Über den Einfluß niedriger und hoher O2-Partialdrucke auf den Sauerstoff- und Kohlendioxidumsatz von Amaranthus und Phaseolus während der Lichtphase

Summary Carbon dioxide and oxygen gas exchange of illuminated Amaranthus and Phaseolus leaves was measured from 0–600 ppm of CO₂ in an open system.At low oxygen concentration (2% O₂) the ratio of CO₂ uptake to O₂ evolution came close to 1.At high oxygen partial pressure (42% O₂) the O₂ compensation point of an Amaranthus leaf was increased and oxygen evolution was depressed. Accordingly the CO₂/O₂ quotients were variable; the lowest value of 1,9 differed significantly from 1,0.The oxygen and carbon dioxide compensation points of a Phaseolus leaf were increased at high oxygen concentration (42% O₂) and oxygen evolution as well as carbon dioxide uptake were reduced. Therefore the ratios CO₂ over O₂ varied and differed greatly from 1,0.It was concluded that the nature of photosynthates is regulated by the gas composition around the leaves.     

Desorption of carbon dioxide from fermentation broth

Rates of CO₂ desorption from fermentation broths under actual operating conditions were determined by measuring the CO₂ partial pressure in the exit gas. The concentrations of CO₂ physically dissolved in the broths were measured by the so-called tubing method. Values of k_La for CO₂ desorption calculated from these values agreed well with the k_La values for oxygen absorption corrected for the difference in gas diffusivities. The dissolved CO₂ concentration in the broth, which seems to bean important operating parameter, can easily be estimated from the CO₂ partial pressure in the exit gas, a more easily measurable quantity, if the k_La value is known. For a given value of k_La, assumption of perfect mixing or plug flow in the gas phase made little difference in the calculated values of the dissolved CO₂ concentration, indicating that the gas phase was probably in between perfect mixing and plug flow. In industrial fermentors, the CO₂ partial pressure in the exit gas can practically be assumed to be in equilibrium with the dissolved CO₂ concentration.     

The length of discontinuous gas exchange cycles in lepidopteran pupae may serve as a mechanism for natural selection

Gas exchange is studied in diapausing pupae of Mamestra brassicae L., whose larvae are reared under identical conditions. The release of CO₂ gas is recorded with infrared gaseous analyzers. Oxygen convective uptake into the tracheae and oxygen consumption rates are recorded by means of a constant‐volume coulometric respirometer. Outputs from both of these respirometry systems are combined with infrared actographs. All 3‐month‐old pupae of M. brassicae display a pattern of discontinuous gas exchange (DGE) cycles of CO₂ gas release by bursts, although the lengths of these cycles varies between individuals. Some pupae exhibit long DGE cycles of at least 20 h in duration, with negligible CO₂ gas release during interburst periods, and there is presumed to be a convective gas exchange at this time. As a result of a partial vacuum inside the tracheae, a large oxygen convective uptake always occurs at the start of the spiracular opening phase. Other pupae have short DGE cycles of less than 3 h in duration, with elevated CO₂ gas release during the interburst period, when gas exchange is predominantly diffusive. The spiracular open phase in these pupae consists of frequent separate convective bursts of CO₂ gas release, with the opening–closing rhythms of the spiracles, which are considered as O phase fluttering. The pupae with long DGE cycles exhibit extremely low metabolic rates and very low total water loss rates, whereas those with short DGE cycles have higher metabolic and total water loss rates. The pupae with long DGE cycles live approximately twice as long as those with short cycles; thus, the present study demonstrates that long DGE cycles confer a fitness benefit on pupae as a result of a lower metabolic rate associated with water economy, conferring on them a longer life.     

An on-line technique for monitoring propionic acid fermentation

An on-line technique, based on measuring the increase in pressure due to CO₂ release in a closed air-tight reactor, was used to evaluate the fermentation of lactate by propionibacteria. The method was applied to batch cultures of Propionibacterium shermanii grown in yeast extract/sodium lactate medium containing lactate as a carbon source under micro-aerophilic conditions. Gas pressure evolution was compared both with substrate consumption and metabolites production and with acidification and growth. Linear relationships were found between gas pressure variation, lactate consumption and propionate and acetate production. The technique also enabled the evaluation of total CO₂ produced, by taking account of pressure, oxygen and pH measurements. These results tend to show that this simple and rapid method could be useful to monitor propionic acid bacteria growth.     

Effect of CO2, CO2 and Diamox on photosynthesis and photorespiration in Chlamydomonas reinhardtii (green alga) and Anacystis nidulans (Cyanobacterium, blue-green alga)

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO₂ uptake and the CO₂ compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO₂, but CO₂ insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO₂, exhibited an CO₂ insensitive net photosynthesis. The CO₂ compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO₂ enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO₂ 1^?1; while Chlamydomonas grown in 5% CO₂ enriched air for 4 days showed a high, but temporary CO₂ compensation point (60 μl CO₂ 1^?1). After a two hour adaptation in low CO₂, a stable, low CO₂ compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO₂, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO₂ concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO₂ (air), the cells behaved as if they had been grown in high CO₂. They showed an oxygen sensitive net photosynthesis and a high CO₂ compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas. The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.     

A field portable system for the measurement of gas exchange of leaves under natural and controlled conditions: examples with field-grown Eucalyptus pauciflora Sieb. ex Spreng. ssp. pauciflora, E. behriana F. Muell. and Pinus radiata R. Don.

Abstract A field portable system is described which measures the response of gas exchange of one leaf to changes in environmental parameters under controlled conditions, and which simultaneously measures the gas exchange of another leaf as the climatic parameters vary naturally. The system consists of two independently operating cuvettes. It enables detailed studies of photosynthesis and stomata/transpiration of leaves attached to the plant in their natural position. It provides control of temperature, humidity, CO₂ and oxygen concentration (or, alternatively, of other gases) as well as of light. Infrared gas analyzers for CO₂ and H₂O are used which allow similar time constants for the measurement of the two gases. Examples of a diurnal course of gas exchange of a leaf in its natural exposition and of experiments with steady-state responses of gas exchange are presented. In Eucalyptus pauciflora Sieb. ex Spreng. ssp. pauciflora, a set of response curves of CO, assimilation (A) to CO₂, as measured at various leaf temperatures and light levels, shows carboxylation efficiency to be light saturated at the lower photon irradiances the lower the leaf temperature is. Carboxylation efficiency is maximal at 25°C. At ambient CO, partial pressure stomata open in a way that CO₂ assimilation occurs at a rate found within the curvature region of the CO₂ response function of A. The light-independent CO² compensation point as a function of temperature is presented. Applying a combined heat/low humidity pulse (15 or 60 min) on leaves of Eucalyptus behriana F. Muell. or Pinus radiata R. Don, respectively, leads to a lower level of intercellular carbon dioxide partial pressure (C_i) during the decline in A and leaf conductance to water vapour (g). A lower C_i level is maintained during recovery of A and g, A almost reaching the pre-pulse level but not g. The existence of an after-effect indicates that the response to the combined high temperature/low humidity pulse is a multi-step process.     

Membrane inlet for mass spectrometric measurement of catalysis by enzymatic decarboxylases

Membrane inlet mass spectrometry (MIMS) uses diffusion across a permeable membrane to detect in solution uncharged molecules of small molecular weight. We point out here the application of MIMS to determine catalytic properties of decarboxylases using as an example catalysis by oxalate decarboxylase (OxDC) from Bacillus subtilis. The decarboxylase activity generates carbon dioxide and formate from the nonoxidative reaction but is accompanied by a concomitant oxidase activity that consumes oxalate and oxygen and generates CO₂ and hydrogen peroxide. The application of MIMS in measuring catalysis by OxDC involves the real-time and continuous detection of oxygen and product CO₂ from the ion currents of their respective mass peaks. Steady-state catalytic constants for the decarboxylase activity obtained by measuring product CO₂ using MIMS are comparable to those acquired by the traditional endpoint assay based on the coupled reaction with formate dehydrogenase, and measuring consumption of O₂ using MIMS also estimates the oxidase activity. The use of isotope-labeled substrate (¹³C₂-enriched oxalate) in MIMS provides a method to characterize the catalytic reaction in cell suspensions by detecting the mass peak for product ¹³CO₂ (m/z 45), avoiding inaccuracies due to endogenous ¹²CO₂.     

An Excel tool for deriving key photosynthetic parameters from combined gas exchange and chlorophyll fluorescence: theory and practice

Combined photosynthetic gas exchange and modulated fluorometres are widely used to evaluate physiological characteristics associated with phenotypic and genotypic variation, whether in response to genetic manipulation or resource limitation in natural vegetation or crops. After describing relatively simple experimental procedures, we present the theoretical background to the derivation of photosynthetic parameters, and provide a freely available Excel‐based fitting tool (EFT) that will be of use to specialists and non‐specialists alike. We use data acquired in concurrent variable fluorescence–gas exchange experiments, where A/C_i and light–response curves have been measured under ambient and low oxygen. From these data, the EFT derives light respiration, initial PSII (photosystem II) photochemical yield, initial quantum yield for CO₂ fixation, fraction of incident light harvested by PSII, initial quantum yield for electron transport, electron transport rate, rate of photorespiration, stomatal limitation, Rubisco (ribulose 1·5‐bisphosphate carboxylase/oxygenase) rate of carboxylation and oxygenation, Rubisco specificity factor, mesophyll conductance to CO₂ diffusion, light and CO₂ compensation point, Rubisco apparent Michaelis–Menten constant, and Rubisco CO₂‐saturated carboxylation rate. As an example, a complete analysis of gas exchange data on tobacco plants is provided. We also discuss potential measurement problems and pitfalls, and suggest how such empirical data could subsequently be used to parameterize predictive photosynthetic models.     

A new method for measuring 13carbon abundance in tracer experiments

Abstract. A new technique for the precise measurement of ¹³C-abundance and concentration is described. It is based on the differences in infra-red spectra between ¹²CO₂ and ¹³CO₂ and can be applied to gas mixtures or organic materials which have been oxidized to CO₂. The gas mixture is first dried and then passed through two infra-red gas analysers (IRGAs) connected in parallel. The two IRGAs are fitted with different optical filters so they differ in their relative sensitivities to ¹²CO₂ and ¹³CO₂. Once these sensitivities are known then simple algebra allows the concentrations of ¹²CO₂ and ¹³CO₂ to be calculated from the two readings. Two variants of this basic system have been tested. In both, one IRGA was a normal commercial instrument with a narrow band pass interference filter making it highly specific for ¹²CO₂; the second instrument was fitted with either a wide-band pass filter covering both the ¹²CO₂ and ¹³CO₂ absorption bands, or a narrow band pass filter specific for ¹³CO₂. These variations convey different advantages in operation. The wide-band system can be easily calibrated using a single natural abundance ¹²CO₂ standard but is only moderately precise at low abundances. It is particularly valuable for continuous monitoring of the relatively high abundance sources used in plant photosynthesis experiments. The narrow-band system gives high precision but requires a more complex standardization procedure. It is recommended for measurements on low-abundance samples resulting from tracer experiments. Here, its high sensitivity permits measurements on samples as small as 3 μmole C, thus enabling plant fractions and individual metabolites to be investigated. While the wide-band system can be manually operated under field conditions, it is necessary for highest precision to use computerized data collection and linearization. These processes are described, as are novel techniques for standardization, the preparation of small quantities of CO₂ of known abundance, and the transfer of gas samples from oxidizer to analyser. Determinations by the wide band system of % abundance in standard gas mixtures gave a standard error of ±0.03% but this increased to over ±0.1% for abundances below 20%. Corresponding values for the narrow-band system were ±0.01% over the whole abundance range an accuracy almost identical to that observed with an organic mass spectrometer. Two pulse-chase experiment with ¹³CO₂ are described in which the technique was used for studies on growth and metabolism of Lemna minor. The first demonstrated that ¹³C-accumulation within the plants matched closely the predictions from the net assimilation rate and measurements of ¹³C-abundance in the gas phase. The second revealed the rapid changes in the ¹³C-labelling of some plant components during pulse and chase phases. These examples demonstrate the potential of the method for studies in plant physiology and biochemistry. In view of its relative cheapness, ease of maintenance and operation, accuracy, and sensitivity, it is suggested that this new method may encourage a wider use of the safe stable ¹³C for biological and medical applications.     

Hydrogen online monitoring based on thermal conductivity for anaerobic microorganisms

H₂-producing microorganisms are a promising source of sustainable biohydrogen. However, most H₂-producing microorganisms are anaerobes, which are difficult to cultivate and characterize. While several methods for measuring H₂ exist, common H₂ sensors often require oxygen, making them unsuitable for anaerobic processes. Other sensors can often not be operated at high gas humidity. Thus, we applied thermal conductivity (TC) sensors and developed a parallelized, online H₂ monitoring for time-efficient characterization of H₂ production by anaerobes. Since TC sensors are nonspecific for H₂, the cross-sensitivity of the sensors was evaluated regarding temperature, gas humidity, and CO₂ concentrations. The systems' measurement range was validated with two anaerobes: a high H₂-producer (Clostridium pasteurianum) and a low H₂-producer (Phocaeicola vulgatus). Online monitoring of H₂ production in shake flask cultivations was demonstrated, and H₂ transfer rates were derived. Combined with online CO₂ and pressure measurements, molar gas balances of the cultivations were closed, and an anaerobic respiration quotient was calculated. Thus, insight into the effect of medium components and inhibitory cultivation conditions on H₂ production with the model anaerobes was gained. The presented online H₂ monitoring method can accelerate the characterization of anaerobes for biohydrogen production and reveal metabolic changes without expensive equipment and offline analysis.     

New Cultural Approaches for Microaerophilic Hyperthermophiles

This article reports on a new culture system designed for studying the effects of nutritional factors on the growth of hyperthermophilic and chemolithotrophic microorganisms. The system comprises 5-l stainless steel jars, an automatic gas dispenser, propylene microplates, and a robotic platform. The culture system was validated using Aquifex aeolicus, a hyperthermophilic, chemolithotrophic, and microaerophilic bacterium, which requires hydrogen, oxygen, CO₂, and minerals for growth. We demonstrated that the cell densities measured on 147 cultures of A. aeolicus microplated in jar at 80°C under partial pressures (in kPa) of water vapor (47), H₂ (117.7), O₂ (28.1), CO₂ (31.4), and N₂ (3.9), followed a normal distribution, with a mean of 0.72 and a standard deviation of 0.04 (variation coefficient: 5.7%). In addition, cross-comparison of the growth kinetics of A. aeolicus in serum bottles and in a jar system highlighted similar kinetics patterns (both mean growth rates were 0.18 and 0.17 h–1, respectively), whereas the maximum cell densities reached were slightly lower in jar than in bottle (0.73 vs. 0.88 OD units, respectively). Furthermore, these results showed that, contrary to bottles, the total pressure of gas in jars remained constant throughout the biotic experiments, even with seven microplates completely filled with grown cultures. In addition, this system has been validated also for hyperthermophilic strictly anaerobes such as Thermotoga maritima or aerobes such as Sulfolobus solfataricus. This new culture system offers an interesting alternative for cultivating hyperthermophiles, using gas as substrate under constant pressure, thus making it possible to miniaturize experiments and study a large number of nutritional factors in one experimental run.     

Jasmonate‐mediated stomatal closure under elevated CO2 revealed by time‐resolved metabolomics

Foliar stomatal movements are critical for regulating plant water loss and gas exchange. Elevated carbon dioxide (CO₂) levels are known to induce stomatal closure. However, the current knowledge on CO₂ signal transduction in stomatal guard cells is limited. Here we report metabolomic responses of Brassica napus guard cells to elevated CO₂ using three hyphenated metabolomics platforms: gas chromatography‐mass spectrometry (MS); liquid chromatography (LC)‐multiple reaction monitoring‐MS; and ultra‐high‐performance LC‐quadrupole time‐of‐flight‐MS. A total of 358 metabolites from guard cells were quantified in a time‐course response to elevated CO₂ level. Most metabolites increased under elevated CO₂, showing the most significant differences at 10 min. In addition, reactive oxygen species production increased and stomatal aperture decreased with time. Major alterations in flavonoid, organic acid, sugar, fatty acid, phenylpropanoid and amino acid metabolic pathways indicated changes in both primary and specialized metabolic pathways in guard cells. Most interestingly, the jasmonic acid (JA) biosynthesis pathway was significantly altered in the course of elevated CO₂ treatment. Together with results obtained from JA biosynthesis and signaling mutants as well as CO₂ signaling mutants, we discovered that CO₂‐induced stomatal closure is mediated by JA signaling.     

A method for controlling the within-root CO2 concentration

Abstract A method is presented for the control of carbon dioxide concentrations within the roots of Fraxinus pennsylvanica Marsh. The results indicated a linear fit of the root CO₂ concentrations to the CO₂ levels of the treatment gases: y= l.l. x+105 (r= 0.98, 18 d.f.). The method presented for controlling CO₂ can be easily modified for other gas mixtures and plant species.     

Net uptake of atmospheric CO2 by coastal submerged aquatic vegetation

‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long‐term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO₂ through air‐sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air‐sea CO₂ fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air‐sea CO₂ flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO_2. This finding is contrary to the conventional perception that most near‐shore ecosystems are sources of atmospheric CO₂. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO₂ may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO₂.     

Influence of buffer system and pH on the amount of oxygen exchanged between solvent H2O and the CO2 produced in the aerobic oxidation of Cypridina luciferin catalyzed by Cypridina luciferase.

Incorporation of ¹⁸O into CO₂ was measured under various buffer conditions when the bioluminescent oxidation of Cypridina luciferin, catalyzed by luciferase, was carried out either in H₂¹⁶O medium with ¹⁸O₂ gas, or in H₂¹⁸O medium with ¹⁶O₂ gas. The results indicate that (1) the exchange of oxygen between CO₂ and solvent H₂O is significantly influenced by the kind of buffer as well as by pH, (2) the exchange of oxygen between solvent H₂O and CO₂ produced from luciferin in a neutral buffer can be reasonably well estimated from the exchange that takes place when the same amount of CO₂ gas is introduced into the same buffer by the presently employed method, and (3) in the Cypridina bioluminescent reaction, one of two oxygens of O₂ is quantitatively incorporated into the product CO₂ prior to the exchange of oxygen between CO₂ and solvent H₂O.     

Growth of Pseudomonas fluorescens in modified atmosphere packaged tofu

Aims: The objective was to study the growth of Pseudomonas in a food product (tofu) where it typically occurs as a spoilage organism, and when this product is stored under modified atmosphere. Methods and Results: A Pseudomonas strain was isolated from the endogenous microflora of tofu. Tofu was inoculated with the strain, packaged in different gas conditions (air, 100% N₂, 30% CO₂/70% N₂ or 100% CO₂) and stored under refrigerated conditions. Microbial loads and the headspace gas composition were monitored during storage. Conclusions: The strain was capable of growing in atmospheres containing no or limited amounts of oxygen and increased amounts of carbon dioxide. Even when 100% CO₂ was used, growth could not be inhibited completely. Significance and Impact of Study: In contrast to the general characteristics of the genus Pseudomonas (strictly aerobic, highly sensitive to CO₂), it should not be expected in the food industry that removing oxygen from the food package and increasing the carbon dioxide content, combined with cold storage, will easily avoid spoilage by Pseudomonas species. Guarantee of hygienic standards and combination of strategies with other microbial growth inhibiting measures should be implemented.     

Use of Chlorella vulgaris for CO(2) mitigation in a photobioreactor

Carbon dioxide (CO₂) is a colorless gas that exists at a concentration of approximately 330 ppm in the atmosphere and is released in great quantities when fossil fuels are burned. The current flux of carbon out of fossil fuels is about 600 times greater than that into fossil fuels. With increased concerns about global warming and greenhouse gas emissions, there have been several approaches proposed for managing the levels of CO₂ emitted into the atmosphere. One of the most understudied methods for CO₂ mitigation is the use of biological processes in engineered systems such as photobioreactors. This research project describes the effectiveness of Chlorella vulgaris, used in a photobioreactor with a very short gas residence time, in sequestering CO₂ from an elevated CO₂ airstream. We evaluated a flow-through photobioreactor's operational parameters, as well as the growth characteristics of the C. vulgaris inoculum when exposed to an airstream with over 1850 ppm CO₂. When using dry weight, chlorophyll, and direct microscopic measurements, it was apparent that the photobioreactor's algal inoculum responded well to the elevated CO₂ levels and there was no build-up of CO₂ or carbonic acid in the photobioreactor. The photobioreactor, with a gas residence time of approximately 2 s, was able to remove up to 74% of the CO₂ in the airstream to ambient levels. This corresponded to a 63.9-g/m³/h bulk removal for the experimental photobioreactor. Consequently, this photobioreactor shows that biological processes may have some promise for treating point source emissions of CO₂ and deserve further study. Received 25 April 2002/ Accepted in revised form 27 July 2002     

Comments on the Role of H2S in the Chemistry of Earth's Early Atmosphere and in Prebiotic Synthesis

Free energy calculations and experimental measurements have been used to show that H₂S/CO₂ mixtures outgassing from a prebiotic Earth's crust would have produced a reducing gas mixture containing CO, H₂, H₂O, and S _x as principal components. Due to rapid recombination of H₂, CO, and S _x to H₂S and CO₂ on cooling from a high temperature to ambient conditions, reducing components would have been retained only if efficient quenching of the reduced gas mixture had been possible. Consequently, subsea vents or vents with efficient infusion of water would have been ideal sites for retention of reduced species and for prebiotic organic synthesis. It is suggested that C/H/O/S ratios are important factors in controlling the degree of prebiotic organic synthesis and, hence, the emergence of life, since if oxygen is abundant, CO₂ and SO₂ would have been dominant species. Received: 5 March 1997 / Accepted: 15 December 1997     

Ecophysiology of photosynthesis in bryophytes: major roles for oxygen photoreduction and non‐photochemical quenching?

CO₂ fixation in mosses saturates at moderate irradiances. Relative electron transport rate (RETR) inferred from chlorophyll fluorescence saturates at similar irradiance in shade species (e.g. Plagiomnium undulatum, Trichocolea tomentella), but many species of unshaded habitats (e.g. Andreaea rothii, Schistidium apocarpum, Sphagnum spp. and Frullania dilatata) show non‐saturating RETR at high irradiance, with high non‐photochemical quenching (NPQ). In P. undulatum and S. apocarpum, experiments in different gas mixtures showed O₂ and CO₂ as interchangeable electron sinks. Nitrogen + saturating CO₂ gave high RETR and depressed NPQ. In S. apocarpum, glycolaldehyde (inhibiting photosynthesis and photorespiration) depressed RETR in air more at low than at high irradiance; in CO₂‐free air RETR was maintained at all irradiances. Non‐saturating electron flow was not suppressed in ambient CO₂ with 1% O₂. The results indicate high capacity for oxygen photoreduction when CO₂ assimilation is limited. Non‐saturating light‐dependent H₂O₂ production, insensitive to glycolaldehyde, suggests that electron transport is supported by oxygen photoreduction, perhaps via the Mehler‐peroxidase reaction. Consistent with this, mosses were highly tolerant to paraquat, which generates superoxide at photosystem I (PSI). Protection against excess excitation energy in mosses involves high capacity for photosynthetic electron transport to oxygen and high NPQ, activated at high irradiance, alongside high reactive oxygen species (ROS) tolerance.     

Influence of vegetation and soil CO2 exchange on the concentration and stable oxygen isotope ratio of atmospheric CO2 within a Pinus resinosa canopy

Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO₂ concentration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO₂ concentrations and more negative ¹⁸O values observed closer to the soil surface. The observed daily increases in ¹⁸O values of forest CO₂ were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO₂ that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isotope ratio of CO₂ diffusing out of a leaf depends primarily on the ¹⁸O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the ¹⁸O values of forest CO₂ at night, because CO₂ released from the soil has equilibrated with soil water which has a lower ¹⁸O content than leaf water. The observed relationship between diurnal changes in CO₂ concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO₂ fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.