The Competition between Hydrogen, Halogen, and Covalent Bonding in Atmospherically Relevant Ammonium Iodate Clusters

Iodine-containing clusters are expected to be central to new particle formation (NPF) events in polar and midlatitude coastal regions. Iodine oxoacids and iodine oxides are observed in newly formed clusters, and in more polluted midlatitude settings, theoretical studies suggest ammonia may increase growth rates. Structural information was obtained via infrared (IR) spectroscopy and quantum chemical calculations for a series of clusters containing ammonia, iodic acid, and iodine pentoxide. Structures for five of the smallest cationic clusters present in the mass spectrum were identified, and four of the structures were found to preferentially form halogen and/or covalent bonds over hydrogen bonds. Ammonia is important in proton transfer from iodic acid components and also provides a scaffold to template the formation of a halogen and covalent bonded backbone. The calculations executed for the two largest clusters studied suggested the formation of a covalent I₃O8- anion within the clusters.

Mechanisms of Reactions between HOI and HY (Y = Cl, Br, I) on a Water Nanodroplet Surface

Iodine chemistry has a broad range of implications for atmospheric processes including new particle formation. Hypoiodous acid (HOI) is a major iodine reservoir species. Its heterogeneous recycling in marine aerosols influences the lifetime of ozone in the troposphere. One important step of such recycling is the reaction between HOI and HY (Y = Cl, Br, I). In this article, we employ ab initio molecular dynamics (AIMD) and quantum chemistry to investigate these reactions at the surface of atmospheric aerosols. Di-halogen (XY) can be formed in a picosecond time scale, with the formation of a loop structure connected by hydrogen and halogen bonds. The photolysis of XY at the surface of an aerosol is faster than in the gas phase. In addition to the formation of di-halogen, a new pathway to forming a [H₂O···I···OH₂]⁺ complex by the direct or indirect proton transition is identified. Results presented in this study deepen our understanding of the faster iodine-heterogeneous recycling at the surface of aerosols.

Bromoform, dibromochloromethane, and dibromomethane over the East China Sea and the western Pacific Ocean: Oceanic emission and spatial variation

Spatial distributions of bromocarbons, including bromoform (CHBr₃), dibromochloromethane (CHBr₂Cl), and dibromomethane (CH₂Br₂), and influential oceanographic parameters that determine their concentrations were measured in the marine atmosphere and seawater of the East China Sea (ECS) and western Pacific Ocean during two cruises from 14 to 24 September, 2017 and from 5 October to 3 December, 2018. The atmospheric concentrations of CHBr₃, CHBr₂Cl, and CH₂Br₂ were 0.33-3.02, 0.16-1.96, and 0.85-1.75 pptv over the western Pacific Ocean and 2.23-4.92, 0.26-1.52, and 0.24-7.47 pptv over the ECS, respectively. There was significant spatial variability in atmospheric bromocarbon concentrations in the study region, with higher concentration over the ECS. The atmospheric mixing ratios of bromocarbons were significantly correlated to the surface seawater bromocarbon concentrations and wind speed. In the ECS, input from terrestrial sources also significantly influenced the distributions of bromocarbons in air. PCA analysis revealed that seawater bromocarbon concentrations were correlated with both water mass and chlorophyll a. Generally lower CH₂Br₂/CHBr₃ ratios were observed in the ECS, which was indicative of mixing and/or dilution in coastal areas. The estimated average sea-to-air fluxes of CHBr₂Cl, CH₂Br₂, and CHBr₃ were 46.86, -3.77, and -6.71 nmol m^-2 d^-1 in the western Pacific Ocean and 111.49, 0.89, and 321.74 nmol m^-2 d^-1 in the ECS, respectively. These results of the net sea-to-air fluxes indicated oceanic net uptake of CH₂Br₂ and CHBr₃ for the western Pacific Ocean and oceanic emission of bromocarbons for the ECS.

Biogenic Iodine and Iodine-Containing Metabolites

This review is intended as a comprehensive survey of iodinated metabolites possessing carbon–iodine covalent bond, which have been obtained from living organisms. Generally thought to be minor components produced by many different organisms these interesting compounds now number more than 110. Many from isolated and identified iodine-containing metabolites showed high biological activities. Recent research, especially in the marine area, indicates this number will increase in the future. Sources of iodinated metabolites include microorganisms, algae, marine invertebrates, and some animals. Their origin and possible biological significance have also been discussed.

Environmental Control of Vanadium Haloperoxidases and Halocarbon Emissions in Macroalgae

Vanadium-dependent haloperoxidases (V-HPO), able to catalyze the reaction of halide ions (Cl^-, Br^-, I^-) with hydrogen peroxide, have a great influence on the production of halocarbons, which in turn are involved in atmospheric ozone destruction and global warming. The production of these haloperoxidases in macroalgae is influenced by changes in the surrounding environment. The first reported vanadium bromoperoxidase was discovered 40 years ago in the brown alga Ascophyllum nodosum. Since that discovery, more studies have been conducted on the structure and mechanism of the enzyme, mainly focused on three types of V-HPO, the chloro- and bromoperoxidases and, more recently, the iodoperoxidase. Since aspects of environmental regulation of haloperoxidases are less well known, the present paper will focus on reviewing the factors which influence the production of these enzymes in macroalgae, particularly their interactions with reactive oxygen species (ROS).

A kinetic study of the CH2OO Criegee intermediate reaction with SO2, (H2O)2, CH2I2 and I atoms using OH laser induced fluorescence

The OH laser induced fluorescence method was used to study the kinetics of CH₂OO reacting with SO₂, (H₂O)₂, CH₂I₂ and I atoms.

VUV photoionization aerosol mass spectrometric study on the iodine oxide particles formed from O3-initiated photooxidation of diiodomethane (CH2I2)

IOPs formed from O₃-initiated photooxidation of CH₂I₂ were investigated based on the combination of a thermal desorption/tunable vacuum ultraviolet time-of-flight photoionization aerosol mass spectrometer with a flow reactor for the first time.

Microbial Transformation of Iodine: From Radioisotopes to Iodine Deficiency

Iodine is a biophilic element that is important for human health, both as an essential component of several thyroid hormones and, on the other hand, as a potential carcinogen in the form of radioiodine generated by anthropogenic nuclear activity. Iodine exists in multiple oxidation states (-1, 0, +1, +3, +5, and +7), primarily as molecular iodine (I₂), iodide (I^-), iodate [Formula: see text] , or organic iodine (org-I). The mobility of iodine in the environment is dependent on its speciation and a series of redox, complexation, sorption, precipitation, and microbial reactions. Over the last 15years, there have been significant advances in iodine biogeochemistry, largely spurred by renewed interest in the fate of radioiodine in the environment. We review the biogeochemistry of iodine, with particular emphasis on the microbial processes responsible for volatilization, accumulation, oxidation, and reduction of iodine, as well as the exciting technological potential of these fascinating microorganisms and enzymes.

Methyl iodine over oceans from the Arctic Ocean to the maritime Antarctic

Studies about methyl iodide (CH₃I), an important atmospheric iodine species over oceans, had been conducted in some maritime regions, but the understanding of the spatial distribution of CH₃I on a global scale is still limited. In this study, we reports atmospheric CH₃I over oceans during the Chinese Arctic and Antarctic Research Expeditions. CH₃I varied considerably with the range of 0.17 to 2.9 pptv with absent of ship emission. The concentration of CH₃I generally decreased with increasing latitudes, except for higher levels in the middle latitudes of the Northern Hemisphere than in the low latitudes. For sea areas, the Norwegian Sea had the highest CH₃I concentrations with a median of 0.91 pptv, while the Central Arctic Ocean had the lowest concentrations with all values below 0.5 pptv. CH₃I concentration over oceans was affected by many parameters, including sea surface temperature, salinity, dissolved organic carbon, biogenic emissions and input from continents, with distinctive dominant factor in different regions, indicating complex biogeochemical processes of CH₃I on a global scale.

A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

Ocean emissions of inorganic and organic iodine compounds drive the biogeochemical cycle of iodine and produce reactive ozone-destroying iodine radicals that influence the oxidizing capacity of the atmosphere. Di-iodomethane (CH₂I₂) and chloro-iodomethane (CH₂ICl) are the two most important organic iodine precursors in the marine boundary layer. Ship-borne measurements made during the TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOC) field campaign in the east tropical Pacific Ocean in January/February 2012 revealed strong diurnal cycles of CH₂I₂ and CH₂ICl in air and of CH₂I₂ in seawater. Both compounds are known to undergo rapid photolysis during the day, but models assume no night-time atmospheric losses. Surprisingly, the diurnal cycle of CH₂I₂ was lower in amplitude than that of CH₂ICl, despite its faster photolysis rate. We speculate that night-time loss of CH₂I₂ occurs due to reaction with NO₃ radicals. Indirect results from a laboratory study under ambient atmospheric boundary layer conditions indicate a k _CH2I2+NO3 of ≤4 × 10^-13 cm³ molecule^-1 s^-1; a previous kinetic study carried out at ≤100 Torr found k _CH2I2+NO3 of 4 × 10^-13 cm³ molecule^-1 s^-1. Using the 1-dimensional atmospheric THAMO model driven by sea-air fluxes calculated from the seawater and air measurements (averaging 1.8 +/- 0.8 nmol m^-2 d^-1 for CH₂I₂ and 3.7 +/- 0.8 nmol m^-2 d^-1 for CH₂ICl), we show that the model overestimates night-time CH₂I₂ by >60 % but reaches good agreement with the measurements when the CH₂I₂ + NO₃ reaction is included at 2-4 × 10^-13 cm³ molecule^-1 s^-1. We conclude that the reaction has a significant effect on CH₂I₂ and helps reconcile observed and modeled concentrations. We recommend further direct measurements of this reaction under atmospheric conditions, including of product branching ratios.

Atmospheric chemistry of alkyl iodides: theoretical studies on the mechanisms and kinetics of CH3I/C2H5I + NO3 reactions

Mechanisms and kinetics of the reactions of the NO₃ radical with CH₃I and C₂H₅I have been investigated from a sound theoretical basis.

Reactivity of CHI3 with OH radicals: X-abstraction reaction pathways (X = H, I), atmospheric chemistry, and nuclear safety

The X-abstraction (X = H, I) pathways in the reaction of CHI3 with OH radical, a possible iodoform removal process relevant to the Earth's atmosphere and conditions prevailing in the case of a nuclear accident, have been studied applying highly correlated ab initio quantum chemistry methods and canonical transition-state theory to obtain reaction energy profiles and rate constants. Geometry optimizations of reactants, products, molecular complexes, and transition states determined at the MP2/cc-pVTZ level of theory have been followed by DK-CCSD(T)/ANO-RCC single-point energy calculations. Further improvement of electronic energies has been achieved by applying spin-orbit coupling, corrections toward full configuration interaction, vibration contributions, and tunneling corrections. Calculated reaction enthalpies at 0 K are -108.2 and -5.1 kJ mol(-1) for the H- and I-abstraction pathways, respectively; the strongly exothermic H-abstraction pathway is energetically favored over the modestly exothermic I-abstraction one. The overall rate constant at 298 K based on our ab initio calculations is 4.90 × 10(-11) cm(3) molecule(-1) s(-1), with the I-abstraction pathway being the major channel over the temperature range of 250-2000 K. The CHI3 atmospheric lifetime with respect to the removal reaction with OH radical is predicted to be about 6 h, very short compared to that of other halomethanes.

HCl yield and chemical kinetics study of the reaction of Cl atoms with CH3I at the 298K temperature using the infra-red tunable diode laser absorption spectroscopy

Pulsed ArF excimer laser (193 nm)-CW infrared (IR) tunable diode laser Herriott type absorption spectroscopic technique has been made for the detection of product hydrochloric acid HCl. Absorption spectroscopic technique is used in the reaction chlorine atoms with methyl iodide (Cl+CH3I) to the study of kinetics on reaction Cl+CH3I and the yield of (HCl). The reaction of Cl+CH3I has been studied with the support of the reaction Cl+C4H10 (100% HCl) at temperature 298 K. In the reaction Cl+CH3I, the total pressure of He between 20 and 125 Torr at the constant concentration of [CH3I] 7.0×10(14) molecule cm(-3). In the present work, we estimated adduct formation is very important in the reaction Cl+CH3I and reversible processes as well and CH3I molecule photo-dissociated in the methyl [CH3] radical. The secondary chemistry has been studied as CH3+CH3ICl = product, and CH3I+CH3ICl = product2. The system has been modeled theoretically for secondary chemistry in the present work. The calculated and experimentally HCl yield nearly 65% at the concentration 1.00×10(14) molecule cm(-3) of [CH3I] and 24% at the concentration 4.0×10(15) molecule cm(-3) of [CH3I], at constant concentration 4.85×10(12) molecule cm(-3) of [CH3], and at 7.3×10(12) molecule cm(-3) of [Cl]. The pressure dependent also studied product of HCl at the constant [CH3], [Cl] and [CH3I]. The experimental results are also very good matching with the modelling work at the reaction CH3+CH3ICl = product (k = (2.75±0.35)×10(-10) s(-1)) and CH3I+CH3ICl = product2 (k = 1.90±0.15)×10(-12) s(-1). The rate coefficients of the reaction CH3+CH3ICl and CH3I+CH3ICl has been made in the present work. The experimental results has been studied by two method (1) phase locked and (2) burst mode.

The development and deployment of a ground-based, laser-induced fluorescence instrument for the in situ detection of iodine monoxide radicals

High abundances of iodine monoxide (IO) are known to exist and to participate in local photochemistry of the marine boundary layer. Of particular interest are the roles IO plays in the formation of new particles in coastal marine environments and in depletion episodes of ozone and mercury in the Arctic polar spring. This paper describes a ground-based instrument that measures IO at mixing ratios less than one part in 10(12). The IO radical is measured by detecting laser-induced fluorescence at wavelengths longer that 500 nm. Tunable visible light is used to pump the A(2)Π3/2 (v(') = 2) ← X(2)Π3/2 (v(″) = 0) transition of IO near 445 nm. The laser light is produced by a solid-state, Nd:YAG-pumped Ti:Sapphire laser at 5 kHz repetition rate. The laser-induced fluorescence instrument performs reliably with very high signal-to-noise ratios (>10) achieved in short integration times (<1 min). The observations from a validation deployment to the Shoals Marine Lab on Appledore Island, ME are presented and are broadly consistent with in situ observations from European Coastal Sites. Mixing ratios ranged from the instrumental detection limit (<1 pptv) to 10 pptv. These data represent the first in situ point measurements of IO in North America.

Naturally occurring organoiodines

This review, with 290 references, presents the fascinating area of iodinated natural products over the past hundred years for the first time.

Spectroscopic study of the I2 formation from the photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at different wavelengths

Emission spectra following the photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at 266 nm were recorded in a slow flow cell. In addition to emission from the electronically excited species including CH (A(2)Δ, B(2)Σ(-), and C(2)Σ(+)), C2 (d(3)Πg), and atomic iodine ((4)P(o)), a series of emission bands was observed in the 12,000-19,000 cm(-1) region. The dominant structure of these emission bands was verified as the I2 B(3)Π(+)(0,u)-X(1)Σ(+)g emission at the 532 nm excitation, and the observed I2 was formed from collisions between iodine atoms generated from the C-I bond dissociation in these iodomethanes. The I2 emission spectra following the photolysis of CH2I2 at different wavelengths were acquired, and the threshold energy for the first C-I bond cleavage was determined to be 208 ± 1 kJ mol(-1). We also obtained the emission spectra of pure I2 at several visible excitation wavelengths for comparison with those from the photolysis of iodomethanes, and a least-squares global fit of the observed I2 emission bands yields more accurate anharmonicity parameters for the vibrational structure in the I2 B-X transition.

The role of vanadium haloperoxidases in the formation of volatile brominated compounds and their impact on the environment

Vanadium haloperoxidases differ strongly from heme peroxidases in substrate specificity and stability and in contrast to a heme group they contain the bare metal oxide vanadate as a prosthetic group. These enzymes specifically oxidize halides in the presence of hydrogen peroxide into hypohalous acids. These reactive halogen intermediates will react rapidly and aspecifically with many organic molecules. Marine algae and diatoms containing these iodo- and bromoperoxidases produce short-lived brominated methanes (bromoform, CHBr3 and dibromomethane CH2Br2) or iodinated compounds. Some seas and oceans are supersaturated with these compounds and they form an important source of bromine to the troposphere and lower stratosphere and contribute significantly to the global budget of halogenated hydrocarbons. This perspective focuses, in particular, on the biosynthesis of these volatile compounds and the direct or indirect involvement of vanadium haloperoxidases in the production of huge amounts of bromoform and dibromomethane. Some of the global sources are discussed and from the literature a picture emerges in which oxidized brominated species generated by phytoplankton, seaweeds and cyanobacteria react with dissolved organic matter in seawater, resulting in the formation of intermediate brominated compounds. These compounds are unstable and decay via a haloform reaction to form an array of volatile brominated compounds of which bromoform is the major component followed by dibromomethane.

Time resolved high frequency spectrum of Br2 molecules using pulsed photoacoustic technique

The paper reports the time resolved spectral distribution of higher order acoustic modes generated in Br2 molecules using pulsed Photoacoustic (PA) technique. New time resolved vibrational spectrum of Br2 molecules are recorded using a single 532nm, pulses of 7ns duration at 10Hz repetition rate obtained from Q-switched Nd:YAG laser. Frank-Condon principle based assignments confirms the presence of 12 numbers of (ν″-ν') vibrational transitions covered by a single 532+2nm pulse profile. Inclusions of higher order zeroth modes in Bassel's function expansion series shows the probability of overlapping of different types of acoustic modes in the designed PA cells. These modes appear in the form of clusters which occupies higher frequency range. The study of decay behavior of PA signal with respect to time confirms the photolysis of Br2 at 532nm wavelength. In addition, the shifting and clustering effect of cavity eigen modes in Br2 molecules have been studied between 1 and 10ms time scale. The estimated Q-factor of PA cell (l=16cm, R=1.4cm) is 145±4 at 27kHz frequency.

Abiotic formation of methyl iodide on synthetic birnessite: a mechanistic study

Methyl iodide is a well-known volatile halogenated organic compound that contributes to the iodine content in the troposphere, potentially resulting in damage to the ozone layer. Most methyl iodide sources derive from biological activity in oceans and soils with very few abiotic mechanisms proposed in the literature. In this study we report that synthetic manganese oxide (birnessite δ-MnO2) can catalyze the formation of methyl iodide in the presence of natural organic matter (NOM) and iodide. Methyl iodide formation was only observed at acidic pH (4-5) where iodide is oxidized to iodine and NOM is adsorbed on δ-MnO2. The effect of δ-MnO2, iodide and NOM concentrations, nature of NOM and ionic strength was investigated. High concentrations of methyl iodide were formed in experiments conducted with the model compound pyruvate. The Lewis acid property of δ-MnO2 leads to a polarization of the iodine molecule, and catalyzes the reaction with natural organic matter. As manganese oxides are strong oxidants and are ubiquitous in the environment, this mechanism could significantly contribute to the global atmospheric input of iodine.

Detection of iodine monoxide in the tropical free troposphere

Atmospheric iodine monoxide (IO) is a radical that catalytically destroys heat trapping ozone and reacts further to form aerosols. Here, we report the detection of IO in the tropical free troposphere (FT). We present vertical profiles from airborne measurements over the Pacific Ocean that show significant IO up to 9.5 km altitude and locate, on average, two-thirds of the total column above the marine boundary layer. IO was observed in both recent deep convective outflow and aged free tropospheric air, suggesting a widespread abundance in the FT over tropical oceans. Our vertical profile measurements imply that most of the IO signal detected by satellites over tropical oceans could originate in the FT, which has implications for our understanding of iodine sources. Surprisingly, the IO concentration remains elevated in a transition layer that is decoupled from the ocean surface. This elevated concentration aloft is difficult to reconcile with our current understanding of iodine lifetimes and may indicate heterogeneous recycling of iodine from aerosols back to the gas phase. Chemical model simulations reveal that the iodine-induced ozone loss occurs mostly above the marine boundary layer (34%), in the transition layer (40%) and FT (26%) and accounts for up to 20% of the overall tropospheric ozone loss rate in the upper FT. Our results suggest that the halogen-driven ozone loss in the FT is currently underestimated. More research is needed to quantify the widespread impact that iodine species of marine origin have on free tropospheric composition, chemistry, and climate.

Coastal iodine emissions. 1. Release of I₂ by Laminaria digitata in chamber experiments

Tidally exposed macroalgae emit large amounts of I(2) and iodocarbons that produce hotspots of iodine chemistry and intense particle nucleation events in the coastal marine boundary layer. Current emission rates are poorly characterized, however, with reported emission rates varying by 3 orders of magnitude. In this study, I(2) emissions from 25 Laminaria digitata samples were investigated in a simulation chamber using incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). The chamber design allowed gradual extraction of seawater to simulate tidal emersion of algae. Samples were exposed to air with or without O(3) and to varying irradiances. Emission of I(2) occurred in four distinct stages: (1) moderate emissions from partially submerged samples; (2) a strong release by fully emerged samples; (3) slowing or stopping of I(2) release; and (4) later pulses of I(2) evident in some samples. Emission rates were highly variable and ranged from 7 to 616 pmol min(-1) gFW(-1) in ozone-free air, with a median value of 55 pmol min(-1) gFW(-1) for 20 samples.

Application of mass spectrometric techniques for the trace analysis of short-lived iodine-containing volatiles emitted by seaweed

Knowledge of the composition and emission rates of iodine-containing volatiles from major widespread seaweed species is important for modeling the impact of halogens on gas-phase atmospheric chemistry, new particle formation, and climate. In this work, we present the application of mass spectrometric techniques for the quantification of short-lived iodine-containing volatiles emitted by eight different seaweeds from the intertidal zone of Helgoland, Germany. A previously developed online time-of-flight aerosol mass spectrometric method was used to determine I(2) emission rates and investigate temporally resolved emission profiles. Simultaneously, iodocarbons were preconcentrated on solid adsorbent tubes and quantified offline using thermodesorption-gas chromatography-mass spectrometry. The total iodine content of the seaweeds was determined using microwave-assisted tetramethylammonium hydroxide extraction followed by inductively coupled-plasma mass spectrometry analysis. The highest total iodine content was found in the Laminariales, followed by the brown algae Ascophyllum nodosum, Fucus vesiculosus, Fucus serratus, and both red algae Chondrus crispus and Delesseria sanguinea. Laminariales were found to be the strongest I(2) emitters. Time series of the iodine release of Laminaria digitata and Laminaria hyperborea showed a strong initial I(2) emission when first exposed to air followed by an exponential decline of the release rate. For both species, I(2) emission bursts were observed. For Laminaria saccharina und F. serratus, a more continuous I(2) release profile was detected, however, F. serratus released much less I(2). A. nodosum and F. vesiculosus showed a completely different emission behavior. The I(2) emission rates of these species were slowly increasing with time during the first 1 to 2 h until a more or less stable I(2) emission rate was reached. The lowest I(2) emission rates were detected for the red algae C. crispus and D. sanguinea. Total iodocarbon emission rates showed almost the same general trend, however, the total iodocarbon emission rates were about one to two orders of magnitude lower than those of molecular iodine, demonstrating that I(2) is the major iodine containing volatile released by the investigated seaweed species. In addition, a clear dependency of iodocarbon emission from the ozone level (0-150 ppb O(3)) was found for L. digitata.

Studies of the Formation and Growth of Aerosol from Molecular Iodine Precursor

The formation and growth of iodine oxide particles (IOPs), originating from molecular iodine precursor, has been studied at room temperature as a function of water vapour, and sulphuric and oxalic acid vapours. A linear variation in total IOP mass was observed over a wide range of iodine atom production rates under both dry and humid formation conditions. Particle formation was also observed in the absence of ozone, and was found to be temperature sensitive, with elevated temperatures resulting in reduced particle number and mass. Electronic structure calculations are used to show that particle formation is initiated by polymerization of I2O4 with I2O3, or with itself. Formation of IOPs in humid conditions results in lower numbers and smaller particles than formed in the absence of water vapour, because H2O forms relatively stable complexes with molecules such as I2O3 and I2O4, inhibiting their polymerization. Addition of H2O to particles formed under dry conditions shows the collapse of fractal-like, aggregate particle structures. The uptake of sulphuric acid vapour onto humidified particles was studied over a wide range of relative humidity (RH) at room temperature, with the calculated accommodation coefficient (α) for this process increasing with RH to a value of 0.75±0.05 at RH = 90%. In contrast, growth of particles exposed to oxalic acid vapour was not observed on the experimental timescales employed, indicating an upper limit for α of 10−3.

The halogenated metabolism of brown algae (Phaeophyta), its biological importance and its environmental significance

Brown algae represent a major component of littoral and sublittoral zones in temperate and subtropical ecosystems. An essential adaptive feature of this independent eukaryotic lineage is the ability to couple oxidative reactions resulting from exposure to sunlight and air with the halogenations of various substrates, thereby addressing various biotic and abiotic stresses i.e., defense against predators, tissue repair, holdfast adhesion, and protection against reactive species generated by oxidative processes. Whereas marine organisms mainly make use of bromine to increase the biological activity of secondary metabolites, some orders of brown algae such as Laminariales have also developed a striking capability to accumulate and to use iodine in physiological adaptations to stress. We review selected aspects of the halogenated metabolism of macrophytic brown algae in the light of the most recent results, which point toward novel functions for iodide accumulation in kelps and the importance of bromination in cell wall modifications and adhesion properties of brown algal propagules. The importance of halogen speciation processes ranges from microbiology to biogeochemistry, through enzymology, cellular biology and ecotoxicology.