Marine iodine emissions in a changing world

Comparison of dissolved iodine measurements in seawater between inductively coupled plasma mass spectrometry and voltammetry

Two analytical methods, inductively coupled plasma mass spectrometry (ICP-MS) combined with high-performance liquid chromatography (HPLC) and voltammetry (VM), for three chemical species of dissolved iodine (iodide, iodate, and total dissolved iodine: TDI) were compared for dozens of coastal seawater samples owing to the compatibility of data between both methods. The median differences in the measured concentrations of TDI, total inorganic dissolved iodine (TII, the sum of iodide and iodate), and iodate between ICP-MS and VM were equivalent to 9.2, 13, and 14%, respectively. These differences were within the ranges that could be explained by the repeated-measurement precision of each measurement method for TDI, TII, and iodate. The difference for iodide was 19%, which was larger than the value based on the repeated-measurement precision for both methods. This is considered to be caused by the chemical instability and lower concentrations of iodide compared to other iodine species in seawater, in addition to the heterogeneity of natural samples. Finally, both methods provided reasonable measurement values for the iodine concentration in natural seawater samples.

Climate changes affecting global iodine status

Global warming is now universally acknowledged as being responsible for dramatic climate changes with rising sea levels, unprecedented temperatures, resulting fires and threatened widespread species loss. While these effects are extremely damaging, threatening the future of life on our planet, one unexpected and paradoxically beneficial consequence could be a significant contribution to global iodine supply. Climate change and associated global warming are not the primary causes of increased iodine supply, which results from the reaction of ozone (O3) arising from both natural and anthropogenic pollution sources with iodide (I-) present in the oceans and in seaweeds (macro- and microalgae) in coastal waters, producing gaseous iodine (I2). The reaction serves as negative feedback, serving a dual purpose, both diminishing ozone pollution in the lower atmosphere and thereby increasing I2. The potential of this I2 to significantly contribute to human iodine intake is examined in the context of I2 released in a seaweed-abundant coastal area. The bioavailability of the generated I2 offers a long-term possibility of increasing global iodine status and thereby promoting thyroidal health. It is hoped that highlighting possible changes in iodine bioavailability might encourage the health community to address this issue.

Spontaneous Appearance of Triiodide Covering the Topmost Layer of the Iodide Solution Interface Without Photo-Oxidation

Ions containing iodine atoms at the vapor-aqueous solution interfaces critically affect aerosol growth and atmospheric chemistry due to their complex chemical nature and multivalency. While the surface propensity of iodide ions has been intensely discussed in the context of the Hofmeister series, the stability of various ions containing iodine atoms at the vapor-water interface has been debated. Here, we combine surface-specific sum-frequency generation (SFG) vibrational spectroscopy with ab initio molecular dynamics simulations to examine the extent to which iodide ions cover the aqueous surface. The SFG probe of the free O-D stretch mode of heavy water indicates that the free O-D group density decreases drastically at the interface when the bulk NaI concentration exceeds ∼2 M. The decrease in the free O-D group density is attributed to the spontaneous appearance of triiodide that covers the topmost interface rather than to the surface adsorption of iodide. This finding demonstrates that iodide is not surface-active, yet the highly surface-active triiodide is generated spontaneously at the water-air interface, even under dark and oxygen-free conditions. Our study provides an important first step toward clarifying iodine chemistry and pathways for aerosol formation.

Environmental iodine as a natural iodine intake in humans and environmental pollution index: a scientometric and updated mini review

Although almost a third of the world's population is exposed to iodine deficiency (ID), and supplementation programs such as enriching table salt have been carried out or are being carried out at the global and national level, in many regions of the world, people are facing an increase in iodine intake, which is mainly due to the presence of large amounts of iodine in water, soil, agricultural products, or high consumption of seafood. Published articles were indexed in the Scopus database (from 2000 to 1 April 2023) were reviewed and analyzed by VOSviewer software. The results showed the growing interest of researchers over the last 20 years in environmental iodine intake. The results of this study can have a significant impact on the planning and policy-making of relevant officials and communities to supply the needed iodine.

Iodide oxidation by ozone at the surface of aqueous microdroplets

The oxidation of iodide by ozone occurs at the sea-surface and within sea spray aerosol, influencing the overall ozone budget in the marine boundary layer and leading to the emission of reactive halogen gases. A detailed account of the surface mechanism has proven elusive, however, due to the difficulty in quantifying multiphase kinetics. To obtain a clearer understanding of this reaction mechanism at the air-water interface, we report pH-dependent oxidation kinetics of I- in single levitated microdroplets as a function of [O3] using a quadrupole electrodynamic trap and an open port sampling interface for mass spectrometry. A kinetic model, constrained by molecular simulations of O3 dynamics at the air-water interface, is used to understand the coupled diffusive, reactive, and evaporative pathways at the microdroplet surface, which exhibit a strong dependence on bulk solution pH. Under acidic conditions, the surface reaction is limited by O3 diffusion in the gas phase, whereas under basic conditions the reaction becomes rate limited on the surface. The pH dependence also suggests the existence of a reactive intermediate IOOO- as has previously been observed in the Br- + O3 reaction. Expressions for steady-state surface concentrations of reactants are derived and utilized to directly compute uptake coefficients for this system, allowing for an exploration of uptake dependence on reactant concentration. In the present experiments, reactive uptake coefficients of O3 scale weakly with bulk solution pH, increasing from 4 × 10-4 to 2 × 10-3 with decreasing solution pH from pH 13 to pH 3.

Abiotic and Biotic Reduction of Iodate Driven by Shewanella oneidensis MR-1

Iodate (IO3-) can be abiotically reduced by Fe(II) or biotically reduced by the dissimilatory Fe(III)-reducing bacterium Shewanella oneidensis (MR-1) via its DmsEFAB and MtrCAB. However, the intermediates and stoichiometry between the Fe(II) and IO3- reaction and the relative contribution of abiotic and biotic IO3- reduction by biogenic Fe(II) and MR-1 in the presence of Fe(III) remain unclear. In this study, we found that abiotic reduction of IO3- by Fe(II) produced intermediates HIO and I- at a ratio of 1:2, followed by HIO disproportionation to I- and IO3-. Comparative analyses of IO3- reduction by MR-1 wild type (WT), MR-1 mutants deficient in DmsEFAB or MtrCAB, and Shewanella sp. ANA-3 in the presence of Fe(III)-citrate, Fe(III) oxides, or clay minerals showed that abiotic IO3- reduction by biogenic Fe(II) predominated under iron-rich conditions, while biotic IO3- reduction by DmsEFAB played a more dominant role under iron-poor conditions. Compared to that in the presence of Fe(III)-citrate, MR-1 WT reduced more IO3- in the presence of Fe(III) oxides and clay minerals. The observed abiotic and biotic IO3- reduction by MR-1 under Fe-rich and Fe-limited conditions suggests that Fe(III)-reducing bacteria could contribute to the transformation of iodine species and I- enrichment in natural iodine-rich environments.

Detoxification of arsenite by iodide in frozen solution

The oxidation of arsenite (As(III)) to arsenate (As(V)) has received significant attention because it helps mitigate the hazardous and adverse effects of As(III) and subsequently improves the effectiveness of arsenic removal. This study developed an efficient freezing technology for the oxidative transformation of As(III) based on iodide (I-). For a sample containing a very low concentration of 20 μM As(III) and 200 μM I- frozen at -20 °C, approximately 19 μM As(V) was formed after reaction for 0.5 h at pH 3. This rapid conversion has never been achieved in previous studies. However, As(V) was not generated in water at 25 °C. The acceleration of the oxidation of As(III) by I- in ice may be attributed to the freeze-concentration effect. During freezing, all components (i.e., As(III), I-, and protons) are highly concentrated in the ice grain boundary regions, resulting in thermodynamically and kinetically favorable conditions for the redox reaction between As(III) and I-. The efficiency of the oxidation of As(III) using I- increased at high I- concentrations and low pH values. The low freezing temperature (below -20 °C) hindered the oxidative transformation of As(III) by I-. The efficiency of the oxidation of As(III) significantly increased using a fixed initial concentration of I- by subjecting the system to six freezing-melting cycles. The outcomes of this study suggest the possibility of the self-detoxification of As(III) in the natural environment, indicating the potential for developing an eco-friendly method for the treatment of As(III)-contaminated areas in regions with a cold climate. It also demonstrates radical remediation to almost completely remove a very small amount of As(III) that was input in As(III)-contaminated wastewater detoxification, a benchmark that existing methods have been unable to achieve.

Spontaneous Iodide Activation at the Air-Water Interface of Aqueous Droplets

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air-water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3- formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir-Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols.

Investigation of changes in the iodine concentrations of oceanic sediment and terrestrial soil samples after thermal drying

To certify the suitability of pretreatment by thermal drying to measure iodine concentrations of oceanic sediment and terrestrial soil samples, changes in iodine concentrations after drying (50, 80 or 85, and 110 °C for 48 h) were examined using the samples in addition to a terrestrial plant (pine needles), which was selected as an intact organic sample. The iodine concentrations per wet weight of the sediment and soil samples processed by thermal drying were comparable to those of the raw samples at all temperatures. However, the concentrations of the plant samples dried at 85 and 110 °C were lower than those of the raw samples. The lower concentrations of the plant samples at higher temperatures were considered to be derived from the volatilization of a part of the plant's organic matter. Finally, these results suggested that the iodine concentrations in oceanic sediment and terrestrial soil samples scarcely change after thermal drying at 110 °C, although the concentrations may decrease when these samples include high contributions of fresh organic matter.

Iodine emission from the reactive uptake of ozone to simulated seawater

The heterogeneous reaction of ozone and iodide is both an important source of atmospheric iodine and dry deposition pathway of ozone in marine environments. While the iodine generated from this reaction is primarily in the form of HOI and I2, there is also evidence of volatile organoiodide compound emissions in the presence of organics without biological activity occuring [M. Martino, G. P. Mills, J. Woeltjen and P. S. Liss, A new source of volatile organoiodine compounds in surface seawater, Geophys. Res. Lett., 2009, 36, L01609, L. Tinel, T. J. Adams, L. D. J. Hollis, A. J. M. Bridger, R. J. Chance, M. W. Ward, S. M. Ball and L. J. Carpenter, Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions, Environ. Sci. Technol., 2020, 54, 13228-13237]. In this study, we evaluate our fundamental understanding of the ozonolysis of iodide which leads to gas-phase iodine emissions. To do this, we compare experimental measurements of ozone-driven gas-phase I2 formation in a flow tube to predictions made with the kinetic multilayer model for surface and bulk chemistry (KM-SUB). The KM-SUB model uses literature rate coefficients used in current atmospheric chemistry models to predict I2(g) formation in pH-buffered solutions of marine composition containing chloride, bromide, and iodide compared to solutions containing only iodide. Experimentally, I2(g) formation was found to be suppressed in solutions containing seawater levels of chloride compared to solutions containing only iodide, but the model does not predict this effect using literature rate constants. However, the model is able to predict this trend upon adjustment of two specific reaction rate constants. To more closely represent true oceanic conditions, we add an organic component to the proxy seawater solutions using material generated from Thalassiosira pseudonana phytoplankton cultures. Whereas the rate of ozone deposition is unaffected, the formation rate of I2(g) is strongly suppressed in the presence of biological organic material, indicative of a sink or reduction of reactive iodine formed during the oxidation process.

Microbial Contributions to Iodide Enrichment in Deep Groundwater in the North China Plain

Microorganisms play crucial roles in the global iodine cycling through iodine oxidation, reduction, volatilization, and deiodination. In contrast to iodate formation in radionuclide-contaminated groundwater by the iodine-oxidizing bacteria, microbial contribution to the formation of high level of iodide in geogenic high iodine groundwater is poorly understood. In this study, our results of comparative metagenomic analyses of deep groundwater with typical high iodide concentrations in the North China Plain revealed the existence of putative dissimilatory iodate-reducing idrABP1P2 gene clusters in groundwater. Heterologous expression and characterization of an identified idrABP1P2 gene cluster confirmed its functional role in iodate reduction. Thus, microbial dissimilatory iodate reduction could contribute to iodide formation in geogenic high iodine groundwater. In addition, the identified iron-reducing, sulfur-reducing, sulfur-oxidizing, and dehalogenating bacteria in the groundwater could contribute to the release and production of iodide through the reductive dissolution of iron minerals, abiotic iodate reduction of derived ferrous iron and sulfide, and dehalogenation of organic iodine, respectively. These microbially mediated iodate reduction and organic iodine dehalogenation processes may also result in the transformation among iodine species and iodide enrichment in other geogenic iodine-rich groundwater systems worldwide.

Effect of the Marine Exercise Retreat Program on Thyroid-Related Hormones in Middle-Aged Euthyroid Women

This study aimed to investigate the effects of a marine exercise retreat program on thyroid-related hormone levels. A total of 62 middle-aged euthyroid women participated in a 6-day marine exercise retreat program. Using thyroid-stimulating hormone (TSH) and free thyroxine (fT4) hormone levels, the participants were divided into high and low-hormone-level groups. Despite decreased TSH and fT4 levels after the program, the factors influencing changes in each group were different. TSH levels were influenced by changes in the normalized low frequency (nLF) of heart rate variability and carbon monoxide (CO) from all the participants, and changes in body fat percentage, nLF, and nitrogen dioxide (NO2) exposure level in the high TSH group. fT4 levels were influenced by changes in body mass index (BMI), NO2 exposure, and particulate matter diameter of 10 µm or less (PM10) exposure in all participants. Changes in BMI and CO exposure influenced the low fT4 group. Lastly, changes in the exercise stress test affected the high fT4 group. Thus, the marine exercise retreat program affected euthyroid thyroid-related hormone levels, and influencing factors differ depending on the initial value of the hormone.

Iodate respiration by Azoarcus sp. DN11 and its potential use for removal of radioiodine from contaminated aquifers

Azoarcus sp. DN11 was previously isolated from gasoline-contaminated groundwater as an anaerobic benzene-degrading bacterium. Genome analysis of strain DN11 revealed that it contained a putative idr gene cluster (idrABP₁P₂ ), which was recently found to be involved in bacterial iodate (IO₃ ^-) respiration. In this study, we determined if strain DN11 performed iodate respiration and assessed its potential use to remove and sequester radioactive iodine (¹²⁹I) from subsurface contaminated aquifers. Strain DN11 coupled acetate oxidation to iodate reduction and grew anaerobically with iodate as the sole electron acceptor. The respiratory iodate reductase (Idr) activity of strain DN11 was visualized on non-denaturing gel electrophoresis, and liquid chromatography-tandem mass spectrometry analysis of the active band suggested the involvement of IdrA, IdrP₁, and IdrP₂ in iodate respiration. The transcriptomic analysis also showed that idrA, idrP₁ , and idrP₂ expression was upregulated under iodate-respiring conditions. After the growth of strain DN11 on iodate, silver-impregnated zeolite was added to the spent medium to remove iodide from the aqueous phase. In the presence of 200 μM iodate as the electron acceptor, more than 98% of iodine was successfully removed from the aqueous phase. These results suggest that strain DN11 is potentially helpful for bioaugmentation of ¹²⁹I-contaminated subsurface aquifers.

Global occurrence of the bacteria with capability for extracellular reduction of iodate

The γ-proteobacterium Shewanella oneidensis MR-1 reduces iodate to iodide extracellularly. Both dmsEFAB and mtrCAB gene clusters are involved in extracellular reduction of iodate by S. oneidensis MR-1. DmsEFAB reduces iodate to hypoiodous acid and hydrogen peroxide (H₂O₂). Subsequently, H₂O₂ is reduced by MtrCAB to facilitate DmsEFAB-mediated extracellular reduction of iodate. To investigate the distribution of bacteria with the capability for extracellular reduction of iodate, bacterial genomes were systematically searched for both dmsEFAB and mtrCAB gene clusters. The dmsEFAB and mtrCAB gene clusters were found in three Ferrimonas and 26 Shewanella species. Coexistence of both dmsEFAB and mtrCAB gene clusters in these bacteria suggests their potentials for extracellular reduction of iodate. Further analyses demonstrated that these bacteria were isolated from a variety of ecosystems, including the lakes, rivers, and subsurface rocks in East and Southeast Asia, North Africa, and North America. Importantly, most of the bacteria with both dmsEFAB and mtrCAB gene clusters were found in different marine environments, which ranged from the Arctic Ocean to Antarctic coastal marine environments as well as from the Atlantic Ocean to the Indian and Pacific Oceans. Widespread distribution of the bacteria with capability for extracellular reduction of iodate around the world suggests their significant importance in global biogeochemical cycling of iodine. The genetic organization of dmsEFAB and mtrCAB gene clusters also varied substantially. The identified mtrCAB gene clusters often contained additional genes for multiheme c-type cytochromes. The numbers of dmsEFAB gene cluster detected in a given bacterial genome ranged from one to six. In latter, duplications of dmsEFAB gene clusters occurred. These results suggest different paths for these bacteria to acquire their capability for extracellular reduction of iodate.

The Roles of DmsEFAB and MtrCAB in Extracellular Reduction of Iodate by Shewanella oneidensis MR-1 with Lactate as the Sole Electron Donor

To investigate their roles in extracellular reduction of iodate (IO₃ ^- ) with lactate as an electron donor, the gene clusters of dmsEFAB, mtrCAB, mtrDEF, and so4360-4357 in Shewanella oneidensis MR-1were systematically deleted. Deletions of dmsEFAB and/or mtrCAB gene clusters diminished the bacterial ability to reduce IO₃ ^- . Furthermore, DmsEFAB and MtrCAB worked collaboratively to reduce IO₃ ^- of which DmsEFAB played a more dominant role than MtrCAB. MtrCAB was involved in detoxifying the reaction intermediate hydrogen peroxide (H₂ O₂ ). The reaction intermediate hypoiodous acid (HIO) was also found to inhibit microbial IO₃ ^- reduction. SO4360-4357 and MtrDEF, however, were not involved in IO₃ ^- reduction. Collectively, these results suggest a novel mechanism of extracellular reduction of IO₃ ^- at molecular level, in which DmsEFAB reduces IO₃ ^- to HIO and H₂ O₂ . The latter is further reduced to H₂ O by MtrCAB to facilitate the DmsEFAB-mediated IO₃ ^- reduction. The extracellular electron transfer pathway of S. oneidensis MR-1is believed to mediate electron transfer from bacterial cytoplasmic membrane, across the cell envelope to the DmsEFAB and MtrCAB on the bacterial outer membrane.

Mechanistic study on photochemical generation of I•/I2•- radicals in coastal atmospheric aqueous aerosol

The reactive iodine species may exhibit significant impacts on many global atmospheric issues and the I^•/I₂^•- radicals play key roles for inducing the formation of these reactive iodine species. However, the current understanding on the formation of I^•/I₂^•- radicals in atmospheric aqueous aerosol is still quite limited. The results reported herein suggest that I^•/I₂^•- can be produced simultaneously in aqueous aerosol by several sunlight-driven photochemical pathways including direct photo-dissociation of soluble organic iodine (SOI) at rates of 0.10-1.34 × 10^-9 M ns^-1 and 0.99-5.68 × 10^-7 M μs^-1, ^•OH-mediated oxidation of I^- at 0.03-1.41 × 10^-8 M ns^-1 and 0.05-4.10 × 10^-6 M μs^-1, and ³DOM^⁎-induced oxidation of I^- at 1.57-1.65 × 10^-9 M ns^-1 and 0.99-5.68 × 10^-7 M μs^-1 for generation of I^• and I₂^•-, respectively. Meanwhile, the pathway of e_aq^--initiated stepwise reduction of IO₃^- to I_2(aq) and further photolyzed into I^• plays negligible role in formation of I^•/I₂^•- due to the low reaction rates and severe quenching effect of e_aq^- by dissolved O₂. Our work presented the new data on mechanism and kinetics for comprehensive elucidation of I^•/I₂^•- formation in coastal atmospheric aqueous aerosol and would help to better understand the transformation mechanism of iodine species, pathways of iodine cycling and the associated environmental impacts involving atmospheric reactive iodine radicals.

Climate changes modulated the history of Arctic iodine during the Last Glacial Cycle

Iodine has a significant impact on promoting the formation of new ultrafine aerosol particles and accelerating tropospheric ozone loss, thereby affecting radiative forcing and climate. Therefore, understanding the long-term natural evolution of iodine, and its coupling with climate variability, is key to adequately assess its effect on climate on centennial to millennial timescales. Here, using two Greenland ice cores (NEEM and RECAP), we report the Arctic iodine variability during the last 127,000 years. We find the highest and lowest iodine levels recorded during interglacial and glacial periods, respectively, modulated by ocean bioproductivity and sea ice dynamics. Our sub-decadal resolution measurements reveal that high frequency iodine emission variability occurred in pace with Dansgaard/Oeschger events, highlighting the rapid Arctic ocean-ice-atmosphere iodine exchange response to abrupt climate changes. Finally, we discuss if iodine levels during past warmer-than-present climate phases can serve as analogues of future scenarios under an expected ice-free Arctic Ocean. We argue that the combination of natural biogenic ocean iodine release (boosted by ongoing Arctic warming and sea ice retreat) and anthropogenic ozone-induced iodine emissions may lead to a near future scenario with the highest iodine levels of the last 127,000 years.