Effect of UV-B radiation on the growth and antioxidant enzymes of Antarctic sea ice microalgae <Emphasis Type="Italic">Chlamydomonas</Emphasis> sp. ICE-L

The effect of ultraviolet-B (UV-B) radiation on Antarctic phytoplankton has become an attractive ecological issue as a result of annual springtime ozone depletion. The effects of UV-B radiation on the growth and antioxidant enzymes were investigated using Antarctic sea ice microalgae Chlamydomonas sp. ICE-L as the material in this study. The results demonstrated that UV-B radiation could notably inhibit the growth, especially at high UV-B radiation intensity (70 μW cm⁻²). Malondialdehyde and O₂ ^·− content in ICE-L increased rapidly in early days (1–3 days) exposed to UV-B radiation enhancement, then decreased rapidly. In the stress of UV-B radiation enhancement, the superoxide dismutase, peroxidase and Catalase activities of 1–4 days in ICE-L were obviously higher than those in the control, and their activities became higher at high UV-B radiation intensity (70 μW cm⁻²). These enzymes activity of 7 days would kept stable at low UV-B radiation intensity (35 μW cm⁻²), but kept high level at high UV-B radiation intensity (70 μW cm⁻²). However, the ascorbate peroxidase activity in ICE-L kept stable under the stress of UV-B radiation enhancement. The above experimental results indicated that the antioxidant enzyme system played an important role in the adaptation of Antarctic ice microalgae under the UV-B radiation change of Antarctic ecosystems.     

Enhanced electrode-reducing rate during the enrichment process in an air-cathode microbial fuel cell

The improvement in electricity generation during the enrichment process of a microbial consortium was analyzed using an air-cathode microbial fuel cell (MFC) repeatedly fed with acetate that was originally inoculated with sludge from an anaerobic digester. The anodic maximum current density produced by the anode biofilm increased from 0.12 mA/cm² at day 28 to 1.12 mA/cm² at day 105. However, the microbial cell density on the carbon cloth anode increased only three times throughout this same time period from 0.21 to 0.69 mg protein/cm², indicating that the biocatalytic activity of the consortium was also enhanced. The microbial activity was calculated to have a per biomass anode-reducing rate of 374 μmol electron g protein⁻¹ min⁻¹ at day 28 and 1,002 μmol electron g protein⁻¹ min⁻¹ at day 105. A bacterial community analysis of the anode biofilm revealed that the dominant phylotype, which was closely related to the known exoelectrogenic bacterium, Geobacter sulfurreducens, showed an increase in abundance from 32% to 70% of the total microbial cells. Fluorescent in situ hybridization observation also showed the increase of Geobacter-like phylotypes from 53% to 72%. These results suggest that the improvement of microbial current generation in microbial fuel cells is a function of both microbial cell growth on the electrode and changes in the bacterial community highly dominated by a known exoelectrogenic bacterium during the enrichment process.     

Leaf carbon gain simulation for tree seedlings acclimatized to microsite light regimes of a temperate pine forest

Leaf carbon gain simulation was performed forQuercus serrata seedlings with previously reported 6 day photosynthetic photon flux density (PPFD) histograms from 20 understorey microsites of a pine forest (Washitani & Tang 1991). This simulation was performed with or without an assumption of the acclimatization of photosynthetic capacity (P_max) to microsite light availability. A constant ratio of respiration rate to P_max, within, the range of 0.07–0.1, was assumed as a constraint. In relatively well illuminated microsites with a diffuse site factor above 0.1, predicted optimal P_max was about 5 μmol m⁻² s⁻¹, with the predicted mean daily net carbon gain being about 50 mmol m⁻² day⁻¹. Each of the predicted optimal P_max and the simulated mean daily net carbon gains with a constant P_max (5 μmol m⁻² s⁻¹) or the predicted optimal P_max was linearly related to the microsite light availability index, diffuse site factor. Simulated net carbon gain was negative at diffuse site factors below 0.04, if the constant of P_max was assumed. The predicted linear relationship between net carbon gain and diffuse site factor could provide an ecophysiological basis for the observed linear dependency of the relative growth rate of biomass ofQ. serrata seedlings on the microsite diffuse site factor (Washitani & Tang 1991).     

Microbial fuel cell with an azo-dye-feeding cathode

Microbial fuel cells (MFCs) were constructed using azo dyes as the cathode oxidants to accept the electrons produced from the respiration of Klebsiella pneumoniae strain L17 in the anode. Experimental results showed that a methyl orange (MO)-feeding MFC produced a comparable performance against that of an air-based one at pH 3.0 and that azo dyes including MO, Orange I, and Orange II could be successfully degraded in such cathodes. The reaction rate constant (k) of azo dye reduction was positively correlated with the power output which was highly dependent on the catholyte pH and the dye molecular structure. When pH was varied from 3.0 to 9.0, the k value in relation to MO degradation decreased from 0.298 to 0.016 μmol min⁻¹, and the maximum power density decreased from 34.77 to 1.51 mW m⁻². The performances of the MFC fed with different azo dyes can be ranked from good to poor as MO > Orange I > Orange II. Furthermore, the cyclic voltammograms of azo dyes disclosed that the pH and the dye structure determined their redox potentials. A higher redox potential corresponded to a higher reaction rate.     

Response of denitrification rate associated with wetting and drying cycles in a littoral wetland area of Lake Biwa,Japan

To clarify the relationship between denitrification activity and dry–wet levels in the littoral wetland sediments of Lake Biwa, Japan, denitrification rates and their regulating parameters (degree of dryness, redox potential, nitrate concentration) were measured on different moisture sediments. Redox potential in sediments was higher in the exposed region in contact with atmosphere than the flooded region covered with water. The nitrate concentration in interstitial waters was undetectable in the flooded region. On the other hand, concentration in the exposed region increased with increase in the degree of sediment dryness. The denitrification rate ranged from <0.001 to 0.88 μg N cm⁻³ h⁻¹ in the exposed region and increased with the increase in the degree of dryness. In the flooded region, on the other hand, no detectable rate (<0.001 μg N cm⁻³ h⁻¹) was observed. This indicates that the rates in the exposed region were mainly influenced by nitrate concentration in the interstitial waters accumulated by desiccation of sediments, whereas rates in the flooded region were strongly limited by no accumulation of nitrate in the anaerobic conditions. The potential denitrification rate, under the application condition of nitrate, ranged from 0.13 to 0.26 μg N cm⁻³ h⁻¹ in the flooded region and from 0.77 to 1.5 μg N cm⁻³ h⁻¹ in the exposed region. The potential rates in the flooded region had a tendency to be lower than those in the exposed region, implying that the number of denitrifying bacteria in the flooded region was low due to inactivation of aerobic respiration and denitrification in the denitrifying bacteria community. Kinetic parameters, maximum rate (V _max) and half-saturation constant (K _s) for denitrification were calculated on the experimental procedures of the wetting–drying cycles of sediments. Both parameters decreased by the wetting treatment and increased by the drying treatment. The fluctuation of V _max values with wetting–drying cycles indicated that the number of denitrifying bacteria was influenced by aerobic respiration and denitrification in the denitrifying bacteria community similar to the potential rates, and denitrifying enzyme was induced by the nitrate supplied by nitrification accelerated through the drying process. On the other hand, the fluctuation of K _s values implied that members of denitrifying bacteria were shifted to members of high nitrate affinity by wetting treatment and of low nitrate affinity by drying treatment.     

Antimicrobial activity of crude extracts from mangrove fungal endophytes

The aim of this work was to select endophytic fungi from mangrove plants that produced antimicrobial substances. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) or minimal fungicidal concentrations (MFC) of crude extracts from 150 isolates were determined against potential human pathogens by a colorimetric microdilution method. Ninety-two isolates (61.3%) produced inhibitory compounds. Most of the extracts (28–32%) inhibited Staphylococcus aureus (MIC/MBC 4–200/64–200 μg ml⁻¹). Only two extracts inhibited Pseudomonas aeruginosa (MIC/MBC 200/>200 μg ml⁻¹). 25.5 and 11.7% inhibited Microsporum gypseum and Cryptococcus neoformans (MIC/MFC 4–200/8–200 μg ml⁻¹ and 8–200/8–200 μg ml⁻¹, respectively), while 7.5% were active against Candida albicans (MIC/MFC 32–200/32–200 μg ml⁻¹). None of the extracts inhibited Escherichia coli. The most active fungal extracts were from six genera, Acremonium, Diaporthe, Hypoxylon, Pestalotiopsis, Phomopsis, and Xylaria as identified using morphological and molecular methods. Phomopsis sp. MA194 (GU592007, GU592018) isolated from Rhizophora apiculata showed the broadest antimicrobial spectrum with low MIC values of 8–32 μg ml⁻¹against Gram-positive bacteria, yeasts and M. gypseum. It was concluded that endophytic fungi from mangrove plants are diverse, many produce compounds with antimicrobial activity and could be suitable sources of new antimicrobial natural products.     

Eco-physiological responses of nitrogen-fixing cyanobacteria to light

The eco-physiological responses of three nitrogen-fixing cyanobacteria (N-fixing cyanobacteria), Aphanizomenon gracile, Anabaena minderi, and Ana. torques-reginae, to light were assessed under nutrient saturation. The N-fixing cyanobacteria were isolated into monocultures from a natural bloom in a shallow colored lake and their growth irradiance parameters and pigment composition were assessed. The different ecological traits related to light use (μ_max, α, I _k) suggest that these N-fixing cyanobacteria are well adapted to low light conditions at sufficient nutrients, yet interspecific differences were observed. Aphanizomenon gracile and Anabaena minderi had high relative growth rates at low irradiances (ca. 70% of those in high light), low half saturation constant for light-limited growth (I _k < 9.09 μmol photon m⁻² s⁻¹) and high efficiency (α < 0.11 day⁻¹ μmol photon⁻¹ m² s). Conversely, Ana. torques-reginae showed poorer light competitiveness: low relative growth rates at low irradiances (ca. 40% of those in high light), low α (0.009 day⁻¹ μmol photon⁻¹ m² s) and higher I _k (35.5 μmol photon m⁻² s⁻¹). Final densities in Aphanizomenon gracile and Anabaena minderi reached bloom densities at irradiances above 30 μmol photon m⁻² s⁻¹ with different hierarchy depending on irradiance, whereas Ana. torques-reginae never achieved bloom densities. All species had very low densities at irradiances ≤17 μmol photon m⁻² s⁻¹, thus no N-fixing blooms would be expected at these irradiances. Also, under prolonged darkness and at lowest irradiance (0 and 3 μmol photon m⁻² s⁻¹) akinetes were degraded, suggesting that in ecosystems with permanently dark sediments, the prevalence of N-fixing cyanobacteria should not be favored. All species displayed peaks of phycocyanin, but no phycoeritrin, probably due to the prevailing red light in the ecosystem from which they were isolated.     

Effects of water stress on photosynthetic activity,dry mass partitioning and some associated metabolic changes in four provenances of neem (<Emphasis Type="Italic">Azadirachta indica</Emphasis> A. Juss)

Chlorophyll (Chl) content, dry mass, relative water content (RWC), leaf mass per area (LMA), proline (Pro) content, malondialdehyde (MDA) content, superoxide dismutase (SOD) and peroxidase (POD) activity, P _N-PAR response curves and gas exchange were studied to determine the effects of water stress on photosynthetic activity, dry mass partitioning and metabolic changes in four provenances of neem (Azadirachta indica A. Juss). The results indicated that provenance differences existed in the adaptation response to water stress that included changes to growth strategies coupled with ecophysiological and metabolic adjustments. As water stress increased, stomatal conductance (g _s), net photosynthetic rate (P _N), transpiration rate (E), and leaf RWC decreased while LMA increased in all provenances. Dry mass was reduced in droughted plants and the percentage increased in dry mass allocated to roots, and enzyme activities of SOD and POD were highest in neem originating from Kalyani (KA) provenance and lowest in neem originating from New Dehli (ND) provenance. In contrast, water stress increased MDA content least in KA and most in ND. Furthermore, neem originating from ND also had the greatest decrease in Chl a/b ratio while the ratio was least affected in neem originating from KA. These findings suggest neem originating from KA may have more drought resistance than neem originating from ND. The data from P _N-PAR response curves are less clear. While these curves showed that drought stress increased compensation irradiance (I _c) and dark respiration (R _D) and decreased saturation irradiance (I _s) and maximum net photosynthetic rate (P _max), the extent of decline in P _max was provenance dependent. P _max under non-waterlimiting conditions was higher in neem originating from Jodhpur (MA) (about 14 μmol m⁻² s⁻¹) than in the other three provenances (all about 10 μmol m⁻² s⁻¹), but mild water stress had minimal effect on P _max of these three provenances whereas P _max of MA provenance declined to 10 μmol m⁻² s⁻¹, i.e. a similar value. However, under severe water stress P _max of MA and KA provenances had declined to 40% of non-stressed values (about 6 and 4 μmol m⁻² s⁻¹, respectively) whereas the decline in P _max of neem originating from Kulapachta (KU) and ND provenances was about 50% of nonstressed values (about 5 μmol m⁻² s⁻¹). These data suggest the P _N responses of KU and ND provenances are most tolerant, and KA and MA least tolerant to increasing water stress, but also suggest MA provenance could be the most desired under both non-water-limiting and water-limiting conditions due to highest P _max in all conditions.     

Green energy from <Emphasis Type="Italic">Rhodopseudomonas palustris</Emphasis> grown at low to high irradiance values,under fed-batch operational conditions

Rhodopseudomonas palustris was grown under continuous irradiances of 36, 56, 75, 151, 320, 500, and 803 W m⁻², for a co-production of both bio-H₂ and biodiesel (lipids) using fed-batch conditions. The highest overall bio-H₂ produced [4.2 l(H₂) l_culture ⁻¹] was achieved at 320 W m⁻², while the highest dry biomass (3.18 g l⁻¹) was attained at 500 W m⁻². Dry biomass contained between 22 and 39% lipid. The total energy conversion efficiency was at its highest (6.9%) at 36 W m⁻².     

Biogeochemical conditions and ice algal photosynthetic parameters in Weddell Sea ice during early spring

Physical, biogeochemical and photosynthetic parameters were measured in sea ice brine and ice core bottom samples in the north-western Weddell Sea during early spring 2006. Sea ice brines collected from sackholes were characterised by cold temperatures (range −7.4 to −3.8°C), high salinities (range 61.4–118.0), and partly elevated dissolved oxygen concentrations (range 159–413 μmol kg⁻¹) when compared to surface seawater. Nitrate (range 0.5–76.3 μmol kg⁻¹), dissolved inorganic phosphate (range 0.2–7.0 μmol kg⁻¹) and silicic acid (range 74–285 μmol kg⁻¹) concentrations in sea ice brines were depleted when compared to surface seawater. In contrast, NH₄ ⁺ (range 0.3–23.0 μmol kg⁻¹) and dissolved organic carbon (range 140–707 μmol kg⁻¹) were enriched in the sea ice brines. Ice core bottom samples exhibited moderate temperatures and brine salinities, but high algal biomass (4.9–435.5 μg Chl a l⁻¹ brine) and silicic acid depletion. Pulse amplitude modulated fluorometry was used for the determination of the photosynthetic parameters F _v/F _m, α, rETR_max and E _k. The maximum quantum yield of photosystem II, F _v/F _m, ranged from 0.101 to 0.500 (average 0.284 ± 0.132) and 0.235 to 0.595 (average 0.368 ± 0.127) in the sea ice internal and bottom communities, respectively. The fluorometric measurements indicated medium ice algal photosynthetic activity both in the internal and bottom communities of the sea ice. An observed lack of correlation between biogeochemical and photosynthetic parameters was most likely due to temporally and spatially decoupled physical and biological processes in the sea ice brine channel system, and was also influenced by the temporal and spatial resolution of applied sampling techniques.     

Properties of native and hydrophobic laccases immobilized in the liquid-crystalline cubic phase on electrodes

Both native Trametes hirsuta laccase and the same laccase modified with palmytic chains to turn it more hydrophobic were prepared and studied with cyclic voltammetry and Raman spectroscopy. Native laccase immobilized in the monoolein cubic phase was characterized with resonance Raman spectroscopy, which demonstrated that the structure at the “blue” copper site of the protein remained intact. The diamond-type monoolein cubic phase prevents denaturation of enzymes on the electrode surface and provides contact of the enzyme with the electrode either directly or through the mediation by electroactive probes. Direct electron transfer for both laccases incorporated into a lyotropic liquid crystal was obtained under anaerobic conditions, whereas bioelectrocatalytic activity was shown only for the native enzyme. The differences in electrochemical behavior of native and hydrophobic laccase as well as possible mechanisms of direct and mediated electron transfers are discussed. The Michaelis constant for 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS²⁻), K _M^app, and the maximal current, I _max, for the native enzyme immobilized onto the electrode were estimated to be 0.24 mM, and 5.3 μA, respectively. The maximal current density and the efficiency of the catalysis, I _max/K _M^app, were found to be 73 μA cm⁻² and 208.2 μA cm⁻² mM⁻¹, respectively, and indicated a high efficiency of oxygen electroreduction by the enzyme in the presence of ABTS²⁻ in the cubic-phase environment. Rate constants were calculated to be 7.5 × 10⁴ and 3.6 × 10⁴ M⁻¹ s⁻¹ for native and hydrophobic laccase, respectively.     

Analysis of ethanol in fermentation samples by a robust nanocomposite-based microbial biosensor

A robust microbial biosensor was constructed from a bionanocomposite prepared by a direct mixing of bacterial cells of Gluconobacter oxydans and carbon nanotubes with ferricyanide employed as a mediator for enhanced sensitivity of ethanol oxidation. A successful integration of the device into flow injection analysis mode of operation provided a high sensitivity of detection of (74 ± 2.7) μA mM⁻¹ cm⁻², a low detection limit of 5 μM and a linear range from 10 μM up to 1 mM. A short response time of the biosensor allowed a sample throughput of 67 h⁻¹ at 0.3 ml min⁻¹. The biosensor exhibited high operational stability with a decrease in the biosensor response of 1.7% during 43 h of continuous operation. The device was used to analyse ethanol in fermentation samples with a good agreement with a HPLC method.     

Catalyzing denitrification of <Emphasis Type="Italic">Paracoccus versutus</Emphasis> by immobilized 1,5-dichloroanthraquinone

The accelerating effect of non-dissolved redox mediator (1,5-dichloroanthraquinone) on the biological denitrification was investigated in this paper using 1,5-dichloroanthraquinone immobilized by calcium alginate (CA) and a heterotrophic denitrification bacterium of Paracoccus versutus (GU111570). The results suggested that the denitrification rate was enhanced 2.1 fold by 25 mmol l⁻¹ 1,5-dichloroanthraquinone of this study, and a positive correlation was found for the denitrification rate and 1,5-dichloroanthraquinone concentrations from 0 to 25 mmol l⁻¹. According to the change characteristic of NO₃ ⁻ and NO₂ ⁻ during the denitrification process, the tentative accelerating mechanism of the denitrification by redox mediators was put forward, and redox mediator might play the role of reduced cofactors like NADH, N(A)DH and SDH, or the similar ubiquinol/ubiquinone (Q/QH₂) role during the denitrification process.     

A mathematical model for describing light-response curves in <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.

A modified exponential model was used to describe light-response curves of Nicotiana tabacum L. The accuracies of an exponential model, a nonrectangular hyperbola model, a rectangular hyperbola model, a modified rectangular hyperbola model and the modified exponential model were evaluated by Mean square error (MSE) and Mean absolute error (MAE). The tests MSE and MAE of the modified exponential model were the lowest among the five models. The light saturation point (LSP) obtained by the exponential model, the nonrectangular hyperbola model and the rectangular hyperbola model were much lower than the measured values, and the maximum net photosynthetic rates (P _max) calculated from these models, were greater than the measured values. P _max at LSP of 1,077 μmol m⁻² s⁻¹ calculated by the modified exponential model was 12.34 μmol(CO₂) m⁻² s⁻¹, which was more accurate than the values obtained from the modified rectangular hyperbola model. The results show that the modified exponential model is superior to other models for describing light-response curves.     

Molecular characterization of phenylalanine ammonia lyase gene from <Emphasis Type="Italic">Cistanche deserticola</Emphasis>

We cloned the gene, CdPAL1, from Cistanche deserticola callus using RACE PCR with degenerate primers that were designed based on a multiple sequence alignment of known PAL genes from other plant species. The gene shows high homology to other known PAL genes registered in GenBank. The recombinant protein exhibited Michaelis–Menten kinetics with a K _m of 0.1013 mM, V _max of 4.858 μmol min⁻¹, K _cat of 3.36 S⁻¹, and K _cat/K _m is 33,168 M⁻¹ S⁻¹. The enzyme had an optimal pH of 8.5 and an activation energy of 38.92 kJ mol⁻¹ when l-Phenylalanine was used as a substrate; l-tyrosine cannot be used as substrate for this protein. The optimal temperature was 55°C, and the thermal stability results showed that, after a treatment at 70°C for 20 min, the protein retained 87% activity, while a treatment at 75°C for 20 min resulted in a loss of over 85% of the enzyme activity. Treatment with heavy metal ions (Hg²⁺, Pb²⁺, and Zn²⁺) showed remarkable inhibitory effects. Among the intermediates from the lignin (cinnamyl alcohol, cinnamyl aldehyde, coniferyl aldehyde, coniferyl alcohol), phenylpropanoid (cinnamic acid, coumaric acid, caffeic acid, and chlorogenic acid) and phenylethanoid (tyrosol and salidroside) biosynthetic pathways, only cinnamic acid showed strong inhibitory effects against CdPAL1 activity with a K _i of 8 μM. Competitive inhibitor AIP exhibited potent inhibition with K _i = 0.056 μM.     

Small scale vertical gradients of Arctic ice algal photophysiological properties

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (F _v/F _m), maximum relative electron transport rate (rETR_max), photosynthetic efficiency (α), and the photoadaptive index (E _k). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0–5 cm from ice–water interface) expressed low F _v/F _m (0.331–0.426) and low α (0.098–0.130 (μmol photons m⁻²s⁻¹)⁻¹) in December. F _v/F _m and α increased in March and May (0.468–0.588 and 0.141–0.438 (μmol photons m⁻²s⁻¹)⁻¹, respectively) indicating increased photosynthetic activity. In addition, increases in rETR_max (3.3–16.4 a.u.) and E _k (20–88 μmol photons m⁻² s⁻¹) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion, photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).     

Photosynthetic characteristics and physiological plasticity of an Aphanizomenon flos-aquae (Cyanobacteria, Nostocaceae) winter bloom in a deep oligo-mesotrophic lake (Lake Stechlin, Germany)

In winter of 2009/2010, Aphanizomenon flos-aquae bloomed in the ice and snow covered oligo-mesotrophic Lake Stechlin, Germany. The photosynthesis of the natural population was measured at eight temperatures in the range of 2–35°C, at nine different irradiance levels in the range of 0–1,320 μmol m⁻² s⁻¹ PAR at each applied temperature. The photoadaptation parameter (I _k) and the maximum photosynthetic rate (P _max) correlated positively with the temperature between 2 and 30°C, and there was a remarkable drop in both parameters at 35°C. The low I _k at low temperatures enabled the active photosynthesis of overwintering populations at low irradiance levels under ice and snow cover. The optimum of the photosynthesis was above 20°C at irradiances above 150 μmol m⁻² s⁻¹. At lower irradiance levels (7.5–30 μmol m⁻² s⁻¹), the photosynthesis was the most intensive in the temperature range of 2–5°C. The interaction between light and temperature allowed the proliferation of A. flos-aquae in Lake Stechlin resulting in winter water bloom in this oligo-mesotrophic lake. The applied 2°C is the lowest experimental temperature ever in the photosynthesis/growth studies of A. flos-aquae, and the results of the P–I and P–T measurements provide novel information about the tolerance and physiological plasticity of this species.     

A comparative analysis of photosynthetic characteristics of hulless barley at two altitudes on the Tibetan Plateau

To determine the photosynthetic characteristics of C₃ plants and their sensitivity to CO₂ at different altitudes on the Tibetan Plateau, hulless barley (Hordeum vulgare L. ssp. vulgare) was grown at altitudes of 4,333 m and 3,688 m. Using gas-exchange measurements, photosynthetic parameters were simulated, including the maximum net photosynthesis (P _max) and the apparent quantum efficiency (α). Plants growing at higher altitude had higher net photosynthetic rates (P _N), photosynthesis parameters (P _max and α) and sensitivities to CO₂ enhancement than plants growing at lower altitude on the Tibetan Plateau. The enhancements of P _N, P _max, and α for plants growing at higher altitude, corresponding with 10 μmol(CO₂) mol⁻¹ increments, were approximately 0.20∼0.45%, 0.05∼0.20% and 0.12∼0.36% greater, respectively, than for plants growing at lower altitude, respectively, where CO₂ levels rose from 10 to 170 μmol(CO₂) mol⁻¹. Therefore, on the Tibetan Plateau, the changes in the photosynthetic capacities and the photosynthetic sensitivities to CO₂ observed in the C₃ plants grown above 3,688 m are likely to increase with altitude despite the decreasing CO₂ partial pressure.     

Physiological responses of Spartina alterniflora to varying environmental conditions in Virginia marshes

Physiological measurements were used to investigate the dependence of photosynthesis on light, temperature, and intercellular carbon dioxide (CO₂) levels in the C₄ marsh grass Spartina alterniflora. Functional relationships between these environmental variables and S. alterniflora physiological responses were then used to improve C₄-leaf photosynthesis models. Field studies were conducted in monocultures of S. alterniflora in Virginia, USA. On average, S. alterniflora exhibited lower light saturation values (~1000 μmol m⁻² s⁻¹) than observed in other C₄ plants. Maximum carbon assimilation rates and stomatal conductance to water vapor diffusion were 36 μmol (CO₂) m⁻² s⁻¹ and 200 mmol (H₂O) m⁻² s⁻¹, respectively. Analysis of assimilation-intercellular CO₂ and light response relationships were used to determine Arrhenius-type temperature functions for maximum rate of carboxylation (V _cmax), phosphoenolpyruvate carboxylase activity (V _pmax), and maximum electron transport rate (J _max). Maximum V _cmax values of 105 μmol m⁻² s⁻¹ were observed at the leaf temperature of 311 K. Optimum V _pmax values (80.6 μmol m⁻² s⁻¹) were observed at the foliage temperature of 308 K. The observed V _pmax values were lower than those in other C₄ plants, whereas V _cmax values were higher, and more representative of C₃ plants. Optimum J _max values reached 138 μmol (electrons) m⁻² s⁻¹ at the foliage temperature of 305 K. In addition, the estimated CO₂ compensation points were in the range of C₃ or C₃–C₄ intermediate plants, not those typical of C₄ plants. The present results indicate the possibility of a C₃–C₄ intermediate or C₄-like photosynthetic mechanism rather than the expected C₄-biochemical pathway in S. alterniflora under field conditions. In a scenario of atmospheric warming and increased atmospheric CO₂ concentrations, S. alterniflora will likely respond positively to both changes. Such responses will result in increased S. alterniflora productivity, which is uncharacteristic of C₄ plants.