible for a large proportion of global carbon fixation, with the possibility that they may fix more carbon under future levels of high CO₂. To determine how increased CO₂ concentrations impact the physiology of the diatom Thalassiosira pseudonana Hasle et Heimdal, nitrate‐limited chemostats were used to acclimate cells to a recent past (333 ± 6 μatm) and two projected future concentrations (476 ± 18 μatm, 816 ± 35 μatm) of CO₂. Samples were harvested under steady‐state growth conditions after either an abrupt (15–16 generations) or a longer acclimation process (33–57 generations) to increased CO₂ concentrations. The use of un‐bubbled chemostat cultures allowed us to calculate the uptake ratio of dissolved inorganic carbon relative to dissolved inorganic nitrogen (DIC:DIN), which was strongly correlated with fCO₂ in the shorter acclimations but not in the longer acclimations. Both CO₂ treatment and acclimation time significantly affected the DIC:DIN uptake ratio. Chlorophyll a per cell decreased under elevated CO₂ and the rates of photosynthesis and respiration decreased significantly under higher levels of CO₂. These results suggest that T. pseudonana shifts carbon and energy fluxes in response to high CO₂ and that acclimation time has a strong effect on the physiological response.     

Effects of NAD kinase 2 overexpression on primary metabolite profiles in rice leaves under elevated carbon dioxide

The concentration of carbon dioxide (CO₂) in the atmosphere is projected to double by the end of the 21st century. In C₃ plants, elevated CO₂ concentrations promote photosynthesis but inhibit the assimilation of nitrate into organic nitrogen compounds. Several steps of nitrate assimilation depend on the availability of ATP and sources of reducing power, such as nicotinamide adenine dinucleotide phosphate (NADPH). Plastid‐localised NAD kinase 2 (NADK2) plays key roles in increasing the ATP/ADP and NADP(H)/NAD(H) ratios. Here we examined the effects of NADK2 overexpression on primary metabolism in rice (Oryza sativa) leaves in response to elevated CO₂. By using capillary electrophoresis mass spectrometry, we showed that the primary metabolite profile of NADK2‐overexpressing plants clearly differed from that of wild‐type plants under ambient and elevated CO₂. In NADK2‐overexpressing leaves, expression of the genes encoding glutamine synthetase and glutamate synthase was up‐regulated, and the levels of Asn, Gln, Arg, and Lys increased in response to elevated CO₂. The present study suggests that overexpression of NADK2 promotes the biosynthesis of nitrogen‐rich amino acids under elevated CO₂.     

NEUTRAL LIPID AND CARBOHYDRATE PRODUCTIVITIES AS A RESPONSE TO NITROGEN STATUS IN ISOCHRYSIS SP. (T‐ISO; HAPTOPHYCEAE): STARVATION VERSUS LIMITATION1

Partitioning of the carbon (C) fixed during photosynthesis between neutral lipids (NL) and carbohydrates was investigated in Isochrysis sp. (Haptophyceae) in relation to its nitrogen (N) status. Using batch and nitrate‐limited continuous cultures, we studied the response of these energy reserve pools to both conditions of N starvation and limitation. During N starvation, NL and carbohydrate quotas increased but their specific growth rates (specific rates of variation, μ_CAR and μ_NL) decreased. When cells were successively deprived and then resupplied with NO₃, both carbohydrates and neutral lipids were inversely related to the N quota (N:C). These negative relationships were not identical during N impoverishment and replenishment, indicating a hysteresis phenomenon between N and C reserve mobilizations. Cells acclimated to increasing degrees of N limitation in steady‐state chemostat cultures showed decreasing NL quota and increasing carbohydrate quota. N starvation led to a visible but only transient increase of NL productivity. In continuous cultures, the highest NL productivity was obtained for the highest experimented dilution rate (D = 1.0 d^?1; i.e., for non N‐limited growth conditions), whereas the highest carbohydrate productivity was obtained at D = 0.67 d^?1. We used these results to discuss the nitrogen conditions that optimize NL productivities in the context of biofuel production.     

Heavy metal sorption by released polysaccharides and whole cultures of two exopolysaccharide-producing cyanobacteria

The metal removal capacity of cultures of two capsulated, exopolysaccharide-producing cyanobacteria, Cyanospira capsulata and Nostoc PCC7936, were tested using copper (II) as the model metal. C. capsulata cultures removed the greatest amount of copper, with a maximum per unit of biomass (q _max) of 115.0±5.1 mg copper g⁻¹ of protein, compared with 85.0±3.2 removed with Nostoc PCC7936 cultures. Water solutions of pure polysaccharides (RPSs) released into the culture medium by C. capsulata and Nostoc PCC7936 achieved q _max values of 20.2±0.8 mg g⁻¹ copper per polysaccharide dry weight with C. capsulata RPS and 11.0±1.5 mg g⁻¹ with Nostoc PCC7936 RPS. Cultures of the two cyanobacteria also removed Zn (II) and Ni (II), in both single-metal systems and in multimetal systems with Cu; in the various single-metal systems more copper was removed than Zn or Ni, while in the multimetal systems a smaller amount of each individual metal was removed but the overall amount of all metal ions sorbed or the amount of copper sorbed in the copper-only system was almost the same with C. capsulata, and slightly higher with Nostoc PCC7936.     

No stimulation of nitrogen fixation by non‐filamentous diazotrophs under elevated CO2 in the South Pacific

Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO₂. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35^oS). High CO₂ treatments, produced using 750 ppmv CO₂ to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO₂ or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN‐A), which may account for the difference in response of nitrogen fixation under elevated CO₂ to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN‐A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO₂. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO₂. If nitrogen fixation by UCYN‐A is not stimulated by elevated pCO₂, then future increases in CO₂ and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.     

Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)

Since the recognition of iron‐limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non‐steady‐state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron‐limited and iron‐replete conditions using artificial seawater medium. Iron‐limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU‐induced variable fluorescence signal (F_v/F_m). Under iron‐replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50‐fold higher. Under iron limitation, cultures excreted up to 100 fmol NO₂^?·cell^?1·d^?1 (about 10% of incorporated N) and NiR activities declined by 50‐fold while internal NO₂^? pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron‐limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO₂^? reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate‐limiting step in nitrogenassimilation.     

Comparison of photosynthetic acclimation to elevated CO2 and limited nitrogen supply in soybean

Plants grown at elevated CO₂ often acclimate such that their photosynthetic capacities are reduced relative to ambient CO₂-grown plants. Reductions in synthesis of photosynthetic enzymes could result either from reduced photosynthetic gene expression or from reduced availability of nitrogen-containing substrates for enzyme synthesis. Increased carbohydrate concentrations resulting from increased photosynthetic carbon fixation at elevated CO₂ concentrations have been suggested to reduce the expression of photosynthetic genes. However, recent studies have also suggested that nitrogen uptake may be depressed by elevated CO₂, or at least that it is not increased enough to keep pace with increased carbohydrate production. This response could induce a nitrogen limitation in elevated-CO₂ plants that might account for the reduction in photosynthetic enzyme synthesis. If CO₂ acclimation were a response to limited nitrogen uptake, the effects of elevated CO₂ and limiting nitrogen supply on photosynthesis and nitrogen allocation should be similar. To test this hypothesis we grew non-nodulating soybeans at two levels each of nitrogen and CO₂ concentration and measured leaf nitrogen contents, photosynthetic capacities and Rubisco contents. Both low nitrogen and elevated CO₂ reduced nitrogen as a percentage of total leaf dry mass but only low nitrogen supply produced significant decreases in nitrogen as a percentage of leaf structural dry mass. The primary effect of elevated CO₂ was to increase non-structural carbohydrate storage rather than to decrease nitrogen content. Both low nitrogen supply and elevated CO₂ also decreased carboxylation capacity (V_cmax) and Rubisco content per unit leaf area. However, when V_cmax and Rubisco content were expressed per unit nitrogen, low nitrogen supply generally caused them to increase whereas elevated CO₂ generally caused them to decrease. Finally, elevated CO₂ significantly increased the ratio of RuBP regeneration capacity to V_cmax whereas neither nitrogen supply nor plant age had a significant effect on this parameter. We conclude that reductions in photosynthetic enzyme synthesis in elevated CO₂ appear not to result from limited nitrogen supply but instead may result from feedback inhibition by increased carbohydrate contents.     

Physiological aspects of surface-immobilized Catharanthus roseus cells

Summary Growth and alkaloid production of surface-immobilized C. roseus cells were studied in a 2-1 bioreactor. Media designed to maximize cell growth or alkaloid production were employed. Nitrate and carbohydrate consumption rates as well as growth rates and biomass yields of immobilized cultures were equal or somewhat lower than for cell suspension cultures. Respiration rate (O₂ consumption and CO₂ production rates) of immobilized C. roseus cell cultures was obtained by on-line analysis of inlet and outlet gas composition using a mass spectrometer. Respiration rate increased during the growth phase and decreased once the nitrogen or the carbon source was depleted from the medium. The respiration rate of immobilized C. roseus cells resembled rates reported in the literature for suspension cultures. Offprint requests to: Denis Rho     

Isolation and characterization of a chlorate-resistant mutant (Clo- R) of the symbiotic cyanobacterium Nostoc ANTH: heterocyst formation and N(2)-fixation in the presence of nitrate,and evidence for separate nitrate and nitrite transport systems

Nostoc ANTH is a filamentous, heterocystous cyanobacterium capable of N₂-fixation in the absence of combined nitrogen. A chlorate-resistant mutant (Clo-R) of Nostoc ANTH was isolated that differentiates heterocysts and fixes N₂ in the presence of nitrate, but not in the presence of nitrite or ammonium. The mutant lacks nitrate uptake and thereby also lacks induction of nitrate reductase activity by nitrate. However, this mutant is able to transport and assimilate nitrite, indicating that there is a transport system for nitrite that is distinct from that for the nitrate. The lack of inhibitory effect of nitrate on N₂-fixation was owing to lack of nitrate uptake and not to lack of enzymes for its assimilation (nitrate reductase and glutamine synthetase) or the lack of an ammonium transport system for retention of ammonia. The mutant has potential for use as a biofertilizer supplementing chemical nitrate fertilizer in rice fields, without N₂-fixation being adversely affected. Received: 16 October 2001 / Accepted: 26 November 2001     

Climate drivers alter nitrogen availability in surface peat and decouple N2 fixation from CH4 oxidation in the Sphagnum moss microbiome

Peat mosses (Sphagnum spp.) are keystone species in boreal peatlands, where they dominate net primary productivity and facilitate the accumulation of carbon in thick peat deposits. Sphagnum mosses harbor a diverse assemblage of microbial partners, including N₂-fixing (diazotrophic) and CH₄-oxidizing (methanotrophic) taxa that support ecosystem function by regulating transformations of carbon and nitrogen. Here, we investigate the response of the Sphagnum phytobiome (plant + constituent microbiome + environment) to a gradient of experimental warming (+0°C to +9°C) and elevated CO₂ (+500 ppm) in an ombrotrophic peatland in northern Minnesota (USA). By tracking changes in carbon (CH₄, CO₂) and nitrogen (NH₄-N) cycling from the belowground environment up to Sphagnum and its associated microbiome, we identified a series of cascading impacts to the Sphagnum phytobiome triggered by warming and elevated CO₂. Under ambient CO₂, warming increased plant-available NH₄-N in surface peat, excess N accumulated in Sphagnum tissue, and N₂ fixation activity decreased. Elevated CO₂ offset the effects of warming, disrupting the accumulation of N in peat and Sphagnum tissue. Methane concentrations in porewater increased with warming irrespective of CO₂ treatment, resulting in a ~10× rise in methanotrophic activity within Sphagnum from the +9°C enclosures. Warming's divergent impacts on diazotrophy and methanotrophy caused these processes to become decoupled at warmer temperatures, as evidenced by declining rates of methane-induced N₂ fixation and significant losses of keystone microbial taxa. In addition to changes in the Sphagnum microbiome, we observed ~94% mortality of Sphagnum between the +0°C and +9°C treatments, possibly due to the interactive effects of warming on N-availability and competition from vascular plant species. Collectively, these results highlight the vulnerability of the Sphagnum phytobiome to rising temperatures and atmospheric CO₂ concentrations, with significant implications for carbon and nitrogen cycling in boreal peatlands.     

Carbon source–sink limitations differ between two species with contrasting growth strategies

Understanding how carbon source and sink strengths limit plant growth is a critical knowledge gap that hinders efforts to maximize crop yield. We investigated how differences in growth rate arise from source–sink limitations, using a model system comparing a fast‐growing domesticated annual barley (Hordeum vulgare cv. NFC Tipple) with a slow‐growing wild perennial relative (Hordeum bulbosum). Source strength was manipulated by growing plants at sub‐ambient and elevated CO₂ concentrations ([CO₂]). Limitations on vegetative growth imposed by source and sink were diagnosed by measuring relative growth rate, developmental plasticity, photosynthesis and major carbon and nitrogen metabolite pools. Growth was sink limited in the annual but source limited in the perennial. RGR and carbon acquisition were higher in the annual, but photosynthesis responded weakly to elevated [CO₂] indicating that source strength was near maximal at current [CO₂]. In contrast, photosynthetic rate and sink development responded strongly to elevated [CO₂] in the perennial, indicating significant source limitation. Sink limitation was avoided in the perennial by high sink plasticity: a marked increase in tillering and root:shoot ratio at elevated [CO₂], and lower non‐structural carbohydrate accumulation. Alleviating sink limitation during vegetative development could be important for maximizing growth of elite cereals under future elevated [CO₂].     

Nitrogenase activity and dark CO2 fixation in the lichen Peltigera aphthosa Willd

The lichen Peltigera aphthosa consists of a fungus and green alga (Coccomyxa) in the main thallus and of a Nostoc located in superficial packets, intermixed with fungus, called cephalodia. Dark nitrogenase activity (acetylene reduction) of lichen discs (of alga, fungus and Nostoc) and of excised cephalodia was sustained at higher rates and for longer than was the dark nitrogenase activity of the isolated Nostoc growing exponentially. Dark nitrogenase activity of the symbiotic Nostoc was supported by the catabolism of polyglucose accumulated in the ligh and which in darkness served to supply ATP and reductant. The decrease in glucose content of the cephalodia paralleled the decline in dark nitrogenase activity in the presence of CO₂; in the absence of CO₂ dark nitrogenase activity declined faster although the rate of glucose loss was similar in the presence and absence of CO₂. Dark CO₂ fixation, which after 30 min in darkness represented 17 and 20% of the light rates of discs and cephalodia, respectively, also facilitated dark nitrogenase activity. The isolated Nostoc, the Coccomyxa and the excised fungus all fixed CO₂ in the dark; in the lichen most dark CO₂ fixation was probably due to the fungus. Kinetic studies using discs or cephalodia showed highest initial incorporation of ¹⁴CO₂ in the dark in to oxaloacetate, aspartate, malate and fumarate; incorporation in to alanine and citrulline was low; incorporation in to sugar phosphates, phosphoglyceric acid and sugar alcohols was not significant. Substantial activities of the enzymes phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and carbamoyl-phosphate synthase (EC 2.7.2.5 and 2.7.2.9) were detected but the activities of PEP carboxykinase (EC 4.1.1.49) and PEP carboxyphosphotransferase (EC 4.1.1.38) were negligible. In the dark nitrogenase activity by the cephalodia, but not by the free-living Nostoc, declined more rapidly in the absence than in the presence of CO₂ in the gas phase. Exogenous NH ₄ ⁺ inhibited nitrogenase activity by cephalodia in the dark especially in the absence of CO₂ but had no effect in the light. The overall data suggest that in the lichen dark CO₂ fixation by the fungus may provide carbon skeletons which accept NH ₄ ⁺ released by the cyanobacterium and that in the absence of CO₂, NH ₄ ⁺ directly, or indirectly via a mechanism which involves glutamine synthetase, inhibits nitrogenase activity.Abbreviations CP carbamoyl phosphate - EDTA ethylenedi-amine tetraacetic acid - PEP phosphoenolpyruvate - RuBP ribulose 1,5 bisphosphate     

Composition of the carbohydrate granules of the cyanobacterium, Cyanothece sp. strain ATCC 51142

Cyanothece sp. strain ATCC 51142 is an aerobic, unicellular, diazotrophic cyanobacterium that temporally separates O₂-sensitive N₂ fixation from oxygenic photosynthesis. The energy and reducing power needed for N₂ fixation appears to be generated by an active respiratory apparatus that utilizes the contents of large interthylakoidal carbohydrate granules. We report here on the carbohydrate and protein composition of the granules of Cyanothece sp. strain ATCC 51142. The carbohydrate component is a glucose homopolymer with branches every nine residues and is chemically identical to glycogen. Granule-associated protein fractions showed temporal changes in the number of proteins and their abundance during the metabolic oscillations observed under diazotrophic conditions. There also were temporal changes in the protein pattern of the granule-depleted supernatant fractions from diazotrophic cultures. None of the granule-associated proteins crossreacted with antisera directed against several glycogen-metabolizing enzymes or nitrogenase, although these proteins were tentatively identified in supernatant fractions. It is suggested that the granule-associated proteins are structural proteins required to maintain a complex granule architecture. Received: 30 August 1996 / Accepted: 24 October 1996     

Impacts of elevated CO2 on expression of plant defensive compounds in Bt‐transgenic cotton in response to infestation by cotton bollworm

• 1 The allocation of defensive compounds of transgenic Bt (cv. GK‐12) and nontransgenic cotton (cv. Simian‐3) grown in elevated CO₂ in response to infestation by cotton bollworm Helicoverpa armigera (Hübner) was studied in closed‐dynamics CO₂ chambers.
• 2 A significant reduction in foliar nitrogen content and Bt toxin protein occurred when transgenic Bt cotton grew under elevated CO₂. A significantly higher carbon/nitrogen ratio as well as condensed tannin and gossypol contents was observed for transgenic Bt (cv. GK‐12) and nontransgenic cotton in elevated CO₂, in partial support of the carbon nutrient balance hypothesis as a result of limiting nitrogen and excess carbon in cotton plants in response to elevated CO₂.
• 3 The CO₂ level and infestation time significantly affected the foliar nitrogen, condensed tannin, gossypol and Bt toxin protein contents of cotton plants after feeding by H. armigera. The interaction between CO₂ levels × cotton variety had a significant effect on foliar nitrogen content after injury by H. armigera.

     

光强和氮源对念珠藻胞外多糖分泌的影响

胞外多糖（EPS）是结皮蓝藻形成生物结皮的胶结剂,为了理解常球状存在的丝状蓝藻Nostoc胶结沙粒的机理,探讨了光强40、80 E/（m2s）和氮源（气态氮,硝态氮）对结皮优势种Nostoc sp.分泌EPS（包括荚膜多糖CPS和释放多糖RPS）的影响规律及其内在机理。结果发现:Nostoc sp.在气态氮和硝态氮下都有相似的快速生长,但其分泌的RPS、CPS及EPS量,在硝态氮下均随光强的增加而增加,在气态氮下却与光强没有关系。相关代谢研究发现,在硝态氮下细胞内有更高含量的可溶性糖和蔗糖。进一步的相关分析发现,在两种氮源下,蔗糖量与RPS量或CPS量间的显著正相关都只发生在80 E/（m2s）下,在气态氮中,两光强下的胞内总糖量都与CPS量显著负相关。以上结果说明,Nostoc sp.在氮源利用和光强适应方面都有明显优势,它即使在快速生长的对数期,也可同时分泌相当量的EPS,这使其在球状藻殖段形成之前胶结沙粒成为可能。由此可推知,Nostoc sp.在贫瘠沙土表面的最初生长过程中,其胞外的EPS均来自胞内的固碳产物,在高光强下,蔗糖很可能是其EPS合成的原料。       

Vertical thin-layer photoreactor for controlled cultivation of cyanobacteria

Nostoc sp. was cultivated in an air-lift reactor with continuous recirculation of the head gas phase that aerated and agitated the cyanobacterial suspension at regulated flow rates. The supply of inorganic carbon for growth was coupled with pH control, in the range of 7.7 to 8.1, by intermittent sparging of CO₂-head gas mixtures. The formation of irregular bubbles with swirling motion at the photostage of the reactor promoted efficient CO₂ transference in dense populations of Nostoc sp. (1.1 g/l) when bubbling at flow rates of 10 l/min. Biomass productivity was almost six-fold higher in the photoreactor (16.4 mg/l.h) than in a conventional system (2.8 mg/l.h). The exponential growth phase of cultures in the photoreactor amounted to 60% of the total growth period.The authors are with the Laboratorio de Alimentos, Area Microbiologia, Facultad de Quimica Bioquimica y Farmacia, Universidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.