Stratospheric ozone depletion

Solar ultraviolet radiation creates an ozone layer in the atmosphere which in turn completely absorbs the most energetic fraction of this radiation. This process both warms the air, creating the stratosphere between 15 and 50 km altitude, and protects the biological activities at the Earth's surface from this damaging radiation. In the last half-century, the chemical mechanisms operating within the ozone layer have been shown to include very efficient catalytic chain reactions involving the chemical species HO, HO2, NO, NO2, Cl and ClO. The NOX and ClOX chains involve the emission at Earth's surface of stable molecules in very low concentration (N2O, CCl2F2, CCl3F, etc.) which wander in the atmosphere for as long as a century before absorbing ultraviolet radiation and decomposing to create NO and Cl in the middle of the stratospheric ozone layer. The growing emissions of synthetic chlorofluorocarbon molecules cause a significant diminution in the ozone content of the stratosphere, with the result that more solar ultraviolet-B radiation (290-320 nm wavelength) reaches the surface. This ozone loss occurs in the temperate zone latitudes in all seasons, and especially drastically since the early 1980s in the south polar springtime-the 'Antarctic ozone hole'. The chemical reactions causing this ozone depletion are primarily based on atomic Cl and ClO, the product of its reaction with ozone. The further manufacture of chlorofluorocarbons has been banned by the 1992 revisions of the 1987 Montreal Protocol of the United Nations. Atmospheric measurements have confirmed that the Protocol has been very successful in reducing further emissions of these molecules. Recovery of the stratosphere to the ozone conditions of the 1950s will occur slowly over the rest of the twenty-first century because of the long lifetime of the precursor molecules.     

Effects of helium group gases and nitrous oxide on HeLa cells

The helium group gases and nitrous oxide at superatomospheric pressures depress multiplication of HeLa cells in monolayer cultures. The effectiveness of these gases in eliciting the pressure-dependent response follows the order N₂O, Xe > Kr > Ar > > Ne and He. The response correlates with lipid solubility of the gases. Depression of growth by 4.2 atm Xe is reversible after exposure for one and two days. Cultures exposed to 7.2 atm Xe show irreversible damage including cytoplasmic vacuolization. Cell attachment is strongly inhibited by Xe; 36% of the cell inoculum were not attached after 24 hours. Affinity for hydrophobic sites in the cell is suggested as determining the order of effectiveness of the gases in evoking the response.     

Changes in tropospheric composition and air quality due to stratospheric ozone depletion and climate change.

It is well-understood that reductions in air quality play a significant role in both environmental and human health. Interactions between ozone depletion and global climate change will significantly alter atmospheric chemistry which, in turn, will cause changes in concentrations of natural and human-made gases and aerosols. Models predict that tropospheric ozone near the surface will increase globally by up to 10 to 30 ppbv (33 to 100% increase) during the period 2000 to 2100. With the increase in the amount of the stratospheric ozone, increased transport from the stratosphere to the troposphere will result in different responses in polluted and unpolluted areas. In contrast, global changes in tropospheric hydroxyl radical (OH) are not predicted to be large, except where influenced by the presence of oxidizable organic matter, such as from large-scale forest fires. Recent measurements in a relatively clean location over 5 years showed that OH concentrations can be predicted by the intensity of solar ultraviolet radiation. If this relationship is confirmed by further observations, this approach could be used to simplify assessments of air quality. Analysis of surface-level ozone observations in Antarctica suggests that there has been a significant change in the chemistry of the boundary layer of the atmosphere in this region as a result of stratospheric ozone depletion. The oxidation potential of the Antarctic boundary layer is estimated to be greater now than before the development of the ozone hole. Recent modeling studies have suggested that iodine and iodine-containing substances from natural sources, such as the ocean, may increase stratospheric ozone depletion significantly in polar regions during spring. Given the uncertainty of the fate of iodine in the stratosphere, the results may also be relevant for stratospheric ozone depletion and measurements of the influence of these substances on ozone depletion should be considered in the future. In agreement with known usage and atmospheric loss processes, tropospheric concentrations of HFC-134a, the main human-made source of trifluoroacetic acid (TFA), is increasing rapidly. As HFC-134a is a potent greenhouse gas, this increasing concentration has implications for climate change. However, the risks to humans and the environment from substances, such as TFA, produced by atmospheric degradation of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are considered minimal. Perfluoropolyethers, commonly used as industrial heat transfer fluids and proposed as chlorohydrofluorocarbon (CHFC) substitutes, show great stability to chemical degradation in the atmosphere. These substances have been suggested as substitutes for CHFCs but, as they are very persistent in the atmosphere, they may be important contributors to global warming. It is not known whether these substances will contribute significantly to global warming and its interaction with ozone depletion but they should be considered for further evaluation.     

Nitrite reduction to nitrous oxide by propionibacteria: Detoxication mechanism

Characteristics of dissimilatory nitrate reduction by Propionibacterium acidi-propionici, P. freudenreichii, P. jensenii, P. shermanii and P. thoenii were studied. All strains reduced nitrate to nitrite and further to N₂O. Recovery of added nitrite-N as N₂O-N approached 100%, so that no other end product existed in a significant quantity. Specific rates of N₂O production were 3 to 6 orders of magnitude lower than specific rates of N₂ production by common denitrifiers. Oxygen but not acetylene inhibited N₂O production in P. acidi-propionici and P. thoenii. Nitrite reduction rates were generally higher than nitrate reduction rates. The enzymes involved in nitrate and nitrite reduction were either constitutive or derepressed by anacrobiosis. Nitrate stimulated synthesis of nitrate reductase in P. acidi-propionici. Specific growth rates and growth yields were increased by nitrate. At 10 mM, nitrite was toxic to all strains, and at 1 mM its effect ranged from none to total inhibition. No distinction was obvious between incomplete forms of denitrification and dissimilatory nitrate reduction to ammonia. N₂O production from nitrite by propionibacteria may represent a detoxication mechanism rather than a part of an energy transformation system.     

Nitric oxide triggers the toxicity due to glutathione depletion in midbrain cultures through 12-lipoxygenase

Glutathione (GSH) depletion is the earliest biochemical alteration shown to date in brains of Parkinson's disease patients. However, data from animal models show that GSH depletion by itself is not sufficient to induce nigral degeneration. We have previously shown that non-toxic inhibition of GSH synthesis with l-buthionine-(S,R)-sulfoximine in primary midbrain cultures transforms a nitric oxide (NO) neurotrophic effect, selective for dopamine neurons, into a toxic effect with participation of guanylate cyclase (GC) and cGMP-dependent protein kinase (PKG) (Canals, S., Casarejos, M. J., de Bernardo, S., Rodríguez-Martín, E., and Mena, M. A. (2001) J. Neurochem. 79, 1183-1195). Here we demonstrate that arachidonic acid (AA) metabolism through the 12-lipoxygenase (12-LOX) pathway is also central for this GSH-NO interaction. LOX inhibitors (nordihydroguaiaretic acid and baicalein), but not cyclooxygenase (indomethacin) or epoxygenase (clotrimazole) ones, prevent cell death in the culture, even when added 10 h after NO treatment. Furthermore, the addition of AA to GSH-depleted cultures precipitates a cell death process that is indistinguishable from that initiated by NO in its morphology, time course, and 12-LOX, GC, and PKG dependence. The first AA metabolite through the 12-LOX enzyme, 12-hydroperoxyeicosatetraenoic acid, induces cell death in the culture, and its toxicity is greatly enhanced by GSH depletion. In addition we show that if GSH synthesis inhibition persists for up to 4 days without any additional treatment, it will induce a cell death process that also depends on 12-LOX, GC, and PKG activation. In this study, therefore, we show that the signaling pathway AA/12-LOX/12-HPETE/GC/PKG may be important in several pathologies in which GSH decrease has been documented, such as Parkinson's disease. The potentiating effect of NO over such a signaling pathway may be of relevance as part of the cascade of events leading to and sustaining nerve cell death.     

Plant reaction to elevated ultraviolet irradiation: potential impacts of stratospheric ozone depletion

The ozone layer depletion evokes the increase of solar UV-B radiation intensity and corresponding reductions of growth (height, leaf area, fresh and dry weight), photosynthetic activity and flowering in higher plants. Competitive interactions also may be altered indirectly by differential growth responses. The UV-B-sensitivity of plants varies both among species and among cultivars of a given species. Photosynthetic activity may be reduced by direct effects on the photosynthetic process or metabolic pathways, or indirectly through effects on photosynthetic pigments or stomatal function. Plants may also respond by accumulating UV-absorbing compounds in their outer tissue layers, which presumably protect sensitive target from UV-damage. The key enzymes in the biosynthetic pathways of these compounds are specifically induced by UV-B irradiation via gene activation. The effects of UV-B radiation on plants can be modified by prevailing microclimatic conditions. Plants tend to be less sensitive to UV-B under drought or mineral deficiency, while sensitivity increases under low levels of visible light. Prognoses of agricultural yield reduction and economic loss for different scenarious of stratospheric ozone depletion are presented.     

Hormonal responses to analgesic nitrous oxide in man

Analgesic concentrations of nitrous oxide were administered to 6 healthy male subjects, and blood samples were assayed for prolactin, ACTH, follicle stimulating hormone, luteinising hormone, growth hormone, cortisol and thyroid hormones. Analgesic nitrous oxide (mean concentration = 48.8%) produced statistically significant elevation of prolactin and depression of cortisol whilst not producing statistically significant changes in the other hormones assayed. The increase in prolactin and decrease in cortisol levels are similar to the hormonal changes associated with administration of opioids in man. We have also confirmed the findings of other workers that cortisol levels may not always be correlated with ACTH levels.     

Electron transport pathways to nitrous oxide in Rhodobacter species

1. Electron transport components involved in nitrous oxide reduction in several strains of Rhodobacter capsulatus and in the denitrifying strain of Rhodobacter sphaeroides (f. sp. denitrificans) have been investigated. Detailed titrations with antimycin A and myxothiazol, inhibitors of the cytochrome bc1 complex, show that part of the electron flow to nitrous oxide passes through this complex. The sensitivity to myxothiazol varies between strains and growth conditions of R. capsulatus; the higher rates of nitrous oxide reduction correlate with the higher sensitivities. Partial inhibition of the nitrous oxide reductase enzyme with azide decreased the sensitivity to myxothiazol of the strains that had the highest nitrous oxide reductase activity. 2. Inhibition of nitrous oxide reduction in cells of R. capsulatus by myxothiazol could be restored under dark conditions by addition of N,N,N',N'-tetramethyl-p-phenylene diamine. The highest activities observed after addition of this electron carrier were found in the strains that had the highest sensitivity to myxothiazol, consistent with the premise that this inhibitor is more effective at the higher flux rates to nitrous oxide. 3. Addition of nitrous oxide to cells of R. capsulatus strain N22DNAR+ under darkness caused oxidation of both b- and c-type cytochromes. The oxidation of b cytochromes was less pronounced in the presence of myxothiazol, consistent with a role for the cytochrome bc1 complex in the electron pathway to nitrous oxide. Ferricyanide, in the absence of myxothiazol, caused a similar extent of oxidation of b cytochromes, but a greater oxidation of c-type, suggesting that there was a pool of c-type cytochrome that was not oxidisable by nitrous oxide. The time course showed that both the b- and c-type cytochromes were oxidised within a few seconds of the addition of nitrous oxide. During the following seconds there was a partial re-reduction of the cytochromes such that after approximately 1 min a lower steady-state of oxidation was attained and this persisted until the nitrous oxide was exhausted. 4. A mutant, MTCBC1, of R. capsulatus that specifically lacked a functional cytochrome bc1 complex reduced nitrous oxide, albeit at 30% of the rate shown by the parent strain MT1131. A reduced minus nitrous-oxide-oxidised difference spectrum for MTCBC1 in the absence of myxothiazol was similar to the corresponding difference spectrum observed for strain N22DNAR+ in the presence of myxothiazol. It is suggested that these difference spectra identify the cytochrome components, including a b-type, involved in a pathway that is alternative to, and independent of, the cytochrome bc1 complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Global oceanic production of nitrous oxide

We use transient time distributions calculated from tracer data together with in situ measurements of nitrous oxide (N(2)O) to estimate the concentration of biologically produced N(2)O and N(2)O production rates in the ocean on a global scale. Our approach to estimate the N(2)O production rates integrates the effects of potentially varying production and decomposition mechanisms along the transport path of a water mass. We estimate that the oceanic N(2)O production is dominated by nitrification with a contribution of only approximately 7 per cent by denitrification. This indicates that previously used approaches have overestimated the contribution by denitrification. Shelf areas may account for only a negligible fraction of the global production; however, estuarine sources and coastal upwelling of N(2)O are not taken into account in our study. The largest amount of subsurface N(2)O is produced in the upper 500 m of the water column. The estimated global annual subsurface N(2)O production ranges from 3.1 ± 0.9 to 3.4 ± 0.9 Tg N yr(-1). This is in agreement with estimates of the global N(2)O emissions to the atmosphere and indicates that a N(2)O source in the mixed layer is unlikely. The potential future development of the oceanic N(2)O source in view of the ongoing changes of the ocean environment (deoxygenation, warming, eutrophication and acidification) is discussed.     

The unaccounted yet abundant nitrous oxide-reducing microbial community: a potential nitrous oxide sink

Nitrous oxide (N₂O) is a major radiative forcing and stratospheric ozone-depleting gas emitted from terrestrial and aquatic ecosystems. It can be transformed to nitrogen gas (N₂) by bacteria and archaea harboring the N₂O reductase (N₂OR), which is the only known N₂O sink in the biosphere. Despite its crucial role in mitigating N₂O emissions, knowledge of the N₂OR in the environment remains limited. Here, we report a comprehensive phylogenetic analysis of the nosZ gene coding the N₂OR in genomes retrieved from public databases. The resulting phylogeny revealed two distinct clades of nosZ, with one unaccounted for in studies investigating N₂O-reducing communities. Examination of N₂OR structural elements not considered in the phylogeny revealed that the two clades differ in their signal peptides, indicating differences in the translocation pathway of the N₂OR across the membrane. Sequencing of environmental clones of the previously undetected nosZ lineage in various environments showed that it is widespread and diverse. Using quantitative PCR, we demonstrate that this clade was most often at least as abundant as the other, thereby more than doubling the known extent of the overall N₂O-reducing community in the environment. Furthermore, we observed that the relative abundance of nosZ from either clade varied among habitat types and environmental conditions. Our results indicate a physiological dichotomy in the diversity of N₂O-reducing microorganisms, which might be of importance for understanding the relationship between the diversity of N₂O-reducing microorganisms and N₂O reduction in different ecosystems.     

Exposure of midwives to nitrous oxide in four hospitals.

The exposure of midwives to nitrous oxide in four hospitals was measured with personal samplers. In three of the four hospitals the average exposure was not significantly less than 100 parts per million (ppm). In one hospital the average exposure was 360 ppm; this was reduced by a factor of about 2.5 when a trial scavenging system was used. Differences in working practices and in the layout, size, and ventilation of the labour suites contributed to the observed differences in average exposure. Midwives and other staff working in the labour room are potentially at risk from excessive occupational exposure to nitrous oxide.     

The reduction of nitrous oxide to dinitrogen by Escherichia coli

Escherichia coli K12 reduces nitrous oxide stoichiometrically to molecular nitrogen with rates of 1.9 mol/h x mg protein. The activity is induced by anaerobiosis and nitrate. N₂+formation from N₂O is inhibited by C₂H₂ (K _i 0.03 mM in the medium) and nitrite (K _i=0.3 mM) but not by azide. A mutant defective in FNR synthesis is unable to reduce N₂O to N₂. The reaction in the wild type could routinely be followed by gas chromatography and alternatively by mass spectrometry measuring the formation of ¹⁵N₂ from ¹⁵N₂O. The enzyme catalyzing N₂O-reduction in E. coli could not be identified; it is probably neither nitrate reductase nor nitrogenase. E. coli does not grow with N₂O as sole respiratory electron acceptor. N₂O-reduction might not have a physiological role in E. coli, and the enzyme involved might catalyze something else in nature, as it has a low affinity for the substrate N₂O (apparent K _m3.0 mM). The capability for N₂O-reduction to N₂ is not restricted to E. coli but is also demonstrable in Yersinia kristensenii and Buttiauxella agrestis of the Enterobacteriaceae. E. coli is able to produce NO and N₂O from nitrite by nitrate reductase, depending on the assay conditions. In such experiments NO _inf2 ^sup- is not reduced to N₂ because of the high demand for N₂O of N₂O-reduction and the inhibitory effect of NO _inf2 ^sup- on this reaction.Dedicated to Professor L. Jaenicke, Köln, on the occassion of his 70th birthday