Appraisal of the Role of Gaseous Signaling Molecules in Thermo-Tolerance Mechanisms in Plants

A significant threat to the ongoing rise in temperature caused by global warming. Plants have many stress-resistance mechanisms, which is responsible for maintaining plant homeostasis. Abiotic stresses largely increase gaseous molecules' synthesis in plants. The study of gaseous signaling molecules has gained attention in recent years. The role of gaseous molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and ethylene, in plants under temperature high-temperature stress are discussed in the current review. Recent studies revealed the critical function that gaseous molecules play in controlling plant growth and development and their ability to respond to various abiotic stresses. Here, we provide a thorough overview of current advancements that prevent heat stress-related plant damage via gaseous molecules. We also explored and discussed the interaction of gaseous molecules. In addition, we provided an overview of the role played by gaseous molecules in high-temperature stress responses, along with a discussion of the knowledge gaps and how this may affect the development of high-temperature-resistant plant species.

Reduced trace gas oxidizers as a response to organic carbon availability linked to oligotrophs in desert fertile islands

Atmospheric trace gases, such as H2 and CO, are important energy sources for microbial growth and maintenance in various ecosystems, especially in arid deserts with little organic substrate. Nonetheless, the impact of soil organic C availability on microbial trace gas oxidation and the underlying mechanisms are unclear at the community level. This study investigated the energy and life-history strategies of soil microbiomes along an organic C gradient inside and out of Hedysarum scoparium islands dispersed in the Mu Us Desert, China. Metagenomic analysis showed that with increasing organic C availability from bare areas into "fertile islands", the abundance of trace gas oxidizers (TGOs) decreased, but that of trace gas nonoxidizers (TGNOs) increased. The variation in their abundance was more related to labile/soluble organic C levels than to stable/insoluble organic C levels. The consumption rates of H2 and CO confirmed that organic C addition, especially soluble organic C addition, inhibited microbial trace gas oxidation. Moreover, microorganisms with distinct energy-acquiring strategies showed different life-history traits. The TGOs had lower 16 S rRNA operon copy numbers, lower predicted maximum growth rates and higher proportions of labile C degradation genes, implying the prevalence of oligotrophs. In contrast, copiotrophs were prevalent in the TGNOs. These results revealed a mechanism for the microbial community to adapt to the highly heterogeneous distribution of C resources by adjusting the abundances of taxa with distinct energy and life-history strategies, which would further affect trace gas consumption and C turnover in desert ecosystems.

Photochemical Production of Carbon Monoxide from Dissolved Organic Matter: Role of Lignin Methoxyarene Functional Groups

Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO₂, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH₃) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH₃ group, regardless of other structural features. We then confirmed that all ArOCH₃-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO₂ and that lignin photodegradation may represent a source of hydroxyl radicals.

Tree phyllospheres are a habitat for diverse populations of CO-oxidizing bacteria

Carbon monoxide (CO) is both a ubiquitous atmospheric trace gas and an air pollutant. While aerobic CO-degrading microorganisms in soils and oceans are estimated to remove ~370 Tg of CO per year, the presence of CO-degrading microorganisms in above-ground habitats, such as the phyllosphere, and their potential role in CO cycling remains unknown. CO-degradation by leaf washes of two common British trees, Ilex aquifolium and Crataegus monogyna, demonstrated CO uptake in all samples investigated. Based on the analyses of taxonomic and functional genes, diverse communities of candidate CO-oxidizing taxa were identified, including members of Rhizobiales and Burkholderiales which were abundant in the phyllosphere at the time of sampling. Based on predicted genomes of phyllosphere community members, an estimated 21% of phyllosphere bacteria contained CoxL, the large subunit of CO-dehydrogenase. In support of this, data mining of publicly available phyllosphere metagenomes for genes encoding CO-dehydrogenase subunits demonstrated that, on average, 25% of phyllosphere bacteria contained CO-dehydrogenase gene homologues. A CO-oxidizing Phyllobacteriaceae strain was also isolated from phyllosphere samples which contains genes encoding both CO-dehydrogenase as well as a ribulose-1,5-bisphosphate carboxylase-oxygenase. These results suggest that the phyllosphere supports diverse and potentially abundant CO-oxidizing bacteria, which are a potential sink for atmospheric CO.

Trace gas oxidizers are widespread and active members of soil microbial communities

Soil microorganisms globally are thought to be sustained primarily by organic carbon sources. Certain bacteria also consume inorganic energy sources such as trace gases, but they are presumed to be rare community members, except within some oligotrophic soils. Here we combined metagenomic, biogeochemical and modelling approaches to determine how soil microbial communities meet energy and carbon needs. Analysis of 40 metagenomes and 757 derived genomes indicated that over 70% of soil bacterial taxa encode enzymes to consume inorganic energy sources. Bacteria from 19 phyla encoded enzymes to use the trace gases hydrogen and carbon monoxide as supplemental electron donors for aerobic respiration. In addition, we identified a fourth phylum (Gemmatimonadota) potentially capable of aerobic methanotrophy. Consistent with the metagenomic profiling, communities within soil profiles from diverse habitats rapidly oxidized hydrogen, carbon monoxide and to a lesser extent methane below atmospheric concentrations. Thermodynamic modelling indicated that the power generated by oxidation of these three gases is sufficient to meet the maintenance needs of the bacterial cells capable of consuming them. Diverse bacteria also encode enzymes to use trace gases as electron donors to support carbon fixation. Altogether, these findings indicate that trace gas oxidation confers a major selective advantage in soil ecosystems, where availability of preferred organic substrates limits microbial growth. The observation that inorganic energy sources may sustain most soil bacteria also has broad implications for understanding atmospheric chemistry and microbial biodiversity in a changing world.

Research Progress on the Functions of Gasotransmitters in Plant Responses to Abiotic Stresses

Abiotic stress is one of the major threats affecting plant growth and production. The harm of abiotic stresses includes the disruption of cellular redox homeostasis, reactive oxygen species (ROS) production, and oxidative stress in the plant. Plants have different mechanisms to fight stress, and these mechanisms are responsible for maintaining the required homeostasis in plants. Recently, the study of gasotransmitters in plants has attracted much attention, especially for abiotic stress. In the present review, abiotic stressors were mostly found to induce gasotransmitter production in plants. Meanwhile, these gasotransmitters can enhance the activity of several antioxidant enzymes, alleviate the harmfulness of ROS, and enhance plant tolerance under various stress conditions. In addition, we introduced the interaction of gasotransmitters in plants under abiotic stress. With their promising applications in agriculture, gasotransmitters will be adopted in the near future.

Solar UV radiation in a changing world: roles of cryosphere-land-water-atmosphere interfaces in global biogeochemical cycles

Global change influences biogeochemical cycles within and between environmental compartments (i.e., the cryosphere, terrestrial and aquatic ecosystems, and the atmosphere). A major effect of global change on carbon cycling is altered exposure of natural organic matter (NOM) to solar radiation, particularly solar UV radiation. In terrestrial and aquatic ecosystems, NOM is degraded by UV and visible radiation, resulting in the emission of carbon dioxide (CO2) and carbon monoxide, as well as a range of products that can be more easily degraded by microbes (photofacilitation). On land, droughts and land-use change can reduce plant cover causing an increase in exposure of plant litter to solar radiation. The altered transport of soil organic matter from terrestrial to aquatic ecosystems also can enhance exposure of NOM to solar radiation. An increase in emission of CO2 from terrestrial and aquatic ecosystems due to the effects of global warming, such as droughts and thawing of permafrost soils, fuels a positive feedback on global warming. This is also the case for greenhouse gases other than CO2, including methane and nitrous oxide, that are emitted from terrestrial and aquatic ecosystems. These trace gases also have indirect or direct impacts on stratospheric ozone concentrations. The interactive effects of UV radiation and climate change greatly alter the fate of synthetic and biological contaminants. Contaminants are degraded or inactivated by direct and indirect photochemical reactions. The balance between direct and indirect photodegradation or photoinactivation of contaminants is likely to change with future changes in stratospheric ozone, and with changes in runoff of coloured dissolved organic matter due to climate and land-use changes.

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Detection of anaerobic carbon monoxide-oxidizing thermophiles in hydrothermal environments

Carboxydotrophic anaerobic thermophiles have been isolated from various hydrothermal environments and are considered to be important carbon monoxide (CO) scavengers or primary producers. However, the ecological factors that influence the distribution, abundance and CO-oxidizing activities of these bacteria are poorly understood. A previous study detected the carboxydotrophic bacteria Carboxydothermus spp. in a hot spring sample and found that they constituted up to 10% of the total bacterial cells. In this study, we investigated environmental features, potential microbial CO-oxidation activities and the abundance of Carboxydothermus spp. in various hot springs to determine environmental factors that affect CO oxidizers and to see whether Carboxydothermus spp. are common in these environments. We detected potential microbial CO-oxidation activities in samples that showed relatively high values of total organic carbon, total nitrogen, oxidation-reduction potential and soil-water content. The abundance of Carboxydothermus spp. did not correlate with the presence of potential microbial CO-oxidation activities; however, Carboxydothermus spp. were detected in a wide range of environments, suggesting that these bacteria are widely distributed in spite of the relatively low population size. This study implies that thermophilic CO oxidizers occur in a wide range of environments and oxidize CO in somewhat oxidative environments rich in organic matter.

Leaf surface wax is a source of plant methane formation under UV radiation and in the presence of oxygen

The terrestrial vegetation is a source of UV radiation-induced aerobic methane (CH4 ) release to the atmosphere. Hitherto pectin, a plant structural component, has been considered as the most likely precursor for this CH4 release. However, most of the leaf pectin is situated below the surface wax layer, and UV transmittance of the cuticle differs among plant species. In some species, the cuticle effectively absorbs and/or reflects UV radiation. Thus, pectin may not necessarily contribute substantially to the UV radiation-induced CH4 emission measured at surface level in all species. Here, we investigated the potential of the leaf surface wax itself as a source of UV radiation-induced leaf aerobic CH4 formation. Isolated leaf surface wax emitted CH4 at substantial rates in response to UV radiation. This discovery has implications for how the phenomenon should be scaled to global levels. In relation to this, we demonstrated that the UV radiation-induced CH4 emission is independent of leaf area index above unity. Further, we observed that the presence of O2 in the atmosphere was necessary for achieving the highest rates of CH4 emission. Methane formation from leaf surface wax is supposedly a two-step process initiated by a photolytic rearrangement reaction of the major component followed by an α-cleavage of the generated ketone.

De-etiolation of wheat seedling leaves: cross talk between heme oxygenase/carbon monoxide and nitric oxide

Greening of etiolated plants is predominantly stimulated by light but the complete molecular mechanism is still unknown. Multiple studies currently focus on the important physiological effects of heme oxygenase (HO)/carbon monoxide (CO) in plants. In this report, firstly, the role of HO/CO in light-induced de-etiolation process was investigated. We discovered that light could significantly increase HO activities, HO-1 gene expression, CO release, and chlorophyll accumulation, all of which were sensitive to zinc protoporphyrin (ZnPPIX), the potent inhibitor of HO-1, respectively. Both HO-1 inducer hematin (H) and CO aqueous solution were able to relieve etiolation in wheat seedling leaves under completely darkness by up-regulating endogenous HO/CO system, so as nitric oxide (NO) donor sodium nitroprusside (SNP) did. Similarly, endogenous NO production was also boost in response to light, SNP, hematin and CO aqueous solution in wheat seedling leaves. Additionally, the restoration of chlorophyll contents was blocked, when the inhibitor of mammalian nitric oxide synthase N(G)-nitro-L-arginine methylester hydrochloride (L-NAME) or the specific scavenger of NO 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO) was added, respectively. Furthermore, the inducible effects of light were different from those of SNP, hematin, and CO on Pfr accumulation and PHYA transcripts. However, all of sodium nitroprusside (SNP), hematin, and CO could accelerate NO emission, which suggested that HO/CO in wheat seedlings de-etiolation under dark-light transition may have a cross talk with NO.

Quantitating carbon monoxide production from heme by vascular plant preparations in vitro

Heme in animals is mainly degraded enzymatically, producing a predictable amount of carbon monoxide (CO). Under some conditions, alternative sources of CO production are important, such as lipid peroxidation and photo-oxidation. Less is known about CO production in plants as a reflection of enzymatic activity or coupled oxidation, but a sensitive assay for CO production in plants would be a valuable tool to explore the various sources in plants as the conditions of the reactions and mechanisms are defined. Using gas chromatography, we determined the requirements for heme-supported in vitro CO generation by exogenous reactants (NADPH, tissue supernatant, oxygen), optimum reaction conditions (time, temperature, pH, light), and effects of various cofactors and substrates using supernatants from Spinacia oleracea (spinach) leaf and Solanum tuberosa (potato) tuber homogenates. We then determined the CO production rate distribution between organ (root, stem, leaf, flower, fruit) supernatants in a number of commercially available plant species. CO production ranged from 4-65 nmol CO/h/g fresh weight and occurred in all vascular plant tissues examined, with the highest rates in chloroplast-containing tissues. In spinach leaves, CO production was concentrated (>2-fold) in the particulate fraction, whereas in potato tubers, the particulate fraction accounted for <50% of the rates in homogenates. We conclude that gas chromatography is uniquely suited for the determination of CO production in pigmented, heterogeneous plant tissue preparations.

Heme oxygenase/carbon monoxide system participates in regulating wheat seed germination under osmotic stress involving the nitric oxide pathway

To investigate the mechanism and signaling pathway of carbon monoxide (CO) and hematin in alleviating seed germination inhibition and lipid peroxidation, polyethylene glycol-6000 (PEG) was used to mimic osmotic stress in a series of experiments. The results showed that wheat seeds pretreated with a lower dose of PEG (12.5%) showed higher tolerance against osmotic stress as well as the up-regulation of heme oxygenase (HO, EC 1.14.99.3) and decreased lipid peroxidation during recuperation, compared to those with a higher dose of PEG (50%). Exposure of wheat seeds to 25% PEG, HO-1 inhibitor or specific scavenger of nitric oxide (NO) alone differentially led to seed germination inhibition. The PEG-induced inhibitory effects on seed germination were ameliorated by the HO-1 inducer hematin, CO or NO donor. Additionally, hematin was able to markedly boost the HO/CO system. However, the addition of the HO-1 inhibitor or the specific scavenger of NO not only reversed the protective effects conferred by hematin, but also blocked the up-regulation of HO/CO. In addition, hematin-driven NO production in wheat seeds under osmotic stress was confirmed. Based on these results, we conclude that the endogenous HO/CO signal system is required for the alleviation of osmotic stress-induced wheat seed germination inhibition and lipid peroxidation, which might have a possible interaction with NO.

Biogenic volatile organic compounds in the Earth system

Biogenic volatile organic compounds produced by plants are involved in plant growth, development, reproduction and defence. They also function as communication media within plant communities, between plants and between plants and insects. Because of the high chemical reactivity of many of these compounds, coupled with their large mass emission rates from vegetation into the atmosphere, they have significant effects on the chemical composition and physical characteristics of the atmosphere. Hence, biogenic volatile organic compounds mediate the relationship between the biosphere and the atmosphere. Alteration of this relationship by anthropogenically driven changes to the environment, including global climate change, may perturb these interactions and may lead to adverse and hard-to-predict consequences for the Earth system.

Carbon monoxide enhances salt tolerance by nitric oxide-mediated maintenance of ion homeostasis and up-regulation of antioxidant defence in wheat seedling roots

Salt stress induced an increase in endogenous carbon monoxide (CO) production and the activity of the CO synthetic enzyme haem oxygenase (HO) in wheat seedling roots. In addition, a 50% CO aqueous solution, applied daily, not only resulted in the enhancement of CO release, but led to a significant reversal in dry weight (DW) and water loss caused by 150 mm NaCl treatment, which was mimicked by the application of two nitric oxide (NO) donors sodium nitroprusside (SNP) and diethylenetriamine NO adduct (DETA/NO). Further analyses showed that CO, as well as SNP, apparently up-regulated H(+)-pump and antioxidant enzyme activities or related transcripts, thus resulting in the increase of K/Na ratio and the alleviation of oxidative damage. Whereas, the CO/NO scavenger haemoglobin (Hb), NO scavenger or synthetic inhibitor methylene blue (MB) or N(G)-nitro-l-arginine methyl ester hydrochloride (l-NAME) differentially blocked these effects. Furthermore, CO was able to mimic the effect of SNP by strongly increasing NO release in the root tips, whereas the CO-induced NO signal was quenched by the addition of l-NAME or cPTIO, the specific scavenger of NO. The results suggested that CO might confer an increased tolerance to salinity stress by maintaining ion homeostasis and enhancing antioxidant system parameters in wheat seedling roots, both of which were partially mediated by NO signal.

Carbon monoxide-induced stomatal closure involves generation of hydrogen peroxide in Vicia faba guard cells

Here the regulatory role of CO during stomatal movement in Vicia faba L. was surveyed. Results indicated that, like hydrogen peroxide (H(2)O(2)), CO donor Hematin induced stomatal closure in dose- and time-dependent manners. These responses were also proven by the addition of gaseous CO aqueous solution with different concentrations, showing the first time that CO and H(2)O(2) exhibit the similar regulation role in the stomatal movement. Moreover, our data showed that ascorbic acid (ASA, an important reducing substrate for H(2)O(2) removal) and diphenylene iodonium (DPI, an inhibitor of the H(2)O(2)-generating enzyme NADPH oxidase) not only reversed stomatal closure by CO, but also suppressed the H(2)O(2) fluorescence induced by CO, implying that CO induced-stomatal closure probably involves H(2)O(2) signal. Additionally, the CO/NO scavenger hemoglobin (Hb) and CO specific synthetic inhibitor ZnPPIX, ASA and DPI reversed the darkness-induced stomatal closure and H(2)O(2) fluorescence. These results show that, perhaps like H(2)O(2), the levels of CO in guard cells of V. faba are higher in the dark than in light, HO-1 and NADPH oxidase are the enzyme systems responsible for generating endogenous CO and H(2)O(2) in darkness respectively, and that CO is involved in darkness-induced H(2)O(2) synthesis in V. faba guard cells.

Carbon monoxide-induced stomatal closure in Vicia faba is dependent on nitric oxide synthesis

Recently, in animals, carbon monoxide (CO), like nitric oxide (NO), was implicated as another important physiological messenger or bioactive molecule. Previous researches indicate that heme oxygenase (HO)-1 (EC 1.14.99.3) catalyzes the oxidative conversion of heme to CO and biliverdin IXa (BV) with the concomitant release of iron. However, little is known about the physiological roles of CO in plant, especially in stomatal movement of guard cells. In the present paper, the regulatory role of CO during stomatal movement in Vicia faba was surveyed. Results indicated that, like sodium nitroprusside (SNP), CO donor hematin induced stomatal closure in dose- and time-dependent manners. These responses were also proved by the addition of gaseous CO aqueous solution with different concentrations, showing for the first time that CO and NO exhibit similar regulation role in the stomatal movement. Moreover, our data showed that 2,4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO)/N(G)-nitro-L-arginine-methyl ester (L-NAME) not only reversed stomatal closure by CO, but also suppressed the NO fluorescence induced by CO, implying that CO-induced stomatal closure probably involves NO/nitric oxide synthase (NOS) signal system. Additionally, the CO/NO scavenger hemoglobin (Hb) and CO-specific synthetic inhibitor zinc protoporphyrin IX (ZnPPIX), NO scavenger cPTIO and NOS inhibitor L-NAME reversed the darkness-induced stomatal closure and NO fluorescence. These results show that, maybe like NO, the levels of CO in guard cells of V. faba is higher in dark than that in light, HO-1 and NOS are the enzyme systems responsible for generating endogenous CO and NO in darkness, respectively, and that CO being from HO-1 mediates darkness-induced NO synthesis in guard cells' stomatal closure of V. faba.

Carbon monoxide from composting due to thermal oxidation of biomass

Emissions of carbon monoxide (CO) were observed from decomposing organic wastes and litter under laboratory, pilot composting plant, and natural conditions. Field studies included air from inside a compost heap of about 200 m3, emissions from composting of livestock wastes at a biologically operating farm, and leaf litter pile air samples. The concentration of CO was up to 120 micromol mol(-1) in the compost piles of green waste, and up to 10 micromol mol(-1) in flux chambers above livestock waste windrow composts. The mean CO flux rates were approximately 20 mg CO m(-2) h(-1) for compost heaps of green waste, and varied from 30 to 100 mg CO m(-2) h(-1) for fresh dung windrows. Laboratory studies using a temperature and ventilation-controlled substrate container were performed to elucidate the origin of CO, and included hay samples of fixed moisture content at temperatures between 5 and 65 degrees C, including nonsterilized as well as sterilized samples. The concentration of CO was up to 160 micromol mol(-1) in these experiments, and Arrhenius-type plot analyses resulted in activation energies of 65 kJ mol(-1) for thermochemically produced CO from the nonsterilized compost substrate. Sterilized samples showed dramatically reduced CO2 but virtually unchanged CO emissions, albeit at a slightly lower activation energy, likely a result of the high-temperature sterilization. Though globally and regionally these CO emissions are only a minor source, thermochemically produced CO emissions might affect local air quality in and near composting facilities.

Soil-atmosphere trace gas exchange in semiarid and arid zones

A review is presented on trace gas exchange of CH4, CO, N2O, and NOx arising from agriculture and natural sources in the world's semiarid and arid zones due to soil processes. These gases are important contributors to the radiative forcing and the chemistry of the atmosphere. Quantitative information is summarized from the available studies. Between 5 and 40% of the global soil-atmosphere exchange for these gases (CH4, CO, N2O, and NOx) may occur in semiarid and arid zones, but for each of these gases there are fewer than a dozen studies to support the individual estimates, and these are from a limited number of locations. Significant differences in the biophysical and chemical processes controlling these trace gas exchanges are identified through the comparison of semiarid and arid zones with the moist temperate or wet/dry savanna land regions. Therefore, there is a poorly quantified understanding of the contribution of these regions to the global trace gas cycles and atmospheric chemistry. More importantly, there is a poor understanding of the feedback between these exchanges, global change, and regional land use and air pollution issues. A set of research issues is presented.

Distribution, diversity and ecology of aerobic CO-oxidizing bacteria

Numerous studies indicate that carbon monoxide (CO) participates in a broader range of processes than any other single molecule, ranging from subcellular to planetary scales. Despite its toxicity to many organisms, a diverse group of bacteria that span multiple phylogenetic lineages metabolize CO. These bacteria are globally distributed and include pathogens, plant symbionts and biogeochemically important lineages in soils and the oceans. New molecular and isolation techniques, as well as genome sequencing, have greatly expanded our knowledge of the diversity of CO oxidizers. Here, we present a newly emerging picture of the distribution, diversity and ecology of aerobic CO-oxidizing bacteria.

Interactive effects of solar UV radiation and climate change on biogeochemical cycling

This report assesses research on the interactions of UV radiation (280-400 nm) and global climate change with global biogeochemical cycles at the Earth's surface. The effects of UV-B (280-315 nm), which are dependent on the stratospheric ozone layer, on biogeochemical cycles are often linked to concurrent exposure to UV-A radiation (315-400 nm), which is influenced by global climate change. These interactions involving UV radiation (the combination of UV-B and UV-A) are central to the prediction and evaluation of future Earth environmental conditions. There is increasing evidence that elevated UV-B radiation has significant effects on the terrestrial biosphere with implications for the cycling of carbon, nitrogen and other elements. The cycling of carbon and inorganic nutrients such as nitrogen can be affected by UV-B-mediated changes in communities of soil organisms, probably due to the effects of UV-B radiation on plant root exudation and/or the chemistry of dead plant material falling to the soil. In arid environments direct photodegradation can play a major role in the decay of plant litter, and UV-B radiation is responsible for a significant part of this photodegradation. UV-B radiation strongly influences aquatic carbon, nitrogen, sulfur and metals cycling that affect a wide range of life processes. UV-B radiation changes the biological availability of dissolved organic matter to microorganisms, and accelerates its transformation into dissolved inorganic carbon and nitrogen, including carbon dioxide and ammonium. The coloured part of dissolved organic matter (CDOM) controls the penetration of UV radiation into water bodies, but CDOM is also photodegraded by solar UV radiation. Changes in CDOM influence the penetration of UV radiation into water bodies with major consequences for aquatic biogeochemical processes. Changes in aquatic primary productivity and decomposition due to climate-related changes in circulation and nutrient supply occur concurrently with exposure to increased UV-B radiation, and have synergistic effects on the penetration of light into aquatic ecosystems. Future changes in climate will enhance stratification of lakes and the ocean, which will intensify photodegradation of CDOM by UV radiation. The resultant increase in the transparency of water bodies may increase UV-B effects on aquatic biogeochemistry in the surface layer. Changing solar UV radiation and climate also interact to influence exchanges of trace gases, such as halocarbons (e.g., methyl bromide) which influence ozone depletion, and sulfur gases (e.g., dimethylsulfide) that oxidize to produce sulfate aerosols that cool the marine atmosphere. UV radiation affects the biological availability of iron, copper and other trace metals in aquatic environments thus potentially affecting metal toxicity and the growth of phytoplankton and other microorganisms that are involved in carbon and nitrogen cycling. Future changes in ecosystem distribution due to alterations in the physical and chemical climate interact with ozone-modulated changes in UV-B radiation. These interactions between the effects of climate change and UV-B radiation on biogeochemical cycles in terrestrial and aquatic systems may partially offset the beneficial effects of an ozone recovery.

Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation

The carbon balance in terrestrial ecosystems is determined by the difference between inputs from primary production and the return of carbon to the atmosphere through decomposition of organic matter. Our understanding of the factors that control carbon turnover in water-limited ecosystems is limited, however, as studies of litter decomposition have shown contradictory results and only a modest correlation with precipitation. Here we evaluate the influence of solar radiation, soil biotic activity and soil resource availability on litter decomposition in the semi-arid Patagonian steppe using the results of manipulative experiments carried out under ambient conditions of rainfall and temperature. We show that intercepted solar radiation was the only factor that had a significant effect on the decomposition of organic matter, with attenuation of ultraviolet-B and total radiation causing a 33 and 60 per cent reduction in decomposition, respectively. We conclude that photodegradation is a dominant control on above-ground litter decomposition in this semi-arid ecosystem. Losses through photochemical mineralization may represent a short-circuit in the carbon cycle, with a substantial fraction of carbon fixed in plant biomass being lost directly to the atmosphere without cycling through soil organic matter pools. Furthermore, future changes in radiation interception due to decreased cloudiness, increased stratospheric ozone depletion, or reduced vegetative cover may have a more significant effect on the carbon balance in these water-limited ecosystems than changes in temperature or precipitation.

The contribution of reactive carbon emissions from vegetation to the carbon balance of terrestrial ecosystems

About 2.4 Pg (1 Pg = 10(15) g) of carbon is emitted annually into the atmosphere as reactive compounds and most of it is eventually oxidized to CO2. Isoprene, alpha-pinene, methanol, carbon monoxide and other compounds emitted by terrestrial vegetation contribute about half of the total flux and are estimated to produce about 1 Pg C as CO2 per year. The global average for vegetated surfaces is about 7 g C m(-2) per year but could exceed 100 g m(-2) per year at some tropical locations. The magnitude of these fluxes on both the landscape and global scales are small relative to the total carbon emission or deposition but are significant relative to the net fluxes. Reactive carbon fluxes are very sensitive to landcover and climate change and may vary significantly due to future perturbations. This paper summarizes what is known about reactive carbon emissions from vegetation including the magnitude of local, landscape, and global scale fluxes and their contribution to atmospheric CO2. Reasons for including this term in carbon flux models are presented as well as the potential importance on various spatial scales. Past, present and future reactive carbon emissions are expected to differ significantly and the implications of this are discussed.

Carbon monoxide oxidation by bacteria associated with the roots of freshwater macrophytes

The potential rates and control of aerobic root-associated carbon monoxide (CO) consumption were assessed by using excised plant roots from five common freshwater macrophytes. Kinetic analyses indicated that the maximum potential uptake velocities for CO consumption ranged from 0.4 to 2.7 &mgr;mol of CO g (dry weight)-1 h-1 for the five species. The observed rates were comparable to previously reported rates of root-associated methane uptake. The apparent half-saturation constants for CO consumption ranged from 50 to 370 nM CO; these values are considerably lower than the values obtained for methane uptake. The CO consumption rates reached maximum values at temperatures between 27 and 32 degreesC, and there was a transition to CO production at >/=44 degreesC, most likely as a result of thermochemical organic matter decomposition. Incubation of roots with organic substrates (e.g., 5 mM syringic acid, glucose, alanine, and acetate) dramatically reduced the rate of CO consumption, perhaps reflecting a shift in metabolism by facultative CO oxidizers. Based on responses to a suite of antibiotics, most of the CO consumption (about 90%) was due to eubacteria rather than fungi or other eucaryotes. Based on the results of acetylene inhibition experiments, methanotrophs and ammonia oxidizers were not active CO consumers.