Absence Of O2 Research Articles

Microscale effects of oxygen and light on bacterial sulfate reduction in organic-rich lacustrine sediments

Biogeochemical cycling involves the exchange of electrons between chemical species through redox reactions. Microorganisms can utilize energy released by redox reactions for their metabolism following a thermodynamic sequence. Among these reactions, the dissimilatory reduction of sulfate (SO42−) to produce hydrogen sulfide (H2S) is one of the most important microbially-mediated pathways. The use of microelectrodes at the water-sediment interface provided a better understanding of sulfate reduction processes in the suboxic and anoxic zones. The goal of this study was to assess the microzonation of H2S and O2 under different conditions of light and oxygen in the water column. For this purpose, organic-rich lacustrine sediments from Pétrola Lake (SE Spain) were used. Sediment incubations were performed in mesocosm devices. The highest production of H2S (up to 0.36 nmol/cm-3s-1) was observed under anoxic and dark conditions. Production under those conditions was several orders of magnitude higher than those measured when oxygen was present in the water column. Furthermore, the absence of O2 in the water column significantly altered the microzonation of H2S in depth. The absence of light seems not to affect the dynamics of O2 and H2S in depth. The study contributes to our understanding of microzonation in organic-rich sediments.

O2 reduction on a Au film electrode in an ionic liquid in the absence and presence of Mg2+ ions: Product formation and adlayer dynamics.

Aiming at a detailed understanding of the interaction between an ionic liquid, O2, and electrodes in Mg-air batteries, we performed a combined differential electrochemical mass spectrometry and in situ infrared spectroscopy model study on the interaction between the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) and a gold film electrode in the presence and absence of O2 and Mg2+ ions in the potential range relevant for the oxygen reduction reaction (ORR) and evolution reaction. Detailed information on the dynamic exchange of adsorbed ions, on the stability/decomposition of the ionic liquid, and on the activity/selectivity/reversibility of the ORR is derived from measurements performed under potentiodynamic and potentiostatic conditions. In neat BMP-TFSI, we find the dynamics of the potential induced exchange of adsorbed ions to depend significantly on the exchange direction. In the presence of O2, the anions formed in the ORR distinctly affect the adsorption characteristics of the IL ions and the exchange dynamics. Furthermore, the ORR changes from reduction to superoxide anions at moderate potentials to reduction to peroxide anion at more negative potentials. In the additional presence of Mg2+ ions, dominant magnesium peroxide and oxide formation result in an irreversible ORR, in contrast to the requirements of an efficient re-chargeable Mg-air battery. In addition, these ions result in the increasing formation of a blocking adlayer, reducing the coverage of adsorbed IL species.

Photocatalytic Selectivities of Ethane, Methane and Dimethylether Controlled by Reaction Conditions and TiO2 Structure in the Degradation of Acetic Acid

AbstractThe impact of photon flux, humidity and residence time using TiO2 P25 and of the surface area of a series of CristalActiv PC TiO2 anatase, on the selectivities of products formed in the continuous photocatalytic degradation of gaseous acetic acid (AA) on the scale of a few hundred ppm, in the presence or absence of O2, was investigated. Under a continuous flow of air HCHO and C2H6 were the most important products detected. CH3OH was also formed but to a lesser extent. Under a continuous flow of N2, the formation of these three products decreases whereas CH3OCH3, an interesting product which can be used as alternative fuel and CH4, is yielded. The impact of the experimental conditions on product formation, taking into account the intermediate formation of H2O2 and the favorable impact of the surface area on the formation of CH4 in the absence of oxygen and its opposite effect on the formation of C2H6 in the presence of air, was discussed.The presence of rutile phase in TiO2 P25 was also highlighted.

Hierarchical Ag-SiO2@Fe3O4 magnetic composites for elemental mercury removal from non-ferrous metal smelting flue gas

Hierarchical Ag-SiO2@Fe3O4 magnetic composites were selected for elemental mercury (Hg0) removal from non-ferrous metal smelting flue gas in this study. Results showed that the hierarchical Ag-SiO2@Fe3O4 magnetic composites had favorable Hg0 removal ability at low temperature. Moreover, the adsorption capacity of hierarchical magnetic composite is much larger than that of pure Fe3O4 and SiO2@Fe3O4. The Hg0 removal efficiency reached the highest value as approximately 92% under the reaction temperature of 150°C, while the removal efficiency sharply reduced in the absence of O2. The characterization results indicated that Ag nanoparticles grew on the surface of SiO2@Fe3O4 support. The large surface area of SiO2 supplied efficient reaction room for Hg and Ag atoms. Ag–Hg amalgam is generated on the surface of the composites. In addition, this magnetic material could be easily separated from fly ashes when adopted for treating real flue gas, and the spent materials could be regenerated using a simple thermal-desorption method.

Experimental study of NO and NO2 adsorption on a fresh or dried NaY zeolite: influence of the gas composition by breakthrough curves measurements

Adsorption of NO, NO2 and NO + NO2 was studied with and without O2 in the gas feed on NaY zeolite (Si/Al = 2.55) at 30 °C and 101.3 kPa. The influence of residual water in NaY was also studied in presence of O2 in the gaseous flux. During adsorption of NO, NO2 is produced in presence of O2 in the gas feed, due to the formation of oxygen species at the surface of the zeolite. Then, created NO2 reacts with residual H2O to form NO. Without O2, the production of NO2 does not occur. If residual H2O is removed, reaction of NO2 with H2O leading to the formation of NO does not happen. Whether with or without O2 in the gas feed during NO2 adsorption, the evolution of NO and NO2 outlet concentrations are similar. First, NO2 is adsorbed, producing NO by reacting with residual H2O and then, the production of NO decreases while NO2 outlet concentration increases. NO2 breakthrough time is slightly higher without O2. In absence of residual water, no NO is produced. Concerning adsorption of NO and NO2 mixture, the evolution of outlet concentrations is different in presence or in absence of O2. With O2, final outlet concentrations of NO and NO2 are not equal to inlet concentrations unlike what happens without O2. With a dried NaY sample, NO2 is adsorbed without forming NO. For all experiments performed, NO is not adsorbed whereas NaY is interesting for NO2 adsorption, even in gaseous mixture adsorption.

Diversifying Anaerobic Respiration Strategies to Compete in the Rhizosphere

The rhizosphere is the interface between plant roots and soil where intense, varied interactions between plants and microbes influence plants’ health and growth through their influence on biochemical cycles, such as the carbon, nitrogen and iron cycles. The rhizosphere is also a changing environment where oxygen can be rapidly limited and anaerobic zones can be established. Microorganisms successfully colonize the rhizosphere when they possess specific traits referred to as rhizosphere competence. Anaerobic respiration flexibility contributes to the rhizosphere competence of microbes. Indeed, a wide range of compounds that are available in the rhizosphere can serve as alternative terminal electron acceptors during anaerobic respiration such as nitrates, iron, carbon compounds, sulfur, metalloids and radionuclides. In the presence of multiple terminal electron acceptors in a complex environment such as the rhizosphere and in the absence of O2, microorganisms will first use the most energetic option to sustain growth. Anaerobic respiration has been deeply studied, and the genes involved in anaerobic respiration have been identified. However, aqueous environment and paddy soils are the most studied environments for anaerobic respiration, even if we provide evidence in this review that anaerobic respiration also occurs in the plant rhizosphere. Indeed, we provide evidence by performing a BLAST analysis on metatranscriptomic data that genes involved in iron, sulfur, arsenate and selenate anaerobic respiration are expressed in the rhizosphere, underscoring that the rhizosphere environment is suitable for the establishment of anaerobic respiration. We thus focus this review on current research concerning the different types of anaerobic respiration that occur in the rhizosphere. We also discuss the flexibility of anaerobic respiration as a fundamental trait for the microbial colonization of roots, environmental and ecological adaptation, persistence and bioremediation in the rhizosphere. Anaerobic respiration appears to be a key process for the functioning of an ecosystem and interactions between plants and microbes.

First Assessments of the Influence of Oxygen Reduction on the Glycerol Electrooxidation Reaction on Pt

The electroactivity of new nanomaterials candidates to be used as anodes for glycerol fuel cells or electrolysers has been largely investigated, but most of the literature reports the use of O2-free solutions. However, the purging process to extract O2 from glycerol solution and the storage at similar condition is costly. Moreover, the lack of knowledge on the glycerol electrooxidation reaction (GEOR) in the presence of O2 prevents alternative applications, as that in single channel microfluidic fuel cells, when the anode must be selective for the alcohol oxidation in the presence of O2. Herein, we accessed the influence of the oxygen reduction reaction on the GEOR by using a flow system with an electrode in wall-jet configuration. The GEOR is investigated on Pt/C nanoparticles (NPs) in the presence of different amounts of O2 in solution as a proof of concept. We also used synthesized Pt@Au to apply the method for a new material candidate to be used as catalyst in glycerol technologies. The presence of oxygen in solution barely influences GEOR on Pt/C or Pt@Au whenever classic stationary measurements are used. We found comparable electrocatalytic parameters and the same carbonyl products, but at slightly different proportions, in the presence or absence of O2 after long-time electrolysis followed by chromatographic analysis. On the other hand, oxygen competes with glycerol by the active sites when the reactants are convectively forced towards the electrode surface, at flow configuration. The competition between GEOR and oxygen reduction results in a net cathodic current density when using O2-saturated glycerol solution at potentials suitable for the reduction reaction on Pt/C. Pt@Au shows remarkable activity for the oxygen reduction, producing high cathodic currents even in air-saturated glycerol solution. Moreover, the presence of O2 greatly decreases the stability of Pt@Au during GEOR, mainly due to the improved oxygen reduction and extended surface accessed by O2, while same material controversially displayed good stability in N2-saturated solution. The use of flow configuration shed a light on the influence of O2 on GEOR, providing new information non-accessible by other classic stationary methods.

62 - Generation of superoxide radical by human cytochrome b5 reductase is modulated by cytochrome b5

Cytochrome b5 reductase (CYB5R) plays a central role in many diverse metabolic reactions by coupling NAD(P)H oxidation to reduction of proteins, such as cytochrome b5 (CYB5). Together, CYB5R and CYB5 transfer electrons to terminal oxidases, such as the mitochondrial amidoxime reducing component (mARC) or hemoglobin. Previous studies have demonstrated that nitric oxide (NO) production by the mARC/CYB5/CYB5R system is quenched by molecular oxygen (O2) and suggested that superoxide radical production was involved, as the addition of Cu,Zn-superoxide(SOD1) dismutase partially recovered NO production. Flavin proteins are known to generate superoxide, suggesting that the CYB5R was the likely site of superoxide radical production. However, the direct formation of superoxide by human CYB5R had not been studied. Here, studies were performed to understand the related mechanisms of radical generation by CYB5R in the absence and presence of CYB5. Electron paramagnetic resonance (EPR) spin trapping studies using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were performed with CYB5R and NADH. An EPR spectrum corresponding to the superoxide radical adduct of DMPO (DMPO-OOH) was obtained. This EPR signal was quenched by the addition of the SOD1. The amount of DMPO-OOH formed from the oxidation of NADH by the CYB5R increased with NADH concentration. Importantly, the formation of DMPO-OOH was abrogated in the presence of CYB5. Spectral kinetics demonstrate that NADH consumption is O2-dependent, as no NADH consumption was observed in the absence of O2. From these results, we propose a mechanism in which NADH-reduced CYB5R catalyzes superoxide radical formation. Our results demonstrate that human CYB5R catalyzes the O2-dependent oxidation of NADH to generate superoxide radical. Additionally, CYB5 may protect biological targets from oxidative damage, by inhibiting CYB5R superoxide radical production. The ability of CYB5R to catalyze superoxide radical production in response to O2 has potential pathophysiological implications for the ischemia/reperfusion induced damages, endothelial dysfunction, and cardiovascular and neurological diseases.

Graphene enhanced Mn-Ce binary metal oxides for catalytic oxidation and adsorption of elemental mercury from coal-fired flue gas

Graphene was used for the enhancement of gaseous elemental mercury (Hg0) catalytic oxidation and adsorption over Mn-based oxides. A series of graphene modified Mn-based and Ce-Mn-based oxides were prepared via a hydrothermal method. Experimental results indicated that Mn-Ce@rGO composite has a Hg0 removal efficiency of 91% at 250 °C which was higher than that or pure MnOx and MnOx@rGO under the same reaction conditions. Hg0 removal efficiency decreased in the absence of O2. The higher SO2 concentration and space velocity resulted in lower Hg0 removal efficiencies. The mechanism study indicated that high valances of Mn4+ and Mn3+ benefit the Hg0 oxidation and the surface oxygen participated in the Hg0 oxidation process. CeO2 supplies sufficient oxygen for mercury surface adsorption. During these processes, graphene-enhanced the catalytic oxidation and adsorption processes through supporting more reaction space, facilitating electron transfer and binding sites. Moreover, the spent materials can be regenerated using a simple thermal-desorption method. Mn-Ce@rGO composite was proved to be a prospective adsorbent for Hg0 removal from coal-fired flue gas.

Synthesis of CeO2-modified activated carbon spheres by grafting and coordinating reactions for elemental mercury removal

Series of resin-based activated carbon spheres with well-dispersed CeO2 particles inside and high surface area were successfully prepared by the grafting of MMA and coordinating reactions of cerium(III) nitrate salt and steam activation. N2 adsorption isotherms, scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction-meter and X-ray photoelectron spectroscopy were applied to study the textural and surface characteristics of the obtained activated carbon spheres, and the Hg0 removal performances were evaluated in a fixed bed reactor. The experimental results indicated that pore structure, surface chemical properties and Hg0 removal ability of activated carbon spheres were observably improved after the modification of grafting and coordinating reactions. The optimal cerium(III) nitrate loading value, reaction temperature and O2 content were 7%, 150 °C and 5%, respectively. Moreover, SO2 showed an obvious inhibitory effect on the Hg0 removal efficiency in the absence of O2, while it facilitated the Hg0 removal in the presence of O2. Nitrogen monoxide promoted the Hg0 removal with or without O2, while water vapor inhibited the Hg0 removal over ACS-M-7%Ce. In addition, both of stable removal performance and excellent recycling ability were shown for ACS-M-7%Ce in ten cycles. The Hg0 removal mechanism analysis indicated that introduced CeO2 significantly promoted the Hg0 removal ability of ACS-M-7%Ce by generating the active species (such as C=O or C–O) and the lattice oxygen through the Ce4+/Ce3+ redox couple, and catalytically oxidized Hg0 into HgO.

Ternary semiconductor metal oxide blends grafted Ag@AgCl hybrid as dimensionally stable anode active layer for photoelectrochemical oxidation of organic compounds: Design strategies and photoelectric synergistic mechanism.

The development of ultra-efficient, sustainable, and easily accessible anode with relative non-precious semiconducting metal oxides is highly significant for application in the practical treatment of organically polluted water. Herein, we report SnO2, TiO2, and Ag2O ternary semiconductor metal oxide blend grafted Ag@AgCl hybrids, prepared with the one-step sol-gel method and applied as a dimensionally stable anode (DSA)-active layer on a SnO2-Sb/Ti electrode. Factors affecting crystal formation, including the presence or absence of O2 during calcination, the calcination temperature, and Ag@AgCl additive dosage were discussed. The micromorphology, phase composition, and photoelectrochemical activity of the newly designed anode were comprehensively characterized. The optimized preparation, which yielded a solid-solution structure with flat and smooth surface and well-crystallized lattice configuration, occurred in the absence of O2 during calcination at 550 ℃ with an Ag@AgCl additive dosage of 0.2 g in the sol-gel precursor. The newly designed DSA displayed improved electrocatalysis (EC) and photoelectrical catalysis (PEC) capacity. The phenol and its TOC removal efficiency reached 90.65% and 58.17% for 10 mA/cm2 current density with a metal halide lamp in 3 h. The lifespan was four times that of SnO2-Sb/Ti electrode. This proposed DSA construction strategy may support improved EC and PEC reactivities toward the decomposition of organic pollutants.

N2O decomposition and reduction on Co-MOR, Fe-MOR and Ni-MOR catalysts: in situ UV–vis DRS and operando FTIR investigation. An insight on the reaction pathways

Co-, Fe- or Ni-exchanged Na-MOR (Si/Al = 9.2) were prepared by ion-exchange method. The catalytic activity for N2O decomposition in the absence or in the presence of NO and for N2O reduction with CH4 in the absence of O2(CRN2O), or in the presence of O2(SCRN2O) was investigated. The catalytic measurements were performed in a flow apparatus with GC analysis of reactants and products. On Fe-MOR, in situ FTIR and UV–vis characterization evidenced coordinatively unsaturated sites (c.u.s.) Fe2+ arising from two families of Fe3+ dimers with different reducibility. Characterization and catalytic results combined with operando FTIR experiments gave an insight into the transition metal ion (tmi) species working during N2O abatement and into the reaction pathways.For N2O decomposition the activity order was Co-MOR > Fe-MOR in the absence of NO and Fe-MOR ≥ Co-MOR in the presence of NO, whereas Ni-MOR was always inactive. The decomposition occurred via redox mechanism passing through the formation of activated surface oxygen species, Oads*. The quasi-oxidic character of this activated oxygen in the Fe3+-O(1+δ)−---Fe(2+δ)+ intermediates with respect to the oxyl character of that in Co3+O‒ intermediates accounted for the lower activity and for the activity enhancement by NO addition to the feed of Fe-MOR with respect to Co-MOR. In Ni-MOR, both isolated or dimeric Ni2+ species were unable to be oxidized by N2O yielding Oads*.For N2O reduction, (CRN2O) and (SCRN2O), the activity order was Fe-MOR > Ni-MOR > Co-MOR. On all catalysts operando FTIR experiments revealed CHxOy intermediates (methoxy, formaldehyde and two types of formate species). The investigation of surface species changing the addition order of the reactants evidenced that the formation of CHxOy intermediates was favored on Co-MOR by pre-saturation with N2O, that yielded Co3+O− able to activate CH4, whereas on Ni-MOR by pre-saturation with CH4, that reduced Ni2+ dimers to Ni+ dimers, able to activate N2O. On Fe-MOR, the Fe2+ dimers formed during activation behaved as Co2+, whereas the Fe2+ dimers formed by reduction with CH4 behaved as Ni+ dimers.The formation and the stability of Oads* surface species were key factors for N2O decomposition and reduction pathways. These factors were affected by the mobility of the tmi electrons, that depends on the oxidation state, nuclearity and location in MOR framework of tmi.

Oxygen content dependent etch rate of single polymer microparticles confined in the sheath region of a low pressure radiofrequency argon/oxygen plasma

To study the etching of polymer microparticles confined in low pressure radiofrequency plasmas, the size and refractive index of single 2 μm particles are experimentally obtained as a function of both time and oxygen content (0%–50%) added to the argon background gas. The etch rate was found to depend heavily on the oxygen (O2) content, especially for mixtures with low fractions of O2. As expected the etch rate was found to be close to zero in absence of O2 and increases to a value of 2 nm min−1 for 0.5% O2 and to roughly 3.5 nm min−1 for 5% O2. Above 5% O2 the etch rate saturates. It is shown that these results are consistent with a steady state etch model taking the effects of both atomic oxygen and positive ions into account.

(Keynote) New p-Type Photocathodes for Solar Fuel Photoelectrochemistry

Semiconductor photocathodes either for the reduction of water or CO2 have received much less attention than their photoanode counterparts. In part, this is due to a lack of p-type materials having good minority carrier dynamics. However, the photostability of p-type materials is also often problematic. These issues are particularly challenging for metal oxide based materials, and thus few examples of p-type metal oxide based photocathodes having any level of practical functionality are available. Nonetheless, such materials are likely to have electrocatalytic activity of interest for the reactions noted above. We have therefore undertaken a search for photostable p-type metal oxides having advantageous electrode properties. To that end, we have focused our investigation on delafossite structures of the form ABO2 where A = Cu+ or Ag+ and B= Fe3+ or Rh3+. Initial studies, focused on p-CuFeO2 evaluating both its solid state properties dopant response and photoelectrochemical capabilities.1 We reported that this material had a conduction band edge well placed for the reduction of either water or CO2 and a band gap (Eg) well matched to the solar spectrum (1.36 eV). Photoelectrochemical stability was found to be passable, but we did observe a slow reduction of Cu(I) to Cu(0), which degraded the electrode over time. In the absence of CO2(aq), this material, under illumination, was found to reduce water to hydrogen. Introduction of aqueous CO2 into the cell caused the photoelectrochemical formation of formate with good faradaic efficiency. We elected to compare the response of this material to that of p-CuRhO2, since we predicted a more negative conduction band edge for the rhodium system, and therefore, anticipated improved CO2 reduction kinetics.2 Surprisingly, this material was highly efficient at water reduction, but incapable of reducing aqueous CO2. However, we did note that the band edge positions of p-CuRhO2 straddled the water oxidation and reduction redox potentials making p-CuRhO2 thermodynamically capable of splitting water without application of an external bias (Eg = 1.9 eV). And, though this is observed, a low optical conversion efficiency was obtained. Photochemical stability in this system was found to be unusual, in that when the electrode was utilized at a basic pH in the presence of air, it was very stable. However, at neutral to acidic pHs or in the absence of O2 (at 3% or higher levels) the system degraded via Cu(0) formation. Based on these results, we have now developed and explored a p-AgRhO2 electrode. Like the Cu counterpart, this material is also excellent at water reduction and inept at CO2 reduction chemistry. The system is quite stable toward photoreductive decomposition at all aqueous pH values. Water reduction conversion efficiency exceeds that reported for the Cu-based system. A measured Eg of 1.7 eV is found. However, the band edges of this material do not straddle both of the water redox potentials. These observations have suggested to us that CO2 reduction only proceeds when an ABO2 surface lacks the ability to efficiently carry out water reduction. To test that hypothesis, we have synthesized a p-AgFeO2, making the prediction that this material will reduce CO2. In preliminary studies, this appears to be the case. The stability and electrocatalytic properties of the delafossite systems studied have led to an interest in p-type chalcopyrite materials in the family [(CuxAg(1-x))(InyGa(1-y))(SzSe(1-z))2] as photocathodes for water reduction.3 These systems have band gaps fairly well matched to the solar spectrum and have been studies as thin film electrodes, however, we are focusing on bulk electrode materials where solid state synthesis allows one to carefully tailor the band edge positions and the bulk electronic properties of the materials. Though we are early in this work, our results point to unusual couplings between the bulk optical and electronic properties of these materials, and the observed electrochemistry. Optimization of these interactions is anticipated to lead to efficient water splitting capabilities under solar irradiation. References (a) Gu, J.; Wuttig, A.; Krizan, J. W.; Hu, Y.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B., The Journal of Physical Chemistry C 2013, 117 (24), 12415-12422; (b) Wuttig, A.; Krizan, J. W.; Gu, J.; Frick, J. J.; Cava, R. J.; Bocarsly, A. B., Journal of Materials Chemistry A 2017, 5 (1), 165-171.Gu, J.; Yan, Y.; Krizan, J. W.; Gibson, Q. D.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B., J Am Chem Soc 2014, 136 (3), 830-3.Frick, J. J.; Kushwaha, S. K.; Cava, R. J.; Bocarsly, A. B., The Journal of Physical Chemistry C 2017, 121 (32), 17046-17052.

Iron catalyzed demethylation of acetic acid*

The iron(IV)-oxo catalyzed cleavage of acetic acid is reported. Iron(II) complexes are used as catalyst precursors and H2O2 as oxidant in reactions in aqueous solution with an excess of chloride and, in some experiments, with ascorbic acid added as an OH. radical scavenger. It is shown that the acetic acid is cleaved into CO2 and a methyl radical. In the presence of O2 and OH. radicals, the initially generated methyl radicals produce methanol and formate. Halogenated alkanes are also formed and, in the absence of O2 and OH. radicals, methane is obtained as the major product. A mechanism for the formation of various environmentally important volatile C1 compounds is deduced from the present results and published studies, and its relevance as an additional source for these compounds in the atmosphere is discussed.

Mathematical Modeling and Mechanism of VUV Photodegradation of H2S in the Absence of O2

The existence of H2S has limited the biogas energy promotion. The traditional photodegradation of H2S is usually conducted in the presence of O2, yet this is unsuitable for biogas desulfurization which should be avoided. Therefore, the ultraviolet degradation of H2S in the absence of O2 was investigated for the first time in the present study from a mathematical point of view. Light wavelength and intensity applied were 185 nm and 2.16 × 10−12 Einstein/cm2·s, respectively. Firstly, the mathematical model of H2S photodegradation was established with MATLAB software, including the gas flow distribution model and radiation model of photoreactor, kinetics model, mass balance model, and calculation model of the degradation rate. Then, the influence of the initial H2S concentration and gas retention time on the photodegradation rate were studied, for verification of the mathematical model. Results indicated that the photodegradation rate decreased with the increase in initial H2S concentration, and the maximum photodegradation rate reached 62.8% under initial concentration of 3 mg/m3. In addition, the photodegradation rate of H2S increased with the increase in retention time. The experimental results were in good accordance with the modeling results, indicating the feasibility of the mathematical model to simulate the photodegradation of H2S. Finally, the intermediate products were simulated and results showed that the main photodegradation products were found to be H2 and elemental S, and concentrations of the two main products were close and agreed well with the reaction stoichiometric coefficients. Moreover, the concentration of free radicals of H• and SH• was rather low.

Contribution of the Excited Triplet State of Humic Acid and Superoxide Radical Anion to Generation and Elimination of Phenoxyl Radical

Contributions of excited triplet state of humic acid (3HA*) and superoxide radical anion (O2•-), which is mainly generated via the reaction of O2 with HA-derived reducing intermediates (HA•-), to phenol transformation were revealed using acetaminophen, 2,4,6-trimethylphenol and tyrosine as probe molecules. Phenol transformation was initiated by 3HA*, leading to the formation of the phenoxyl radical (PhO•), but the distribution of transformation intermediates was codetermined by 3HA* and HA•-. The influence of HA•- essentially resulted from the production of O2•-, which affected the fate of PhO•. PhO• could undergo dimerization, or react with O2•-, leading to either phenol peroxide formation (radical addition) or phenol regeneration (electron transfer). In addition, PhO• could bind to HA or react with HA radicals, particularly in the absence of O2 and O2•-. These PhO• reactions were dependent on the reduction potential and structure of PhO•. This study also proved that the reaction of phenol with 1O2 and the reaction of PhO• with O2•- lead to the same oxidation product. The contributions of 3HA* and its generated 1O2, HA•- and its generated O2•- to phenol transformation were pH-dependent.

Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment.

We describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O2 without life being present. Consequently, our ability to interpret both the presence and absence of O2 in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O2 release into the atmosphere for several billion years, producing a false negative for biologically generated O2. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O2, planetary processes that may generate abundant atmospheric O2 without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures—Oxygenic photosynthesis—Exoplanets—Planetary atmospheres. Astrobiology 18, 630–662.

Comparative analysis of strategies to prepare electron sinks in aquatic photoautotrophs.

While subject to illumination, photosystem I (PSI) has the potential to produce reactive oxygen species (ROS) that can cause photo-oxidative damage in oxygenic photoautotrophs. The reaction center chlorophyll in PSI (P700) is kept oxidized in excess light conditions to limit over-excitation of PSI and alleviate the production of ROS. Oxidation of P700 requires a sufficient electron sink for PSI, which is responsible for flavodiiron proteins (FLV) safely dissipating electrons to O2 in cyanobacteria, green algae, and land plants except for angiosperms during short-pulse light (SP) illumination under which photosynthesis and photorespiration do not occur. This fact implies that O2 usage is essential for P700 oxidation but also raises the question why angiosperms lost FLV. Here, we first found that aquatic photoautotrophs in red plastid lineage, in which no gene for FLV has been found, could keep P700 oxidized during SP illumination alleviating the photo-oxidative damage in PSI even without O2 usage. We comprehensively assessed P700 oxidation during SP illumination in the presence and absence of O2 in cyanobacteria (Cyanophyta), green algae (Chlorophyta), angiosperms (Streptophyta), red algae (Rhodophyta), and secondary algae (Cryptophyta, Haptophyta, and Heterokontophyta). A variety of dependencies of P700 oxidation on O2 among these photoautotrophs clearly suggest that O2 usage and FLV are not universally required to oxidize P700 for protecting PSI against ROS damage. Our results expand the understanding of the diverse strategies taken by oxygenic photoautotrophs to oxidize P700 and mitigate the risks of ROS.

P-67 - Diverse strategies in P700 oxidation system for suppressing ROS production were acquired during the evolution of photosynthetic organisms

We developed the method to produce ROS intrinsically in photosynthetic organisms: repetitive short-pulse (rSP) illumination. rSP illumination fulfills electrons in photosynthetic electron transport (PET) system, where O2 is reduced to superoxide radical and O2 is also excited to singlet O2 at photosystem I (PSI) of PET system. These ROS oxidatively damage PSI in angiosperms. By contrast, mosses/liverworts, ferns, and gymnosperms do not suffer from PSI damages, where a reaction center chlorophyll in PSI, P700, was oxidized to the maximum in the presence of O2, on the other hand P700 was reduced in angiosperm leaves. The reduced P700 shows electron accumulation in PSI to stimulate ROS production. These organisms, other than angiosperms, have flavodiiron (FLV) proteins, which catalyze O2 reduction to H2O using ferredoxin/NADPH. The mutant of liverwort, Marchantia polymorpha, deficient in FLV, could not oxidize P700 and suffered from PSI oxidative damage by rSP-illumination. That is, these photosynthetic organisms rapidly oxidize P700 to suppress ROS production. Recently, we found that diatoms, red algae, brown algae oxidize P700 even in the absence of O2 and did not suffer from any oxidative damages of PSI. These photosynthetic organisms might have a new mechanism to oxidize P700.