Relationships between carbonyl sulfide (COS) and CO2 during leaf gas exchange

Restricted internal diffusion weakens transpiration-photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange

Increasingly frequent and intense heatwaves threaten ecosystem health in a warming climate. However, plant responses to heatwaves are poorly understood. A key uncertainty concerns the intensification of transpiration when heatwaves suppress photosynthesis, known as transpiration-photosynthesis decoupling. Field observations of such decoupling are scarce, and the underlying physiological mechanisms remain elusive. Here, we use carbonyl sulphide (COS) as a leaf gas exchange tracer to examine potential mechanisms leading to transpiration-photosynthesis decoupling on a coast live oak in a southern California woodland in spring 2013. We found that heatwaves suppressed both photosynthesis and leaf COS uptake but increased transpiration or sustained it at non-heatwave levels throughout the day. Despite statistically significant decoupling between transpiration and photosynthesis, stomatal sensitivity to environmental factors did not change during heatwaves. Instead, midday photosynthesis during heatwaves was restricted by internal diffusion, as indicated by the lower internal conductance to COS. Thus, increased evaporative demand and nonstomatal limitation to photosynthesis act jointly to decouple transpiration from photosynthesis without altering stomatal sensitivity. Decoupling offered limited potential cooling benefits, questioning its effectiveness for leaf thermoregulation in xeric ecosystems. We suggest that adding COS to leaf and ecosystem flux measurements helps elucidate diverse physiological mechanisms underlying transpiration-photosynthesis decoupling.

Increased belowground tree carbon allocation in a mature mixed forest in a dry versus a wet year

Tree species differ in their carbon (C) allocation strategies during environmental change. Disentangling species-specific strategies and contribution to the C balance of mixed forests requires observations at the individual tree level. We measured a complete set of C pools and fluxes at the tree level in five tree species, conifers and broadleaves, co-existing in a mature evergreen mixed Mediterranean forest. Our study period included a drought year followed by an above-average wet year, offering an opportunity to test the effect of water availability on tree C allocation. We found that in comparison to the wet year, C uptake was lower in the dry year, C use was the same, and allocation to belowground sinks was higher. Among the five major C sinks, respiration was the largest (ca. 60%), while root exudation (ca. 10%) and reproduction (ca. 2%) were those that increased the most in the dry year. Most trees relied on stored starch for maintaining a stable soluble sugars balance, but no significant differences were detected in aboveground storage between dry and wet years. The detailed tree-level analysis of nonstructural carbohydrates and δ13 C dynamics suggest interspecific differences in C allocation among fluxes and tissues, specifically in response to the varying water availability. Overall, our findings shed light on mixed forest physiological responses to drought, an increasing phenomenon under the ongoing climate change.

Sulfur nutrition and its role in plant growth and development

Sulfur is one of the essential nutrients that is required for the adequate growth and development of plants. Sulfur is a structural component of protein disulfide bonds, amino acids, vitamins, and cofactors. Most of the sulfur in soil is present in organic matter and hence not accessible to the plants. Anionic form of sulfur (SO42-) is the primary source of sulfur for plants that are generally present in minimal amounts in the soil. It is water-soluble, so readily leaches out of the soil. Sulfur and sulfur-containing compounds act as signaling molecules in stress management as well as normal metabolic processes. They also take part in crosstalk of complex signaling network as a mediator molecule. Plants uptake sulfate directly from the soil by using their dedicated sulfate transporters. In addition, plants also use the sulfur transporter of a symbiotically associated organism like bacteria and fungi to uptake sulfur from the soil especially under sulfur depleted conditions. So, sulfur is a very important component of plant metabolism and its analysis with different dimensions is highly required to improve the overall well-being of plants, and dependent animals as well as human beings. The deficiency of sulfur leads to stunted growth of plants and ultimately loss of yield. In this review, we have focused on sulfur nutrition, uptake, transport, and inter-organismic transfer to host plants. Given the strong potential for agricultural use of sulfur sources and their applications, we cover what is known about sulfur impact on the plant health. We identify opportunities to expand our understanding of how the application of soil microbes like AMF or other root endophytic fungi affects plant sulfur uptake and in turn plant growth and development.

Root and rhizosphere contribution to the net soil COS exchange

Background and aims

Partitioning the measured net ecosystem carbon dioxide (CO2) exchange into gross primary productivity (GPP) and ecosystem respiration remains a challenge, which scientists try to tackle by using the properties of the trace gas carbonyl sulfide (COS). Its similar pathway into and within the leaf makes it a potential photosynthesis proxy. The application of COS as an effective proxy depends, among other things, on a robust inventory of potential COS sinks and sources within ecosystems. While the soil received some attention during the last couple of years, the role of plant roots is mostly unknown. In our study, we investigated the effects of live roots on the soil COS exchange.

Methods

An experimental setup was devised to measure the soil and the belowground plant parts of young beech trees observed over the course of 9 months.

Results

During the growing season, COS emissions were significantly lower when roots were present compared to chambers only containing soil, while prior to the growing season, with photosynthetically inactive trees, the presence of roots increased COS emissions. The difference in the COS flux between root-influenced and uninfluenced soil was fairly constant within each month, with diurnal variations in the COS flux driven primarily by soil temperature changes rather than the presence or absence of roots.

Conclusion

While the mechanisms by which roots influence the COS exchange are largely unknown, their contribution to the overall ground surface COS exchange should not be neglected when quantifying the soil COS exchange.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11104-023-06438-0.

Intercomparison of methods to estimate gross primary production based on CO2 and COS flux measurements

Separating the components of ecosystem-scale carbon exchange is crucial in order to develop better models and future predictions of the terrestrial carbon cycle. However, there are several uncertainties and unknowns related to current photosynthesis estimates. In this study, we evaluate four different methods for estimating photosynthesis at a boreal forest at the ecosystem scale, of which two are based on carbon dioxide (CO2) flux measurements and two on carbonyl sulfide (COS) flux measurements. The CO2-based methods use traditional flux partitioning and artificial neural networks to separate the net CO2 flux into respiration and photosynthesis. The COS-based methods make use of a unique 5-year COS flux data set and involve two different approaches to determine the leaf-scale relative uptake ratio of COS and CO2 (LRU), of which one (LRUCAP) was developed in this study. LRUCAP was based on a previously tested stomatal optimization theory (CAP), while LRUPAR was based on an empirical relation to measured radiation. For the measurement period 2013-2017, the artificial neural network method gave a GPP estimate very close to that of traditional flux partitioning at all timescales. On average, the COS-based methods gave higher GPP estimates than the CO2-based estimates on daily (23% and 7% higher, using LRUPAR and LRUCAP, respectively) and monthly scales (20% and 3% higher), as well as a higher cumulative sum over 3 months in all years (on average 25% and 3% higher). LRUCAP was higher than LRU estimated from chamber measurements at high radiation, leading to underestimation of midday GPP relative to other GPP methods. In general, however, use of LRUCAP gave closer agreement with CO2-based estimates of GPP than use of LRUPAR. When extended to other sites, LRUCAP may be more robust than LRUPAR because it is based on a physiological model whose parameters can be estimated from simple measurements or obtained from the literature. In contrast, the empirical radiation relation in LRUPAR may be more site-specific. However, this requires further testing at other measurement sites.

Leaf relative uptake of carbonyl sulfide to CO2 seen through the lens of stomatal conductance-photosynthesis coupling

Carbonyl sulfide (COS) has emerged as a multi-scale tracer for terrestrial photosynthesis. To infer ecosystem-scale photosynthesis from COS fluxes often requires knowledge of leaf relative uptake (LRU), the concentration-normalized ratio between leaf COS uptake and photosynthesis. However, current mechanistic understanding of LRU variability remains inadequate for deriving robust COS-based estimates of photosynthesis. We derive a set of closed-form equations to describe LRU responses to light, humidity and CO₂ based on the Ball-Berry stomatal conductance model and the biochemical model of photosynthesis. This framework reproduces observed LRU responses: decreasing LRU with increasing light or decreasing humidity; it also predicts that LRU increases with ambient CO₂ . By fitting the LRU equations to flux measurements on a C₃ reed (Typha latifolia), we obtain physiological parameters that control LRU variability, including an estimate of the Ball-Berry slope of 7.1 without using transpiration measurements. Sensitivity tests reveal that LRU is more sensitive to photosynthetic capacity than to the Ball-Berry slope, indicating stomatal response to photosynthesis. This study presents a simple framework for interpreting observed LRU variability and upscaling LRU. The stoma-regulated LRU response to CO₂ suggests that COS may offer a unique window into long-term stomatal acclimation to elevated CO₂ .

Light and Water Conditions Co-Regulated Stomata and Leaf Relative Uptake Rate (LRU) during Photosynthesis and COS Assimilation: A Meta-Analysis

As a trace gas involved in hydration during plant photosynthesis, carbonyl sulfide (COS) and its leaf relative uptake rate (LRU) is used to reduce the uncertainties in simulations of gross primary productivity (GPP). In this study, 101 independent observations were collected from 22 studies. We extracted the LRU, stomatal conductance (gs), canopy COS and carbon dioxide (CO2) fluxes, and relevant environmental conditions (i.e., light, temperature, and humidity), as well as the atmospheric COS and CO2 concentrations (Ca,COS and Ca,CO2). Although no evidence was found showing that gs regulates LRU, they responded in opposite ways to diurnal variations of environmental conditions in both mixed forests (LRU: Hedges’d = −0.901, LnRR = −0.189; gs: Hedges’d = 0.785, LnRR = 0.739) and croplands dominated by C3 plants (Hedges’d = −0.491, LnRR = −0.371; gs: Hedges’d = 1.066, LnRR = 0.322). In this process, the stomata play an important role in COS assimilation (R2 = 0.340, p = 0.020) and further influence the interrelationship of COS and CO2 fluxes (R2 = 0.650, p = 0.000). Slight increases in light intensity (R2 = 1, p = 0.002) and atmospheric drought (R2 = 0.885, p = 0.005) also decreased the LRU. The LRU saturation points of Ca,COS and Ca,CO2 were observed when ΔCa,COS ≈ 13 ppt (R2 = 0.580, p = 0.050) or ΔCa,CO2 ≈ −18 ppm (R2 = 0.970, p = 0.003). This study concluded that during plant photosynthesis and COS assimilation, light and water conditions co-regulated the stomata and LRU.

Fungal Carbonyl Sulfide Hydrolase of Trichoderma harzianum Strain THIF08 and Its Relationship with Clade D β-Carbonic Anhydrases

Carbonyl sulfide (COS) is the most abundant and long-lived sulfur-containing gas in the atmosphere. Soil is the main sink of COS in the atmosphere and uptake is dominated by soil microorganisms; however, biochemical research has not yet been conducted on fungal COS degradation. COS hydrolase (COSase) was purified from Trichoderma harzianum strain THIF08, which degrades COS at concentrations higher than 10,000 parts per million by volume from atmospheric concentrations, and its gene cos (492 bp) was cloned. The recombinant protein purified from Escherichia coli expressing the cos gene converted COS to H₂S. The deduced amino acid sequence of COSase (163 amino acids) was assigned to clade D in the phylogenetic tree of the β-carbonic anhydrase (β-CA) family, to which prokaryotic COSase and its structurally related enzymes belong. However, the COSase of strain THIF08 differed from the previously known prokaryotic COSase and its related enzymes due to its low reactivity to CO₂ and inability to hydrolyze CS₂. Sequence comparisons of the active site amino acids of clade D β-CA family enzymes suggested that various Ascomycota, particularly Sordariomycetes and Eurotiomycetes, possess similar enzymes to the COSase of strain THIF08 with >80% identity. These fungal COSase were phylogenetically distant to prokaryotic clade D β-CA family enzymes. These results suggest that various ascomycetes containing COSase contribute to the uptake of COS by soil.

Differential responses to two heatwave intensities in a Mediterranean citrus orchard are identified by combining measurements of fluorescence and carbonyl sulfide (COS) and CO2 uptake

The impact of extreme climate episodes such as heatwaves on plants physiological functioning and survival may depend on the event intensity, which requires quantification. We unraveled the distinct impacts of intense (HW) and intermediate (INT) heatwave days on carbon uptake, and the underlying changes in the photosynthetic system, in a Mediterranean citrus orchard using leaf active (pulse amplitude modulation; PAM) and canopy level passive (sun-induced; SIF) fluorescence measurements, together with CO₂ , water vapor, and carbonyl sulfide (COS) exchange measurements. Compared to normal (N) days, gross CO₂ uptake fluxes (gross primary production, GPP) were significantly reduced during HW days, but only slightly decreased during INT days. By contrast, COS uptake flux and SIF_A (at 760 nm) decreased during both HW and INT days, which was reflected in leaf internal CO₂ concentrations and in nonphotochemical quenching, respectively. Intense (HW) heatwave conditions also resulted in a substantial decrease in electron transport rates, measured using leaf-scale fluorescence, and an increase in the fractional energy consumption in photorespiration. Using the combined proxy approach, we demonstrate a differential ecosystem response to different heatwave intensities, which allows the trees to preserve carbon assimilation during INT days but not during HW days.

Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes

Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO₂ uptake) are needed. Carbonyl sulfide (COS) is the major long-lived sulfur-bearing gas in the atmosphere and a promising proxy for GPP. Large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Sulfur isotope measurements (³⁴S/³²S; δ³⁴S) have been suggested as a useful tool to constrain COS sources. Yet such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. Here we present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ³⁴S ± SE) value of 13.9 ± 0.1‰ for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of -1.9 ± 0.3‰ which we measured in plant-chamber experiments. Air samples with strong anthropogenic influence indicated an anthropogenic COS isotopic value of 8 ± 1‰. Samples of seawater-equilibrated-air indicate that the marine COS source has an isotopic value of 14.7 ± 1‰. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.

Measurement Device for Ambient Carbonyl Sulfide by Means of Catalytic Reduction Followed by Wet Scrubbing/Fluorescence Detection

A portable chemical analysis system for monitoring ambient carbonyl sulfide (COS) was investigated for the first time. COS is paid attention to from the perspectives of photosynthesis tracer, breath diagnosis marker, and new process-use in the manufacture of semiconductors. Recently, the threshold level value of COS was settled at 5 ppm in volume ratio (ppmv) for workplace safety management. In this work, COS was converted to H₂S by a small column packed with alumina catalyzer at 65 °C. Then, the H₂S produced was collected in a small channel scrubber to react with fluorescein mercuric acetate (FMA), and the resulting fluorescence quenching was monitored using an LED/photodiode-based miniature detector. The miniature channel scrubber was re-examined to determine its robustness and easy fabrication, and conditions of the catalyzer were optimized. When the FMA concentration used was 1 μM, the limit of detection and dynamic range, which were both proportional to the FMA concentration, were 0.07 and 25 ppbv, respectively. Ambient COS in the background level and even contaminated COS in the nitrogen gas cylinder could be detected. If necessary, H₂S was removed selectively by reproducible adsorbent columns. COS concentrations of engine exhaust were measured by the proposed method and by cryo-trap-gas chromatography-flame photometric detection, and the results obtained (0.5-5.9 ppbv) by the two methods agreed well (R ² = 0.945, n = 19). COS in ambient air and exhaust gases was successfully measured without any batchwise pretreatment.

A top-down approach of sources and non-photosynthetic sinks of carbonyl sulfide from atmospheric measurements over multiple years in the Paris region (France)

Carbonyl sulfide (COS) has been proposed as a proxy for carbon dioxide (CO2) taken up by plants at the leaf and ecosystem scales. However, several additional production and removal processes have been identified which could complicate its use at larger scales, among which are soil uptake, dark uptake by plants, and soil and anthropogenic emissions. This study evaluates the significance of these processes at the regional scale through a top-down approach based on atmospheric COS measurements at Gif-sur-Yvette (GIF), a suburban site near Paris (France). Over a period of four and a half years, hourly measurements at 7 m above ground level were performed by gas chromatography and combined with 222Radon measurements to calculate nocturnal COS fluxes using the Radon-Tracer Method. In addition, the vertical distribution of COS was investigated at a second site, 2 km away from GIF, where a fast gas analyzer deployed on a 100 m tower for several months during winter 2015-2016 recorded mixing ratios at 3 heights (15, 60 and 100 m). COS appears to be homogeneously distributed both horizontally and vertically in the sampling area. The main finding is that the area is a persistent COS sink even during wintertime episodes of strong pollution. Nighttime net uptake rates ranged from -1.5 to -32.8 pmol m-2 s-1, with an average of -7.3 ± 4.5 pmol m-2 s-1 (n = 253). However, episodes of biogenic emissions happened each year in June-July (11.9 ± 6.2 pmol m-2 s-1, n = 24). Preliminary analyses of simulated footprints of source areas influencing the recorded COS data suggest that long-range transport of COS from anthropogenic sources located in Benelux, Eastern France and Germany occasionally impacts the Paris area during wintertime. These production and removal processes may limit the use of COS to assess regional-scale CO2 uptake in Europe by plants through inverse modeling.

From Elemental Sulfur to Hydrogen Sulfide in Agricultural Soils and Plants

Sulfur is an essential element in determining the productivity and quality of agricultural products. It is also an element associated with tolerance to biotic and abiotic stress in plants. In agricultural practice, sulfur has broad use in the form of sulfate fertilizers and, to a lesser extent, as sulfite biostimulants. When used in the form of bulk elemental sulfur, or micro- or nano-sulfur, applied both to the soil and to the canopy, the element undergoes a series of changes in its oxidation state, produced by various intermediaries that apparently act as biostimulants and promoters of stress tolerance. The final result is sulfate S⁺⁶, which is the source of sulfur that all soil organisms assimilate and that plants absorb by their root cells. The changes in the oxidation states of sulfur S⁰ to S⁺⁶ depend on the action of specific groups of edaphic bacteria. In plant cells, S⁺⁶ sulfate is reduced to S⁻² and incorporated into biological molecules. S⁻² is also absorbed by stomata from H₂S, COS, and other atmospheric sources. S⁻² is the precursor of inorganic polysulfides, organic polysulfanes, and H₂S, the action of which has been described in cell signaling and biostimulation in plants. S⁻² is also the basis of essential biological molecules in signaling, metabolism, and stress tolerance, such as reactive sulfur species (RSS), SAM, glutathione, and phytochelatins. The present review describes the dynamics of sulfur in soil and plants, considering elemental sulfur as the starting point, and, as a final point, the sulfur accumulated as S⁻² in biological structures. The factors that modify the behavior of the different components of the sulfur cycle in the soil–plant–atmosphere system, and how these influences the productivity, quality, and stress tolerance of crops, are described. The internal and external factors that influence the cellular production of S⁻² and polysulfides vs. other S species are also described. The impact of elemental sulfur is compared with that of sulfates, in the context of proper soil management. The conclusion is that the use of elemental sulfur is recommended over that of sulfates, since it is beneficial for the soil microbiome, for productivity and nutritional quality of crops, and also allows the increased tolerance of plants to environmental stresses.

Soil carbonyl sulfide exchange in relation to microbial community composition: insights from a managed grassland soil amendment experiment

The viability of carbonyl sulfide (COS) measurements for partitioning ecosystem-scale net carbon dioxide (CO2) fluxes into photosynthesis and respiration critically depends on our knowledge of non-leaf sinks and sources of COS in ecosystems. We combined soil gas exchange measurements of COS and CO2 with next-generation sequencing technology (NGS) to investigate the role of soil microbiota for soil COS exchange. We applied different treatments (litter and glucose addition, enzyme inhibition and gamma sterilization) to soil samples from a temperate grassland to manipulate microbial composition and activity. While untreated soil was characterized by consistent COS uptake, other treatments reduced COS uptake and even turned the soil into a net COS source. Removing biotic processes through sterilization led to positive or zero fluxes. We used NGS to link changes in the COS response to alterations in the microbial community composition, with bacterial data having a higher explanatory power for the measured COS fluxes than fungal data. We found that the genera Arthrobacter and Streptomyces were particularly abundant in samples exhibiting high COS emissions. Our results indicate co-occurring abiotic production and biotic consumption of COS in untreated soil, the latter linked to carbonic anhydrase activity, and a strong dependency of the COS flux on the activity, identity, abundance of and substrate available to microorganisms.

Influences of light and humidity on carbonyl sulfide-based estimates of photosynthesis

Carbonyl sulfide (COS) measurements enable quantification of terrestrial photosynthesis, which cannot be directly measured at scales greater than the leaf level. The accuracy of COS-based estimates of gross primary production (GPP) depends on how we relate the COS uptake to that of CO₂. This study shows that COS-based GPP estimates will be significantly overestimated if the different environmental responses of COS and CO₂ uptake are not taken into account. These findings are relevant for studies that rely on COS to quantify ecosystem to regional scale GPP, and support the use of a COS-based approach to constrain ecosystem flux partitioning. Moreover, the strong stomatal control on COS uptake shown in this study makes COS a suitable tracer for stomatal diffusion.

Understanding climate controls on gross primary productivity (GPP) is crucial for accurate projections of the future land carbon cycle. Major uncertainties exist due to the challenge in separating GPP and respiration from observations of the carbon dioxide (CO₂) flux. Carbonyl sulfide (COS) has a dominant vegetative sink, and plant COS uptake is used to infer GPP through the leaf relative uptake (LRU) ratio of COS to CO₂ fluxes. However, little is known about variations of LRU under changing environmental conditions and in different phenological stages. We present COS and CO₂ fluxes and LRU of Scots pine branches measured in a boreal forest in Finland during the spring recovery and summer. We find that the diurnal dynamics of COS uptake is mainly controlled by stomatal conductance, but the leaf internal conductance could significantly limit the COS uptake during the daytime and early in the season. LRU varies with light due to the differential light responses of COS and CO₂ uptake, and with vapor pressure deficit (VPD) in the peak growing season, indicating a humidity-induced stomatal control. Our COS-based GPP estimates show that it is essential to incorporate the variability of LRU with environmental variables for accurate estimation of GPP on ecosystem, regional, and global scales.

Sun-induced fluorescence and gross primary productivity during a heat wave

Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.

Assessing canopy performance using carbonyl sulfide measurements

Carbonyl sulfide (COS) is a tracer of ecosystem photosynthesis that can advance carbon cycle research from leaf to global scales; however, a range of newly reported caveats related to sink/source strength of various ecosystem components hinder its application. Using comprehensive eddy-covariance and chamber measurements, we systematically measure ecosystem contributions from leaf, stem, soil, and litter and were able to close the ecosystem COS budget. The relative contributions of nonphotosynthetic components to the overall canopy-scale flux are relatively small (~4% during peak activity season) and can be independently estimated based on their responses to temperature and humidity. Converting COS to photosynthetic CO₂ fluxes based on the leaf relative uptake of COS/CO₂ , faces challenges due to observed daily and seasonal changes. Yet, this ratio converges around a constant value (~1.6), and the variations, dominated by light intensity, were found unimportant on a flux-weighted daily time-scale, indicating a mean ratio of daytime gross-to-net primary productivity of ~2 in our ecosystem. The seasonal changes in the leaf relative uptake ratio may indicate a reduction in mesophyll conductance in winter, and COS-derived canopy conductance permitted canopy temperature estimate consistent with radiative skin temperature. These results support the feasibility of using COS as a powerful and much-needed means of assessing ecosystem function and its response to change.

Isotopic Fractionation of Sulfur in Carbonyl Sulfide by Carbonyl Sulfide Hydrolase of Thiobacillus thioparus THI115

Carbonyl sulfide (COS) is one of the major sources of stratospheric sulfate aerosols, which affect the global radiation balance and ozone depletion. COS-degrading microorganisms are ubiquitous in soil and important for the global flux of COS. We examined the sulfur isotopic fractionation during the enzymatic degradation of COS by carbonyl sulfide hydrolase (COSase) from Thiobacillus thioparus THI115. The isotopic fractionation constant (34ɛ value) was -2.2±0.2‰. Under experimental conditions performed at parts per million by volume level of COS, the 34ɛ value for intact cells of T. thioparus THI115 was -3.6±0.7‰, suggesting that, based on Rees' model, the 34ɛ value mainly depended on COS transport into the cytoplasm. The 34ɛ value for intact cells of T. thioparus THI115 was similar to those for Mycobacterium spp. and Williamsia sp., which are known to involve the conserved region of nucleotide sequences encoding the clade D of β-class carbonic anhydrase (β-CA) including COSase. On the other hand, the 34ɛ value was distinct from those for bacteria in the genus Cupriavidus. These results provide an insight into biological COS degradation, which is indispensable for estimating the COS global budget based on the isotope because of the significant contribution of COS degradation by microorganisms harboring β-CA family enzymes.

Eddy covariance carbonyl sulphide flux measurements with a quantum cascade laser absorption spectrometer

The trace gas carbonyl sulphide (COS) has lately received growing interest in the eddy covariance (EC) community due to its potential to serve as an independent approach for constraining gross primary production and canopy stomatal conductance. Thanks to recent developments of fast-response high-precision trace gas analysers (e.g. quantum cascade laser absorption spectrometers (QCLAS)), a handful of EC COS flux measurements have been published since 2013. To date, however, a thorough methodological characterisation of QCLAS with regard to the requirements of the EC technique and the necessary processing steps has not been conducted. The objective of this study is to present a detailed characterization of the COS measurement with the Aerodyne QCLAS in the context of the EC technique, and to recommend best EC processing practices for those measurements. Data were collected from May to October 2015 at a temperate mountain grassland in Tyrol, Austria. Analysis of the Allan variance of high-frequency concentration measurements revealed sensor drift to occur under field conditions after an averaging time of around 50 s. We thus explored the use of two high-pass filtering approaches (linear detrending and recursive filtering) as opposed to block averaging and linear interpolation of regular background measurements for covariance computation. Experimental low-pass filtering correction factors were derived from a detailed cospectral analysis. The CO₂ and H₂O flux measurements obtained with the QCLAS were compared against those obtained with a closed-path infrared gas analyser. Overall, our results suggest small, but systematic differences between the various high-pass filtering scenarios with regard to the fraction of data retained in the quality control and flux magnitudes. When COS and CO₂ fluxes are combined in the so-called ecosystem relative uptake rate, systematic differences between the high-pass filtering scenarios largely cancel out, suggesting that this relative metric represents a robust key parameter comparable between studies relying on different post-processing schemes.

Bryophyte gas-exchange dynamics along varying hydration status reveal a significant carbonyl sulphide (COS) sink in the dark and COS source in the light

Carbonyl sulphide (COS) is a potential tracer of gross primary productivity (GPP), assuming a unidirectional COS flux into the vegetation that scales with GPP. However, carbonic anhydrase (CA), the enzyme that hydrolyses COS, is expected to be light independent, and thus plants without stomata should continue to take up COS in the dark. We measured net CO₂ (A^C ) and COS (A^S ) uptake rates from two astomatous bryophytes at different relative water contents (RWCs), COS concentrations, temperatures and light intensities. We found large A^S in the dark, indicating that CA activity continues without photosynthesis. More surprisingly, we found a nonzero COS compensation point in light and dark conditions, indicating a temperature-driven COS source with a Q₁₀ (fractional change for a 10°C temperature increase) of 3.7. This resulted in greater A^S in the dark than in the light at similar RWC. The processes underlying such COS emissions remain unknown. Our results suggest that ecosystems dominated by bryophytes might be strong atmospheric sinks of COS at night and weaker sinks or even sources of COS during daytime. Biotic COS production in bryophytes could result from symbiotic fungal and bacterial partners that could also be found on vascular plants.

Large historical growth in global terrestrial gross primary production

Growth in terrestrial gross primary production (GPP)-the amount of carbon dioxide that is 'fixed' into organic material through the photosynthesis of land plants-may provide a negative feedback for climate change. It remains uncertain, however, to what extent biogeochemical processes can suppress global GPP growth. As a consequence, modelling estimates of terrestrial carbon storage, and of feedbacks between the carbon cycle and climate, remain poorly constrained. Here we present a global, measurement-based estimate of GPP growth during the twentieth century that is based on long-term atmospheric carbonyl sulfide (COS) records, derived from ice-core, firn and ambient air samples. We interpret these records using a model that simulates changes in COS concentration according to changes in its sources and sinks-including a large sink that is related to GPP. We find that the observation-based COS record is most consistent with simulations of climate and the carbon cycle that assume large GPP growth during the twentieth century (31% ± 5% growth; mean ± 95% confidence interval). Although this COS analysis does not directly constrain models of future GPP growth, it does provide a global-scale benchmark for historical carbon-cycle simulations.

In situ soil COS exchange of a temperate mountain grassland under simulated drought

During recent years, carbonyl sulfide (COS), a trace gas with a similar diffusion pathway into leaves as carbon dioxide (CO2), but with no known "respiration-like" leaf source, has been discussed as a promising new approach for partitioning net ecosystem-scale CO2 fluxes into photosynthesis and respiration. The utility of COS for flux partitioning at the ecosystem scale critically depends on the understanding of non-leaf sources and sinks of COS. This study assessed the contribution of the soil to ecosystem-scale COS fluxes under simulated drought conditions at temperate grassland in the Central Alps. We used transparent steady-state flow-through chambers connected to a quantum cascade laser spectrometer to measure the COS and CO2 gas exchange between the soil surface and the atmosphere. Soils were a source of COS during the day, emissions being mainly driven by incoming solar radiation and to a lesser degree soil temperature. Soil water content had a negligible influence on soil COS exchange and thus the drought and control treatment were statistically not significantly different. Overall, daytime fluxes were large (12.5 ± 13.8 pmol m-2 s-1) in their magnitude and consistently positive compared to the previous studies, which predominantly used dark chambers. Nighttime measurements revealed soil COS fluxes around zero, as did measurements with darkened soil chambers during daytime reinforcing the importance of incoming solar radiation. Our results suggest that abiotic drivers play a key role in controlling in situ soil COS fluxes of the investigated grassland.

Degradation of carbonyl sulfide by Actinomycetes and detection of clade D of β-class carbonic anhydrase

Carbonyl sulfide (COS) is an atmospheric trace gas and one of the sources of stratospheric aerosol contributing to climate change. Although one of the major sinks of COS is soil, the distribution of COS degradation ability among bacteria remains unclear. Seventeen out of 20 named bacteria belonging to Actinomycetales had COS degradation activity at mole fractions of 30 parts per million by volume (ppmv) COS. Dietzia maris NBRC 15801T and Mycobacterium sp. THI405 had the activity comparable to a chemolithoautotroph Thiobacillus thioparus THI115 that degrade COS by COS hydrolase for energy production. Among 12 bacteria manifesting rapid degradation at 30 ppmv COS, D. maris NBRC 15801T and Streptomyces ambofaciens NBRC 12836T degraded ambient COS (∼500 parts per trillion by volume). Geodermatophilus obscurus NBRC 13315T and Amycolatopsis orientalis NBRC 12806T increased COS concentrations. Moreover, six of eight COS-degrading bacteria isolated from soils had partial nucleotide sequences similar to that of the gene encoding clade D of β-class carbonic anhydrase, which included COS hydrolase. These results indicate the potential importance of Actinomycetes in the role of soils as sinks of atmospheric COS.

Degradation and emission of carbonyl sulfide, an atmospheric trace gas, by fungi isolated from forest soil

Soil is thought to be important both as a source and a sink of carbonyl sulfide (COS) in the troposphere, but the mechanism affecting COS uptake, especially for fungi, remains uncertain. Fungal isolates that were collected randomly from forest soil showed COS-degrading ability at high frequencies: 38 out of 43 isolates grown on potato dextrose agar showed degradation of 30 ppmv COS within 24 h. Of these isolates, eight degraded 30 ppmv of COS to below the detection limit within 2 h. These isolates also showed an ability to degrade COS included in ambient air (around 500 pptv) and highly concentrated (12 500 ppmv) level, even though the latter is higher than the lethal level for mammals. COS-degrading activity was estimated by using ergosterol as a biomass index for fungi. Trichoderma sp. THIF08 had the highest COS-degrading activity of all the isolates. Interestingly, Umbelopsis/Mortierella spp. THIF09 and THIF13 were unable to degrade 30 ppmv COS within 24 h, and actually emitted COS during the cultivation in ambient air. These results indicate a fungal contribution to the flux of COS between the terrestrial and atmospheric environments.

DFT Study of the Reaction Mechanisms of Carbon Dioxide and its Isoelectronic Molecules CS2 and OCS Dissolved in Pyrrolidinium and Imidazolium Acetate Ionic Liquids

The reaction mechanisms of CO2 and its isoelectronic molecules OCS and CS2 dissolved in N-butyl-N-methylpyrrolidinium acetate and in 1-butyl-3-methylimidazolium acetate were investigated by DFT calculations in "gas phase". The analysis of predicted multistep pathways allowed calculating energies of reaction and energy barriers of the processes. The major role played by the acetate anion in the degradation of the solutes CS2 and OCS as well as in the capture of OCS and CO2 by the imidazolium ring is highlighted. In both ionic liquids, this anion governs the conversion of CS2 into OCS and of OCS into CO2 through interatomic S-O exchanges between the anion and the solutes with formation of thioacetate anions. In imidazolium acetate, the selective capture of CS2 and OCS by the imidazolium ring competes with the S-O exchanges. From the calculated values of the energy barriers a basicity scale of the anions is proposed. The (13)C NMR chemical shifts of the predicted adducts were calculated and agree well with the experimental observations. It is argued that the scenario issued from the calculated pathways is shown qualitatively to be independent from the functionals and basis set used, constitute a valuable tool in the understanding of chemical reactions taking place in liquid phase.

Seasonal fluxes of carbonyl sulfide in a midlatitude forest

Carbonyl sulfide (OCS), the most abundant sulfur gas in the atmosphere, has a summer minimum associated with uptake by vegetation and soils, closely correlated with CO2. We report the first direct measurements to our knowledge of the ecosystem flux of OCS throughout an annual cycle, at a mixed temperate forest. The forest took up OCS during most of the growing season with an overall uptake of 1.36 ± 0.01 mol OCS per ha (43.5 ± 0.5 g S per ha, 95% confidence intervals) for the year. Daytime fluxes accounted for 72% of total uptake. Both soils and incompletely closed stomata in the canopy contributed to nighttime fluxes. Unexpected net OCS emission occurred during the warmest weeks in summer. Many requirements necessary to use fluxes of OCS as a simple estimate of photosynthesis were not met because OCS fluxes did not have a constant relationship with photosynthesis throughout an entire day or over the entire year. However, OCS fluxes provide a direct measure of ecosystem-scale stomatal conductance and mesophyll function, without relying on measures of soil evaporation or leaf temperature, and reveal previously unseen heterogeneity of forest canopy processes. Observations of OCS flux provide powerful, independent means to test and refine land surface and carbon cycle models at the ecosystem scale.

Bidirectional exchange of biogenic volatiles with vegetation: emission sources, reactions, breakdown and deposition

Biogenic volatile organic compound (BVOC) emissions are widely modelled as inputs to atmospheric chemistry simulations. However, BVOC may interact with cellular structures and neighbouring leaves in a complex manner during volatile diffusion from the sites of release to leaf boundary layer and during turbulent transport to the atmospheric boundary layer. Furthermore, recent observations demonstrate that the BVOC emissions are bidirectional, and uptake and deposition of BVOC and their oxidation products are the rule rather than the exception. This review summarizes current knowledge of within-leaf reactions of synthesized volatiles with reactive oxygen species (ROS), uptake, deposition and storage of volatiles, and their oxidation products as driven by adsorption on leaf surface and solubilization and enzymatic detoxification inside leaves. The available evidence indicates that because of the reactions with ROS and enzymatic metabolism, the BVOC gross production rates are much larger than previously thought. The degree to which volatiles react within leaves and can be potentially taken up by vegetation depends upon compound reactivity, physicochemical characteristics, as well as upon their participation in leaf metabolism. We argue that future models should be based upon the concept of bidirectional BVOC exchange and consider modification of BVOC sink/source strengths by within-leaf metabolism and storage.

Sources and sinks of carbonyl sulfide in an agricultural field in the Southern Great Plains

Net photosynthesis is the largest single flux in the global carbon cycle, but controls over its variability are poorly understood because there is no direct way of measuring it at the ecosystem scale. We report observations of ecosystem carbonyl sulfide (COS) and CO2 fluxes that resolve key gaps in an emerging framework for using concurrent COS and CO2 measurements to quantify terrestrial gross primary productivity. At a wheat field in Oklahoma we found that in the peak growing season the flux-weighted leaf relative uptake of COS and CO2 during photosynthesis was 1.3, at the lower end of values from laboratory studies, and varied systematically with light. Due to nocturnal stomatal conductance, COS uptake by vegetation continued at night, contributing a large fraction (29%) of daily net ecosystem COS fluxes. In comparison, the contribution of soil fluxes was small (1-6%) during the peak growing season. Upland soils are usually considered sinks of COS. In contrast, the well-aerated soil at the site switched from COS uptake to emissions at a soil temperature of around 15 °C. We observed COS production from the roots of wheat and other species and COS uptake by root-free soil up to a soil temperature of around 25 °C. Our dataset demonstrates that vegetation uptake is the dominant ecosystem COS flux in the peak growing season, providing support of COS as an independent tracer of terrestrial photosynthesis. However, the observation that ecosystems may become a COS source at high temperature needs to be considered in global modeling studies.

Hydrogen sulfide: environmental factor or signalling molecule?

Hydrogen sulfide (H₂S) has traditionally been thought of as a phytotoxin, having deleterious effects on the plant growth and survival. It is now recognized that plants have enzymes which generate H₂S, cysteine desulfhydrase, and remove it, O-acetylserine lyase. Therefore, it has been suggested that H₂S is considered as a signalling molecule, alongside small reactive compounds such as hydrogen peroxide (H₂O₂) and nitric oxide (NO). Exposure of plants to low of H₂S, for example from H₂S donors, is revealing that many physiological effects are seen. H₂S seems to have effects on stomatal apertures. Intracellular effects include increases in glutathione levels, alterations of enzyme activities and influences on NO and H₂O₂ metabolism. Work in animals has shown that H₂S may have direct effects on thiol modifications of cysteine groups, work that will no doubt inform future studies in plants. It appears therefore, that instead of thinking of H₂S as a phytotoxin, it needs to be considered as a signalling molecule that interacts with reactive oxygen species and NO metabolism, as well as having direct effects on the activity of proteins. The future may see H₂S being used to modulate plant physiology in the field or to protect crops from postharvest spoilage.

Sensitivity of plants to changing atmospheric CO2 concentration: from the geological past to the next century

The rate of CO(2) assimilation by plants is directly influenced by the concentration of CO(2) in the atmosphere, c(a). As an environmental variable, c(a) also has a unique global and historic significance. Although relatively stable and uniform in the short term, global c(a) has varied substantially on the timescale of thousands to millions of years, and currently is increasing at seemingly an unprecedented rate. This may exert profound impacts on both climate and plant function. Here we utilise extensive datasets and models to develop an integrated, multi-scale assessment of the impact of changing c(a) on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling c(a) is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that tends to maintain 1 - c(i)/c(a), the relative gradient for CO(2) diffusion into the leaf, relatively constant. This is achieved through predictable adjustments to stomatal anatomy and chloroplast biochemistry. Importantly, the long-term response to changing c(a) can be described by simple equations rooted in the formulation of more commonly studied short-term responses.

Carbonyl sulfide (COS) as a tracer for canopy photosynthesis, transpiration and stomatal conductance: potential and limitations

The theoretical basis for the link between the leaf exchange of carbonyl sulfide (COS), carbon dioxide (CO(2)) and water vapour (H(2)O) and the assumptions that need to be made in order to use COS as a tracer for canopy net photosynthesis, transpiration and stomatal conductance, are reviewed. The ratios of COS to CO(2) and H(2)O deposition velocities used to this end are shown to vary with the ratio of the internal to ambient CO(2) and H(2)O mole fractions and the relative limitations by boundary layer, stomatal and internal conductance for COS. It is suggested that these deposition velocity ratios exhibit considerable variability, a finding that challenges current parameterizations, which treat these as vegetation-specific constants. COS is shown to represent a better tracer for CO(2) than H(2)O. Using COS as a tracer for stomatal conductance is hampered by our present poor understanding of the leaf internal conductance to COS. Estimating canopy level CO(2) and H(2)O fluxes requires disentangling leaf COS exchange from other ecosystem sources/sinks of COS. We conclude that future priorities for COS research should be to improve the quantitative understanding of the variability in the ratios of COS to CO(2) and H(2)O deposition velocities and the controlling factors, and to develop operational methods for disentangling ecosystem COS exchange into contributions by leaves and other sources/sinks. To this end, integrated studies, which concurrently quantify the ecosystem-scale CO(2), H(2)O and COS exchange and the corresponding component fluxes, are urgently needed.

Hydrogen sulfide effects on stomatal apertures

Hydrogen sulfide (H(2)S) has recently been reported to be a signaling molecule in plants. It has been well established that is has such roles in animals and it has been suggested that it is included into the group of gasotransmitters. We have recently shown that hydrogen sulfide causes stomatal opening in the model plant Arabidopsis thaliana. H(2)S can be supplied to the plant tissues from donors such as sodium hydrosulfide (NaSH) or more recently from slow release H(2)S donor molecules such as GYY4137. Both give similar effects, that is, they cause stomatal opening. Furthermore both H(2)S donors reduced the accumulation of nitric oxide (NO) induced by abscisic acid (ABA) treatment of leaf tissues. Here similar work has been repeated in a crop plant, Capsium anuum, and similar data has been obtained, suggesting that such effects of hydrogen sulfide on plants is not confined to model species.