Cycling Of Iodine Research Articles

Impact of abiotic and biogeochemical processes on halogen concentrations (Cl, Br, F, I) in mineral soil along a climatic gradient.

In contrast to earlier ideas that halogens behave inertly in soil, extensive biogeochemical cycling of fluorine (F), chlorine (Cl), bromine (Br) and iodine (I) has been shown for temperate forests. To further advance our understanding of halogen behaviour in soil beyond humid temperate forests, we sampled soil profiles in protected areas along the Chilean Coastal Cordillera, representing a pronounced climatic gradient spanning from arid to humid. Halogen concentrations in soil were analysed by combustion ion chromatography. Highest average total halogen concentrations occurred at the arid site (Cl, F: 4270 and 897 mg kg-1) as well as the humid end of the climatic gradient (Br, I: 42.6 and 9.8 mg kg-1). Vertical distribution patterns of halogens were most pronounced at the humid end of the gradient and became less distinct under drier climate. The climatic gradient demonstrates the important role of biotic processes (e.g. the halogenation of organic matter) on the retention of halogens in the soil. However, this climate-specific role may be overridden by mainly abiotic processes within a given climate zone (e.g. weathering, leaching, sorption to secondary soil minerals, evaporative enrichment), resulting in vertical relocation of halogens in the soil. Since some of these processes oppose each other, complex interactions and depth distributions of F, Cl, Br and I occur in the soil. In summary, our findings provide new insights into the fate of halogens in mineral soil of different climatic zones, which is important, for example, when radiohalogens are deposited on a large scale after nuclear accidents.

Theoretical treatment of IO-X (X = N2, CO, CO2, H2O) complexes.

Iodine monoxide (IO) is an important component of the biogeochemical cycle of iodine. For instance, it is present in the troposphere, where it plays a crucial role in the physical chemical processes involving iodine containing compounds. Here, we present a theoretical study on a series of atmospherically relevant complexes of IO with N2, CO, CO2 and H2O, where their structural and spectroscopic properties and their interaction energies are computed. Calculations are carried out by means of ab initio post Hartree-Fock (RCCSD(T) and RMP2) methods and density functional theory DFT (PBE0 and M05-2X) based approaches with and without the inclusion of dispersion correction. After comparison to RCCSD(T), we highlight the good performance of M05-2X(+D3) DFT in describing the bonding between IO and X (X = N2, CO, CO2, H2O). Moreover, we found that the IO-X (X = N2, CO, CO2, H2O) complexes are formed by non-covalent interactions between the two monomers. In sum, we characterized two types of complexes: I-bonded and O-bonded, where the former is more stable. The atmospheric implications of the present findings are also discussed such as in the formation of the iodine oxide particles (IOPs).

Oxidation of iodide to iodate by cultures of marine ammonia-oxidising bacteria

Reaction with iodide (I−) at the sea surface is an important sink for atmospheric ozone, and causes sea-air emission of reactive iodine which in turn drives further ozone destruction. To incorporate this process into chemical transport models, improved understanding of the factors controlling marine iodine speciation, and especially sea-surface iodide concentrations, is needed. The oxidation of I− to iodate (IO3−) is the main sink for oceanic I−, but the mechanism for this remains unknown. We demonstrate for the first time that marine nitrifying bacteria mediate I− oxidation to IO3−. A significant increase in IO3− concentrations compared to media-only controls was observed in cultures of the ammonia-oxidising bacteria Nitrosomonas sp. (Nm51) and Nitrosoccocus oceani (Nc10) supplied with 9–10 mM I−, indicating I− oxidation to IO3−. Cell-normalised production rates were 15.69 (±4.71) fmol IO3− cell−1 d−1 for Nitrosomonas sp., and 11.96 (±6.96) fmol IO3− cell−1 d−1 for Nitrosococcus oceani, and molar ratios of iodate-to-nitrite production were 9.2 ± 4.1 and 1.88 ± 0.91 respectively. Preliminary experiments on nitrite-oxidising bacteria showed no evidence of I− to IO3− oxidation. If the link between ammonia and I− oxidation observed here is representative, our ocean iodine cycling model predicts that future changes in marine nitrification could alter global sea surface I− fields with potential implications for atmospheric chemistry and air quality.

Direct field evidence of autocatalytic iodine release from atmospheric aerosol

Reactive iodine plays a key role in determining the oxidation capacity, or cleansing capacity, of the atmosphere in addition to being implicated in the formation of new particles in the marine boundary layer. The postulation that heterogeneous cycling of reactive iodine on aerosols may significantly influence the lifetime of ozone in the troposphere not only remains poorly understood but also heretofore has never been observed or quantified in the field. Here, we report direct ambient observations of hypoiodous acid (HOI) and heterogeneous recycling of interhalogen product species (i.e., iodine monochloride [ICl] and iodine monobromide [IBr]) in a midlatitude coastal environment. Significant levels of ICl and IBr with mean daily maxima of 4.3 and 3.0 parts per trillion by volume (1-min average), respectively, have been observed throughout the campaign. We show that the heterogeneous reaction of HOI on marine aerosol and subsequent production of iodine interhalogens are much faster than previously thought. These results indicate that the fast formation of iodine interhalogens, together with their rapid photolysis, results in more efficient recycling of atomic iodine than currently considered in models. Photolysis of the observed ICl and IBr leads to a 32% increase in the daytime average of atomic iodine production rate, thereby enhancing the average daytime iodine-catalyzed ozone loss rate by 10 to 20%. Our findings provide direct field evidence that the autocatalytic mechanism of iodine release from marine aerosol is important in the atmosphere and can have significant impacts on atmospheric oxidation capacity.

The formation of marine red beds and iron cycling on the Mesoproterozoic North China Platform

Abstract Marine red beds (MRBs) are common in sedimentary records, but their genesis and environmental implications remain controversial. Genetic models proposed for MRBs variably invoke diagenetic or primary enrichments of iron, with vastly different implications for the redox state of the contemporaneous water column. The Xiamaling Formation (ca. 1.4 Ga) in the North China Platform hosts MRBs that offer insights into the iron cycling and redox conditions during the Mesoproterozoic Era. In the Xiamaling MRBs, well-preserved, nanometer-sized flaky hematite particles are randomly dispersed in the clay (illite) matrix, within the pressure shadow of rigid detrital grains. The presence of hematite flake aggregates with multiple face-to-edge (“cardhouse”) contacts indicates that the hematite particles were deposited as loosely bound, primary iron oxyhydroxide flocs. No greenalite or other ferrous iron precursor minerals have been identified in the MRBs. Early diagenetic ankerite concretions hosted in the MRBs show non-zero I/(Ca+Mg) values and positive Ce anomalies (>1.3), suggesting active redox cycling of iodine and manganese and therefore the presence of molecular oxygen in the porewater and likely in the water column during their formation. These observations support the hypothesis that iron oxyhydroxide precipitation occurred in moderately oxygenated marine waters above storm wave base (likely <100 m). Continentally sourced iron reactivated through microbial dissimilatory iron reduction, and distal hydrothermal fluids may have supplied Fe(II) for the iron oxyhydroxide precipitation. The accumulation of the Xiamaling MRBs may imply a slight increase of seawater oxygenation and the existence of long-lasting adjacent ferruginous water mass.

A Global Model for Iodine Speciation in the Upper Ocean

AbstractAn ocean iodine cycling model is presented, which predicts upper ocean iodine speciation. The model comprises a three‐layer advective and diffusive ocean circulation model of the upper ocean and an iodine cycling model embedded within this circulation. The two primary reservoirs of iodine are represented, iodide and iodate. Iodate is reduced to iodide in the mixed layer in association with primary production, linked by an iodine to carbon (I:C) ratio. A satisfactory model fit with observations cannot be obtained with a globally constant I:C ratio, and the best fit is obtained when the I:C ratio is dependent on sea surface temperature, increasing at low temperatures. Comparisons with observed iodide distributions show that the best model fit is obtained when oxidation of iodide back to iodate is associated with mixed layer nitrification. Sensitivity tests, where model parameters and processes are perturbed, reveal that primary productivity, mixed layer depth, oxidation, advection, surface freshwater flux, and the I:C ratio all have a role in determining surface iodide concentrations, and the timescale of iodide in the mixed layer is sufficiently long for nonlocal processes to be important. Comparisons of the modeled iodide surface field with parameterizations by other authors show good agreement in regions where observations exist but significant differences in regions without observations. This raises the question of whether the existing parameterizations are capturing the full range of processes involved in determining surface iodide and shows the urgent need for observations in regions where there are currently none.

Surface Inorganic Iodine Speciation in the Indian and Southern Oceans From 12°N to 70°S

Marine iodine speciation has emerged as a potential tracer of primary productivity, sedimentary inputs, and ocean oxygenation. The reaction of iodide with ozone at the sea surface has also been identified as the largest deposition sink for tropospheric ozone and the dominant source of iodine to the atmosphere. Accurate incorporation of these processes into atmospheric models requires improved understanding of iodide concentrations at the air-sea interface. Observations of sea surface iodide are relatively sparse and are particularly lacking in the Indian Ocean basin. Here we examine 127 new sea surface (≤10 m depth) iodide and iodate observations made during three cruises in the Indian Ocean and the Indian sector of the Southern Ocean. The observations span latitudes from ∼12°N to ∼70°S, and include three distinct hydrographic regimes: the South Indian subtropical gyre, the Southern Ocean and the northern Indian Ocean including the southern Bay of Bengal. Concentrations and spatial distribution of sea surface iodide follow the same general trends as in other ocean basins, with iodide concentrations tending to decrease with increasing latitude (and decreasing sea surface temperature). However, the gradient of this relationship was steeper in subtropical waters of the Indian Ocean than in the Atlantic or Pacific, suggesting that it might not be accurately represented by widely used parameterizations based on sea surface temperature. This difference in gradients between basins may arise from differences in phytoplankton community composition and/or iodide production rates. Iodide concentrations in the tropical northern Indian Ocean were higher and more variable than elsewhere. Two extremely high iodide concentrations (1241 and 949 nM) were encountered in the Bay of Bengal and are thought to be associated with sedimentary inputs under low oxygen conditions. Excluding these outliers, sea surface iodide concentrations ranged from 20 to 250 nM, with a median of 61 nM. Controls on sea surface iodide concentrations in the Indian Ocean were investigated using a state-of-the-art iodine cycling model. Multiple interacting factors were found to drive the iodide distribution. Dilution via vertical mixing and mixed layer depth shoaling are key controls, and both also modulate the impact of biogeochemical iodide formation and loss processes.

Senescence as the main driver of iodide release from a diverse range of marine phytoplankton

Abstract. The reaction between ozone and iodide at the sea surface is now known to be an important part of atmospheric ozone cycling, causing ozone deposition and the release of ozone-depleting reactive iodine to the atmosphere. The importance of this reaction is reflected by its inclusion in chemical transport models (CTMs). Such models depend on accurate sea surface iodide fields, but measurements are spatially and temporally limited. Hence, the ability to predict current and future sea surface iodide fields, i.e. sea surface iodide concentration on a narrow global grid, requires the development of process-based models. These models require a thorough understanding of the key processes that control sea surface iodide. The aim of this study was to explore if there are common features of iodate-to-iodide reduction amongst diverse marine phytoplankton in order to develop models that focus on sea surface iodine and iodine release to the troposphere. In order to achieve this, rates and patterns of changes in inorganic iodine speciation were determined in 10 phytoplankton cultures grown at ambient iodate concentrations. Where possible these data were analysed alongside results from previous studies. Iodate loss and some iodide production were observed in all cultures studied, confirming that this is a widespread feature amongst marine phytoplankton. We found no significant difference in log-phase, cell-normalised iodide production rates between key phytoplankton groups (diatoms, prymnesiophytes including coccolithophores and phaeocystales), suggesting that a phytoplankton functional type (PFT) approach would not be appropriate for building an ocean iodine cycling model. Iodate loss was greater than iodide formation in the majority of the cultures studied, indicating the presence of an as-yet-unidentified “missing iodine” fraction. Iodide yield at the end of the experiment was significantly greater in cultures that had reached a later senescence stage. This suggests that models should incorporate a lag between peak phytoplankton biomass and maximum iodide production and that cell mortality terms in biogeochemical models could be used to parameterise iodide production.

Experimental observations of marine iodide oxidation using a novel sparge-interface MC-ICP-MS technique

Iodine speciation is used as a redox indicator and water mass tracer. However, the kinetics and mechanisms of oxidation of the reduced iodine species, iodide (I−), in seawater remain largely unresolved. Here, we provide constraints on I− oxidation rates from seawater incubation experiments using a sensitive radiotracer–incubation approach with high-precision mass spectrometry. Samples are inoculated with iodine-129 as I−, incubated under controlled conditions, then oxidized and reduced iodine species are isolated using liquid chromatographic separation. Samples are subsequently aspirated using a novel sparging interface, and the 129I/127I ratio of both iodine species is determined via multi-collector inductively-coupled plasma mass spectrometry. This highly-sensitive analytical approach is optimized for estimating the predicted slow rates of iodine redox transformations in small volume seawater samples at marine-relevant total iodine concentrations. Application of this method to coastal samples from Martha's Vineyard Sound, USA, evidences the heretofore-elusive oxidation of I− under marine-analogue conditions. Our results demonstrate an increase in the 129I/127I of aqueous oxidized iodine species in unamended seawater, most likely due to formation of hypoiodous acid and then dissolved organic iodine, at rates of 118–189 nM/ yr. These rates are comparable to those inferred from previous open ocean mass balance approaches but have the benefit of directly linking iodine speciation changes to in situ I− oxidation. Though our treatment experiments do not definitively isolate the oxidation mechanism, they provide preliminary insight into important roles for both abiotic and biotic factors as well as iodine transformations between dissolved, particulate, and gaseous forms. Ultimately, the derived rates and novel experimental technique have potential to improve marine iodine cycling models and provide constraints on the importance of in situ processes in setting patterns of iodine speciation.

Diurnal cycle of iodine, bromine, and mercury concentrations in Svalbard surface snow

Abstract. Sunlit snow is highly photochemically active and plays a key role in the exchange of gas phase species between the cryosphere and the atmosphere. Here, we investigate the behaviour of two selected species in surface snow: mercury (Hg) and iodine (I). Hg can deposit year-round and accumulate in the snowpack. However, photo-induced re-emission of gas phase Hg from the surface has been widely reported. Iodine is active in atmospheric new particle formation, especially in the marine boundary layer, and in the destruction of atmospheric ozone. It can also undergo photochemical re-emission. Although previous studies indicate possible post-depositional processes, little is known about the diurnal behaviour of these two species and their interaction in surface snow. The mechanisms are still poorly constrained, and no field experiments have been performed in different seasons to investigate the magnitude of re-emission processes Three sampling campaigns conducted at an hourly resolution for 3 d each were carried out near Ny-Ålesund (Svalbard) to study the behaviour of mercury and iodine in surface snow under different sunlight and environmental conditions (24 h darkness, 24 h sunlight and day–night cycles). Our results indicate a different behaviour of mercury and iodine in surface snow during the different campaigns. The day–night experiments demonstrate the existence of a diurnal cycle in surface snow for Hg and iodine, indicating that these species are indeed influenced by the daily solar radiation cycle. Differently, bromine did not show any diurnal cycle. The diurnal cycle also disappeared for Hg and iodine during the 24 h sunlight period and during 24 h darkness experiments supporting the idea of the occurrence (absence) of a continuous recycling or exchange at the snow–air interface. These results demonstrate that this surface snow recycling is seasonally dependent, through sunlight. They also highlight the non-negligible role that snowpack emissions have on ambient air concentrations and potentially on iodine-induced atmospheric nucleation processes.

Using organic compounds to assess the dominant controls on seasonal iodine variability in the brown alga Ecklonia cava in the northwestern Pacific coast of central Japan

Brown algal species are among the most efficient iodine accumulators and thus heavily influence the biogeochemical cycling of iodine in coastal environments. Previously, we reported seasonal variability in iodine concentrations in the brown alga Ecklonia cava collected from the Japanese coast over a 2-year period. In this study, we sought to identify the factors controlling this observed variability by investigating the relationships between the concentrations of iodine and major organic compounds (carbohydrates and fatty acids). The concentrations of total carbohydrates and individual constituent monosaccharides showed no significant correlations with iodine concentration, implying that carbohydrate concentration and composition are not the dominant controls on iodine concentration in E. cava. However, fucose concentrations were significantly correlated when data points of highest iodine concentrations were removed, potentially suggesting some influence of structural carbohydrate on iodine variability. Significant positive correlations with iodine concentration were only observed in odd-chain fatty acids, which are known to be bacterial biomarkers; in particular, the C17:1 fatty acid showed the highest correlation (r = 0.83). The concentrations of the remaining fatty acids, as well as total concentration of fatty acids, did not correlate significantly with iodine concentrations. Our results suggest that, in general, the concentration and composition of fatty acids do not directly influence iodine variability and that iodine concentrations in E. cava are predominantly controlled by bacteria living on its surface.

Metal Reduction and Protein Secretion Genes Required for Iodate Reduction by Shewanella oneidensis

The metal-reducing gammaproteobacterium Shewanella oneidensis reduces iodate (IO3 -) as an anaerobic terminal electron acceptor. Microbial IO3 - electron transport pathways are postulated to terminate with nitrate (NO3 -) reductase, which reduces IO3 - as an alternative electron acceptor. Recent studies with S. oneidensis, however, have demonstrated that NO3 - reductase is not involved in IO3 - reduction. The main objective of the present study was to determine the metal reduction and protein secretion genes required for IO3 - reduction by Shewanella oneidensis with lactate, formate, or H2 as the electron donor. With all electron donors, the type I and type V protein secretion mutants retained wild-type IO3 - reduction activity, while the type II protein secretion mutant lacking the outer membrane secretin GspD was impaired in IO3 - reduction. Deletion mutants lacking the cyclic AMP receptor protein (CRP), cytochrome maturation permease CcmB, and inner membrane-tethered c-type cytochrome CymA were impaired in IO3 - reduction with all electron donors, while deletion mutants lacking c-type cytochrome MtrA and outer membrane β-barrel protein MtrB of the outer membrane MtrAB module were impaired in IO3 - reduction with only lactate as an electron donor. With all electron donors, mutants lacking the c-type cytochromes OmcA and MtrC of the metal-reducing extracellular electron conduit MtrCAB retained wild-type IO3 - reduction activity. These findings indicate that IO3 - reduction by S. oneidensis involves electron donor-dependent metal reduction and protein secretion pathway components, including the outer membrane MtrAB module and type II protein secretion of an unidentified IO3 - reductase to the S. oneidensis outer membrane.IMPORTANCE Microbial iodate (IO3 -) reduction is a major component in the biogeochemical cycling of iodine and the bioremediation of iodine-contaminated environments; however, the molecular mechanism of microbial IO3 - reduction is poorly understood. Results of the present study indicate that outer membrane (type II) protein secretion and metal reduction genes encoding the outer membrane MtrAB module of the extracellular electron conduit MtrCAB are required for IO3 - reduction by S. oneidensis On the other hand, the metal-reducing c-type cytochrome MtrC of the extracellular electron conduit is not required for IO3 - reduction by S. oneidensis These findings indicate that the IO3 - electron transport pathway terminates with an as yet unidentified IO3 - reductase that associates with the outer membrane MtrAB module to deliver electrons extracellularly to IO3.

Iodine distribution and cycling in a beech (Fagus sylvatica) temperate forest

Radioiodine is of health concerns in case of nuclear events. Possible pathways and rates of flow are essential information for risk assessment. Forest ecosystems could influence the global cycle of long-lived radioiodine isotope (129I) with transfer processes similar to stable isotope (127I). Understanding iodine cycling in forest involves study of the ecosystem as a whole. In this context, we determined the 127I contents and distribution in soil, tree compartments and atmospheric inputs during a three years in situ monitoring of a temperate beech forest stand. The iodine cycle was first characterized in terms of stocks by measuring its concentrations in: tree, litterfall, humus, soil, rainfall, throughfall, stemflow and soil solutions. Main annual fluxes (requirement, uptake and internal transfers) and forest input-output budget were also estimated using conceptual model calculations. Our findings show that: (i) soil is the main I reservoir accounting for about 99.9% of ecosystem total stock; (ii) iodine uptake by tree represents a minor fraction of the available pool in soil (<0.2%); (iii) iodine allocation between tree compartments involves low immobilization in wood and restricted location in the roots; (iv) translocation of excess iodine towards senescing foliage appears as an elimination process for trees, and (v) litterfall is a major pathway in the I biological cycling. In our soil conditions, the input – output budget shows that the ecosystem behaves as a potential source of I for groundwater.

Characterization of Iodine-Related Molecular Processes in the Marine Microalga Tisochrysis lutea (Haptophyta)

Iodine metabolism is essential for the antioxidant defense of marine algae and in the biogeochemical cycle of iodine. Moreover, some microalgae can synthetize thyroid hormone-like compounds that are essential to sustain food webs. However, knowledge regarding iodine-related molecular processes in microalgae is still scarce. In this study, a de novo transcriptome of Tisochrysis lutea cultured under high iodide concentrations (5 mM) was assembled using both long and short reads. A database termed IsochrysisDB was established to host all genomic information. Gene expression analyses during microalgal growth showed that most of the antioxidant- (aryl, ccp, perox, sod1, sod2, sod3, apx3, ahp1) and iodide-specific deiodinase (dio) genes increased their mRNA abundance progressively until the stationary phase to cope with oxidative stress. Moreover, the increase of dio mRNA abundance in aging cultures indicated that this enzyme was also involved in senescence. Cell treatments with iodide modified the expression of perox whereas treatments with iodate changed the transcript levels of gpx1 and ccp. To test the dependence of perox on iodide, microalgae cells were treated with hydrogen peroxide (H2O2) either in presence or absence of iodide observing that several genes related to reactive oxygen species deactivation (perox, gpx1, apx2, apx3, ahp1, ahp2, sod1, sod3 and aryl) were transcriptionally activated although with some temporal differences. However, only the expression of perox was dependent on iodide levels indicating this enzyme, acquired by horizontal gene transfer, could act as a haloperoxidase. All these data indicate that T. lutea activates coordinately the expression of antioxidant genes to cope with oxidative stress. The identification of a phase-regulated deiodinase and a novel haloperoxidase provide new clues about the origin and evolution of thyroid signaling and the antioxidant role of iodine in the marine environment.

Iodate and nitrate transformation by Agrobacterium/Rhizobium related strain DVZ35 isolated from contaminated Hanford groundwater

Nitrate and radioiodine (129I) contamination is widespread in groundwater underneath the Central Plateau of the Hanford Site. 129I, a byproduct of nuclear fission, is of concern due to a 15.7 million year half-life, and toxicity. The Hanford 200 West Area contains plumes covering 4.3 km2 with average 129I concentrations of 3.5 pCi/L. Iodate accounts for 70.6% of the iodine present and organo-iodine and iodide make up 25.8% and 3.6%, respectively. Nitrate plumes encompassing the 129I plumes have a surface area of 16 km2 averaging 130 mg/L. A nitrate and iodate reducing bacterium closely related to Agrobacterium, strain DVZ35, was isolated from sediment incubated in a 129I plume. Iodate removal efficiency was 36.3% in transition cultures, and 47.8% in anaerobic cultures. Nitrate (10 mM) was also reduced in the microcosm. When nitrate was spiked into the microcosms, iodate removal efficiency was 84.0% and 69.2% in transition and anaerobic cultures, respectively. Iodate reduction was lacking when nitrate was absent from the growth medium. These data indicate there is simultaneous reduction of nitrate and iodate by DVZ35, and iodate is reduced to iodide. Results provide the scientific basis for combined nitrogen and iodine cycling throughout the Hanford Site.

Microbial megacities fueled by methane oxidation in a mineral spring cave

Massive biofilms have been discovered in the cave of an iodine-rich former medicinal spring in southern Germany. The biofilms completely cover the walls and ceilings of the cave, giving rise to speculations about their metabolism. Here we report on first insights into the structure and function of the biofilm microbiota, combining geochemical, imaging and molecular analytics. Stable isotope analysis indicated that thermogenic methane emerging into the cave served as an important driver of biofilm formation. The undisturbed cavern atmosphere contained up to 3000 p.p.m. methane and was microoxic. A high abundance and diversity of aerobic methanotrophs primarily within the Methylococcales (Gammaproteobacteria) and methylotrophic Methylophilaceae (Betaproteobacteria) were found in the biofilms, along with a surprising diversity of associated heterotrophic bacteria. The highest methane oxidation potentials were measured for submerged biofilms on the cavern wall. Highly organized globular structures of the biofilm matrix were revealed by fluorescent lectin staining. We propose that the extracellular matrix served not only as an electron sink for nutrient-limited biofilm methylotrophs but potentially also as a diffusive barrier against volatilized iodine species. Possible links between carbon and iodine cycling in this peculiar habitat are discussed.

The potential role of kelp forests on iodine speciation in coastal seawater.

Kelps have a major role in marine and atmospheric iodine cycling in the coastal zone of temperate regions, with potential wide-ranging impacts on ozone destruction in the coastal marine boundary layer. However, little is known about the impact of kelp forests on iodine speciation in coastal sea water. To address this, we examined iodide and iodate concentrations in seawater in and around a giant kelp forest near San Diego, CA, USA, and a nearby site that was not influenced by kelp biology. Our data shows that while both iodide and iodate concentrations remained unchanged during the year at the nearby site, these concentrations changed significantly in and around the kelp forest, and were strongly related to changes in kelp canopy biomass. In particular, iodide reached its highest concentration and iodate reached its lowest concentration during the summer when the kelp canopies were near their maximum, while the opposite pattern was observed during the winter and spring when the kelp canopies were near their minimum. Further, comparisons of these changes with corresponding changes in seawater temperature and wind speed indicated that these relationships were relatively small compared to those with changes in kelp biomass. Together, our data show a strong relationship between kelp biomass and iodine metabolism.

Aging and Substrate Type Effects on Iodide and Iodate Accumulation by Barley (Hordeum vulgare L.)

The mechanism of iodine uptake by plants from soil is the interest of many studies, though the information about the uptake mechanism and iodine mobility in soil is still insufficient. The uptake of the most common naturally occurring iodine species in soil—iodide and iodate by barley (Hordeum vulgare L.) from iodine-spiked soil and agar cultivation media was evaluated in our study based on laboratory pot and petri dish experiments. Soil pot experiments were conducted in two stages: after iodine spiking and after a 3-month aging period taking account on the processes that can influence iodine bioavailability. Our results indicate that iodine bioavailability does depend on the form of iodine present in the growth substrate, the character of the substrate itself, and the time between iodine application and plant cultivation. The loss of iodine from soil via volatilization can also be a limiting factor to iodine bioavailability. Our results provide additional information on iodine mobility and behavior in soil and suggest plants’ essential role in biogeochemical cycle of iodine. We also highlighted importance of iodine source and speciation on the base level of the food chain where iodine mobility and bioavailability is limited by its chemical form, substrate characteristics, and intensity of natural biogeochemical processes related to iodine biotransformation.

Cycling of iodine by microalgae: Iodine uptake and release by a microalgae biofilm in a groundwater holding pond

Biological iodine cycling has been widely studied in marine environments, but rarely considered in terrestrial waters. In this work, a microalgae biofilm, with an iodine content of 350±29mgkg−1, is shown to potentially be a significant iodine cycler in a groundwater holding pond. Cultivation of the biofilm in extracted groundwater was carried out at bench scale. In parallel, iodine release from an iodine-rich biofilm kept in the dark was monitored. Iodine uptake and release were found to be proportional to algal growth and decay, with the maximum growth rate and specific decay rate being 0.53±0.05gm−2d−1 and 0.12±0.02d−1 respectively. Iodine update and release in an engineered groundwater holding pond (1m3:1m2×1m) was simulated. The results indicated that bioaccumulation causes non-steady state conditions in such ponds, and in extreme circumstances decay events can elevate iodine concentrations to problematic levels.

Iodine balance, growth and biochemical composition of three marine microalgae cultured under various inorganic iodine concentrations

Marine phytoplankton have been found to be involved in the biogeochemical cycle of iodine in the ocean. However, the role of iodine in the metabolism of marine microalgae has hardly been studied and whether iodine is an element required for growth is not known. In this study, three species of marine microalgae were grown in batch cultures at various concentrations of iodide (I−) or iodate (IO3 −) in order to evaluate their effect on growth rate and biochemical composition. Changes in inorganic iodine concentrations in the culture medium were also determined. Growth rate was not affected by external inorganic iodine concentration in cultures of Dunaliella salina and Tisochrysis lutea. In cultures of Phaeodactylum tricornutum, growth was inhibited at 2500 µM IO3 −. No clear effects of I− or IO3 − were observed on biochemical composition, except for the particulate iodine content of the microalgae, which increased with increasing dissolved iodine concentration. Results of dissolved inorganic iodine measurements indicated that IO3 − was reduced to I− during the exponential growth phase of microalgae cultures, under both natural and higher IO3 − concentrations. Iodine balance calculations showed that the amount of disappeared IO3 − was higher than the amount of I− produced in several cultures and that this discrepancy could not be explained by the formation of particulate iodine. I incorporation rates into biomass were low compared to I− production rates. We conclude that although iodine is clearly taken up from the external medium and it is transformed by microalgae, it seems unlikely that it is an element required for growth or that it plays an essential role in microalgal metabolism.