Impact of cyanogen iodide in killing of Escherichia coli by the lactoperoxidase-hydrogen peroxide-(pseudo)halide system

In the presence of hydrogen peroxide, the heme protein lactoperoxidase is able to oxidize thiocyanate and iodide to hypothiocyanite, reactive iodine species, and the inter(pseudo)halogen cyanogen iodide. The killing efficiency of these oxidants and of the lactoperoxidase-H₂O₂-SCN^?/I^? system was investigated on the bioluminescent Escherichia coli K12 strain that allows time-resolved determination of cell viability. Among the tested oxidants, cyanogen iodide was most efficient in killing E. coli, followed by reactive iodine species and hypothiocyanite. Thereby, the killing activity of the LPO-H₂O₂-SCN^?/I^? system was greatly enhanced in comparison to the sole application of iodide when I^? was applied in two- to twenty-fold excess over SCN^?. Further evidence for the contribution of cyanogen iodide in killing of E. coli was obtained by applying methionine. This amino acid disturbed the killing of E. coli mediated by reactive iodine species (partial inhibition) and cyanogen iodide (total inhibition), but not by hypothiocyanite. Changes in luminescence of E. coli cells correlate with measurements of colony forming units after incubation of cells with the LPO-H₂O₂-SCN^?/I^? system or with cyanogen iodide. Taken together, these results are important for the future optimization of the use of lactoperoxidase in biotechnological applications.     

Propranolol has direct antithyroid activity: Inhibition of iodide transport in cultured thyroid follicles

The effect of propranolol on the process of thyroid hormone formation was studied in a physiological culture system. Porcine thyroid follicles were preincubated with propranolol for 24 h. Iodide transport, iodine organification, and de novo thyroid hormone formation were measured by incubating these follicles with the mixture of carrier-free 0·1 μCi Na ¹²⁵I and 50 nM NaI for 2 to 6 h at 37°C. A concentration of propranolol greater than 100 μM inhibited iodide transport in a dose-dependent manner; this inhibition was non-competitive with iodide and independent of thyrotropin (TSH). Reduced iodine organification and thyroid hormone formation was seen with 150 μM propranolol or greater. The inhibitory action of propranolol was not caused by beta-blocking activity, since D -propranolol (devoid of beta-blocking activity) inhibited iodide transport, and other beta-blockers (metoprolol, atenolol, and labetalol) did not inhibit iodide transport. The inhibition of iodide transport was most likely caused by membrane stabilizing activity since quinidine, which possess the same membrane stabilizing activity as propranolol, also inhibited iodide transport. TSH-mediated cAMP generation and Na ⁺K⁺ ATPase activity, membrane functions for iodide transport, were unaffected by propranolol. Our study has shown, for the first time, that propranolol has a direct antithyroid action, namely inhibition of iodide transport in the intact thyroid follicle.     

Availability of iodide and iodate to spinach (Spinacia oleracea L.) in relation to total iodine in soil solution

A greenhouse pot experiment was carried out to investigate the availability of iodide and iodate to soil-grown spinach (Spinacia oleracea L.) in relation to total iodine concentration in soil solution. Four iodine concentrations (0, 0.5, 1, 2 mg kg⁻¹) for iodide (I⁻) and iodate (IO₃⁻) were used. Results showed that the biomass productions of spinach were not significantly affected by the addition of iodate and iodide to the soil, and that iodine concentrations in spinach plants on the basis of fresh weights increased with increasing addition of iodine. Iodine concentrations in tissues were much greater for plants grown with iodate than with iodide. In contrast to the iodide treatments, in iodate treatment leaves accounted for a larger fraction of the total plant iodine. The soil-to-leaf transfer factors (TF_leaf) for plants grown with iodate were about tenfold higher than those grown with iodide. Iodine concentrations in soil solution increased with increasing iodine additions to the soil irrespective of iodine species. However, total iodine in soil solution was generally higher for iodate treatments than iodide both in pots with and without spinach. According to these results, iodate can be considered as potential iodine fertilizer to increase iodine content in vegetables.     

Evaluation of Iodide and Iodate for Adsorption–Desorption Characteristics and Bioavailability in Three Types of Soil

Adsorption–desorption of iodine in two forms, viz., iodide (I⁻) and iodate (IO₃⁻), in three types of soil were investigated. The soils were: red soil developed on Quaternary red earths (REQ)— clayey, kaolintic thermic plinthite Aquult, Inceptisol soil (IS) and alluvial soil (AS)—Fluvio-marine yellow loamy soil. The isothermal curves of iodine adsorption on soils were described by Langmuir and Freundlich equation, and the maximum adsorption values (y _m) were obtained from the simple Langmuir model. As compared with the iodide, the iodate was adsorbed in higher amounts by the soils tested. Among three soils, the REQ soil adsorbed more iodine (I⁻ and IO₃⁻) than the IS and AS. The distribution coefficient (K _d) of iodine in the soils decreased exponentially with increasing iodine loading concentration. Desorption of iodine in soil was increased correspondingly with increasing adsorption values. The REQ soil had a greater affinity for iodine than the IS and AS at the same iodine loadings. In the pot experiment cultivated with pakchoi (Brassica chinensis L.) and added with two exogenous iodine sources, the iodide form was quickly taken up by pakchoi and caused more toxicity to the vegetable. The rate of iodine loss from soil was higher for iodide form as compared with the iodate. The iodine bioavailability was the highest but the persistence was the weakest in AS among the three soils tested, and the REQ soil showed just the opposite trend to that of the AS soil. This study is of theoretical importance to understand the relationship between iodine adsorption–desorption characteristics and their bioavailability in different soils and it also has practical implications for seeking effective alternatives of iodine biofortification to prevent iodine deficiency disorders.     

Reduction of iodate in iodated salt to iodide during cooking with iodine as measured by an improved HPLC/ICP–MS method

Background: Iodate is a strong oxidant, and some animal studies indicate that iodate intake may cause adverse effects. A key focus of the safety assessment of potassium iodate as a salt additive is determining whether iodate is safely reduced to iodide in food. Objective: To study the reduction of iodate in table salt to iodide and molecular iodine during cooking. Materials and Methods: Fifteen food samples cooked with and without iodated salt were prepared in duplicate. The iodine in the cooked food was extracted with deionized water. The iodine species in the extracts were determined by using an improved high-performance liquid chromatography/inductively coupled plasma–mass spectrometry (HPLC/ICP–MS). The cooking temperature and the pH of the food were determined. Results: The conversion rate of iodate in iodated salt to iodide and molecular iodine was 96.4%±14.7% during cooking, with 86.8%±14.5% of the iodate converted to iodide ions and 9.6% ±6.2% converted to molecular iodine to lose. The limit of detection, limit of quantification, relative standard deviation and recovery rate of the method HPLC/ICP–MS were 0.70 μg/L for I⁻ (0.69 μg/L for IO₃⁻), 2.10 μg/L for I⁻ (2.06 μg/L for IO₃⁻), 2.6% and 101.6%±2.6%, respectively. Conclusion: Almost all iodate added to food was converted into iodide and molecular iodine during cooking. The improved HPLC/ICP–MS was reliable in the determination of iodine species in food extracts.     

Synthesis of [3,5-125I]triiodo-l-thyronine of high specific activity

Two methods for the synthesis of [3,5-¹²⁵I]triiodo-l-thyronine of high specific activity are described. This triiodthyronine which carries the iodine label exclusively in the nonphenolic ring has not been available so far. Both methods start from [3,5-¹²⁵I]diiodo-l-thyronine which is iodinated either with iodine in potassium iodide or with iodide and chloramine T. The concentration of the iodinating agent is critical in both methods and the pH of the reaction mixture must be high enough (～11) to cause complete ionization of the phenolic group of the substrate. The triiodothyronine obtained in over 70% yield is purified by ion-exchange chromatography.     

Flow‐injection method for the determination of iodide/iodine using Ru(bpy)33+–NADH chemiluminescence detection

A two‐channel flow‐injection (FI) method is reported for the determination of iodide and iodine by its enhancement effect on the Ru(bpy)₃³⁺–NADH chemiluminescence (CL) system. The limit of detection (3 s of blank) was 1.0 × 10^–9 mol/L iodide/iodine, with a sample throughput of 60/h. The calibration graphs over the range 1.0–50 × 10^–8 mol/L gave correlation coefficients of 0.9994 and 0.999 (n = 5) with relative standard deviations (RSD; n = 4) of 1.0–2.5%, respectively. The effects of interfering cations, anions and some organic compounds were also studied. The method was applied to iodized salts and pharmaceutical samples and the results obtained were in good agreement with the value quoted. The CL method developed was compared with spectrophotometric method. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of lodine in Various Formulations on the Growth of Barley and Pea Plants in Nutrient Solution Culture

The forms of iodine added to cultures of barley were potassiumiodide, potassium iodate, potassium periodate, and iodoaceticacid at iodine concentrations of 1.0 ppm and 10.0 ppm. Withpea, only iodide and iodate at 1.0 ppm iodine concentrationwere used. For both species, comparisons were made with culturesto which no iodine was added. In barley, growth was increased by 1.0 ppm iodine, the relativeeffectiveness of the different formulations being in the order:iodoacetic acid > iodide > iodate > periodate. With10.0 ppm, iodide and iodoacetic acid treatments gave reducedgrowth, iodate was without effect, and periodate enhanced growth. In pea, 1.0 ppm iodine was inhibitory, iodide being more toxicthan iodate. Analysis of dry matter showed iodine content according to treatmentto be in the order: iodide > iodoacetic acid > iodate> periodate     

Thyroid autoregulation. Inhibitory effects of iodinated derivatives of arachidonic acid on iodine metabolism

Thyroid autoregulation has been linked to an organified iodocompound. Since several iodolipids are produced by the gland their possible role in thyroid autoregulation was examined. The following pure synthetic compounds were prepared: 1) 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-OH-A); 2) its omega lactone (IL-ω);3)5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid delta lactone (IL-δ). Their action on iodine metabolism was studied. Iodine uptake was measured in calf thyroid slices. At 10-⁴M I-OH-A caused a 64% decrease in the T/M ratio while IL-ω inhibited it by 36% and IL-δ was without effect. At 10⁻⁵M the inhibition was 44% for I-OH-A and 19% for IL-ω, while T₃ was without action. A possible isotopic dilution effect was excluded, and no change in iodine efflux was observed. The inhibition by I-OH-A of iodine uptake was observed after only 15 min preincubation. This compound alse decreased ¹²⁵I accumulation in rats.In calf thyroid slice, I-OH-A at 10⁻⁴M, inhibited PB¹²⁵I formation by 80%, IL-ω 62% and IL-ω by 37% and arachidonic acid were without action. I-OH-A also caused a dose-dependent inhibition of TSH-stimulated iodide organification.The present results demonstrate, for the first time, that iodinated derivatives of arachidonic acid inhibition thyroid function and mimic the effect of iodine on thyroid autoregulation.     

Iodine in Raw and Pasteurized Milk of Dairy Cows Fed Different Amounts of Potassium Iodide

Relation between iodine (I) intake by lactating Holstein cows and iodine concentrations in raw and pasteurized milk were investigated. Four treatment groups with eight cows assigned to each treatment were fed a basal diet containing 0.534 mg I/kg alone or supplemented with potassium iodide at 2.5, 5 or 7.5 mg/kg in 7-week period. Iodine concentrations in raw milk increased with each increase in dietary I from 162.2 ng/ml for basal diet to 534.5, 559.8 and 607.5 ng/ml when 2.5, 5 and 7.5 mg/kg was fed as potassium iodide (P < 0.05). This trend was found for blood plasma and urine iodine concentration. Iodine supplementation had no significant effect on thyroidal hormones. high-temperature short-time (HTST) pasteurization process reduced I concentration. The mean iodine content found in the milk prior to heating processing was 466.0 ± 205.0 ng/ml, whereas for the processed milk this level was 349.5 ± 172.8 ng/ml. It was concluded that iodine supplementation above of NRC recommendation (0.5 mg/kg diet DM) resulted in significant increases in iodine concentrations in milk, although the effect of heating in HTST pasteurization process on iodine concentration was not negligible.     

Effects of potassium iodide on the growth and metabolite accumulation of two planktonic diatoms

In this study the effects of potassium iodide on the growth and metabolite accumulation of Nitzschia closterium (Ehr.) W. Smith and Phaedactylum tricornutum Bolin were investigated to assess its possible application to the mass culture of the two diatoms in open environment, extensive systems. The results indicated that supplementation of potassium iodide at a concentration of 1000 mg L⁻¹ resulted in a reduction of the induction phase in cultures of N. closterium and P. tricornutum and led to an increase in the accumulation of biomass and extracellular polymeric substances. Conversely, the addition of potassium iodide, at all concentrations tested, showed no obvious effect on the fatty acid profiles of the two diatoms, particularly in the content of eicosapentaenoic and decosahexaenoic acid. Potassium iodide was also found to inhibit the growth of Dunaliella salina, Cryptomonas sp. and Chlorella sp. at minimum inhibitory concentrations of 356.8, 475.9 and 696.2 mg L⁻¹, respectively. It also inhibited bacteria, including species isolated from the two diatom cultures, at a minimum concentration of 400 mg L⁻¹. These results suggest that potassium iodide is an effective agent for inhibiting the proliferation of certain flagellate and non-flagellate algae, and bacteria, thus forming a favorable environment for diatoms to proliferate and consequently improving accumulation of biomass and EPS. These properties of potassium iodide provide a possible solution for preventing contamination from flagellate and non-flagellate algae in mass culture of the two diatoms without causing significant changes in their fatty acid composition.     

Physicochemical studies of amylose and its derivatives in aqueous solutions: Thermodynamics of the iodine–triiodide complex

Thermodynamic properties of the amylose–iodine–triiodide complex have been studied by spectrophotometry and by calorimetry using previously studied samples of amylose ionic derivatives, carboxymethylamylose and diethylaminoethylamylose. The ratio of triiodide to total molecular iodine ([I₃]_b/[I]_b + [I₂]_b) in the complex is ca. 0.3 over a range of iodide concentration from 10^?5 to 10^?4M, and there is no evidence for an increasing charge at slightly higher iodide concentration. Direct calorimetric experiments have been carried out in different conditions of polymer, iodine, and iodide concentration in order to study the dependence of the heat of the complexation as a function of the above parameters. It is shown that the dependence of the measured ΔH on the iodide concentration simply derives from the rearrangement of the triiodide equilibrium because of the uptake of a fixed ratio of iodine and triiodide molecules in the complex.     

Influence of Incubation Conditions on Hydrolysis Efficiency and Iodine Enrichment in Baker’s Yeast

The influence of incubation conditions, enzyme type, hydrolysis time, and potassium iodide concentration on hydrolysis and iodine enrichment were studied in supernatant and pellets of Saccharomyces cervisiae hydrolysates. The type of enzyme used and incubation time significantly influence hydrolysis efficiency and protein concentration in supernatant and pellet. The highest protein hydrolysis efficiency was obtained by 24-h incubation with papain. Significantly lower values were observed for pepsin and autolysis. The potassium iodide concentration influences the iodine content of supernatant and pellet, but not hydrolysis. Iodide enrichment of supernatant and pellet depends on the concentration of iodide using during incubation. High concentration of iodide and long incubation times were the conditions for optimal iodide enrichment and high-protein hydrolysates. The optimal hydrolysis efficiency and iodine enrichment were obtained during 24-h incubation with papain in a 4.5-mM potassium iodide medium. The efficiency reached 98.22% with iodine concentrations of 2,664.91 and 9,200.67 μg/g iodine in pellet and supernatant, respectively.     

Some quantitative aspects of the labelling of proteins with 125I by the iodine monochloride method

The labelling of proteins by the iodine monochloride method was studied by using a mathematical model. The equations used were primarily derived from the mass law equation of the isotopic exchange reaction between [¹²⁵I]iodide and iodine monochloride. For convenient application, all equations were programmed into a computing desk-top calculator. To support the validity of the theoretical model, a series of iodinations of insulin were performed under various labelling conditions. The results of these experiments compare well with the theoretically derived values. Deviations from the theoretical values occurring at molar ratios of [¹²⁵I]iodide to iodine monochloride < 0.1 and > 4.0 are explained and suggestions made about how to prevent them. The mathematical model was used to simulate the isotopic exchange, and the iodination reaction under various conditions, to study (a) the influence of the amount of [¹²⁵I]iodide on the amount of [¹²⁵I]iodine monochloride formed, (b) the influence of the specific radioactivity of [¹²⁵I]iodide on the amount of [¹²⁵I]iodine monochloride formed, and (c) the influence of the specific radioactivity of [¹²⁵I]iodide on the number of millicuries needed for labelling to a desired extent.     

Isolation of Iodide-Oxidizing Bacteria from Iodide-Rich Natural Gas Brines and Seawaters

Iodide-oxidizing bacteria (IOB), which oxidize iodide (I⁻) to molecular iodine (I₂), were isolated from iodide-rich (63 μM to 1.2 mM) natural gas brine waters collected from several locations. Agar media containing iodide and starch were prepared, and brine waters were spread directly on the media. The IOB, which appeared as purple colonies, were obtained from 28 of the 44 brine waters. The population sizes of IOB in the brines were 10² to 10⁵ colony-forming units (CFU) mL⁻¹. However, IOB were not detected in natural seawaters and terrestrial soils (fewer than 10 CFU mL⁻¹ and 10² CFU g wet weight of soils⁻¹, respectively). Interestingly, after the enrichment with 1 mM iodide, IOB were found in 6 of the 8 seawaters with population sizes of 10³ to 10⁵ CFU mL⁻¹. 16S rDNA sequencing and phylogenetic analyses showed that the IOB strains are divided into two groups within the α-subclass of the Proteobacteria. One of the groups was phylogenetically most closely related to Roseovarius tolerans with sequence similarities between 94% and 98%. The other group was most closely related to Rhodothalassium salexigens, although the sequence similarities were relatively low (89% to 91%). The iodide-oxidizing reaction by IOB was mediated by an extracellular enzyme protein that requires oxygen. Radiotracer experiments showed that IOB produce not only I₂ but also volatile organic iodine, which were identified as diiodomethane (CH₂I₂) and chloroiodomethane (CH₂ClI). These results indicate that at least two types of IOB are distributed in the environment, and that they are preferentially isolated in environments in which iodide levels are very high. It is possible that IOB oxidize iodide in the natural environment, and they could significantly contribute to the biogeochemical cycling of iodine.     

Iodine from bacterial iodide oxidization by Roseovarius spp. inhibits the growth of other bacteria

Microbial activities in brine, seawater, or estuarine mud are involved in iodine cycle. To investigate the effects of the microbiologically induced iodine on other bacteria in the environment, a total of 13 bacteria that potentially participated in the iodide-oxidizing process were isolated from water or biofilm at a location containing 131 μg ml^?1 iodide. Three distinct strains were further identified as Roseovarius spp. based on 16 S rRNA gene sequences after being distinguished by restriction fragment length polymorphism analysis. Morphological characteristics of these three Roseovarius spp. varied considerably across and within strains. Iodine production increased with Roseovarius spp. growth when cultured in Marine Broth with 200 μg ml^?1 iodide (I^?). When 10⁶ CFU/ml Escherichia coli, Pseudomonas aeruginosa, and Bacillus pumilus were exposed to various concentrations of molecular iodine (I₂), the minimum inhibitory concentrations (MICs) were 0.5, 1.0, and 1.0 μg ml^?1, respectively. However, fivefold increases in the MICs for Roseovarius spp. were obtained. In co-cultured Roseovarius sp. IOB-7 and E. coli in Marine Broth containing iodide (I^?), the molecular iodine concentration was estimated to be 0.76 μg ml^?1 after 24 h and less than 50 % of E. coli was viable compared to that co-cultured without iodide. The growth inhibition of E. coli was also observed in co-cultures with the two other Roseovarius spp. strains when the molecular iodine concentration was assumed to be 0.52 μg ml^?1.     

Uptake of iodide in the marine haptophyte Isochrysis sp. (T.ISO) driven by iodide oxidation

Uptake of iodide was studied in the marine microalga Isochrysis sp. (isol. Haines, T.ISO) during short‐term incubations with radioactive iodide (¹²⁵I^?). Typical inhibitors of the sodium/iodide symporter (NIS) did not inhibit iodide uptake, suggesting that iodide is not taken up through this transport protein, as is the case in most vertebrate animals. Oxidation of iodide was found to be an essential step for its uptake by T.ISO and it seemed likely that hypoiodous acid (HOI) was the form of iodine taken up. Uptake of iodide was inhibited by the addition of thiourea and of other reducing agents, like L‐ascorbic acid, L‐glutathione and L‐cysteine and increased after the addition of oxidized forms of the transition metals Fe and Mn. The simultaneous addition of both hydrogen peroxide (H₂O₂) and a known iodide‐oxidizing myeloperoxidase (MPO) significantly increased iodine uptake, but the addition of H₂O₂ or MPO separately, had no effect on uptake. This confirms the observation that iodide is oxidized prior to uptake, but it puts into doubt the involvement of H₂O₂ excretion and membrane‐bound or extracellular haloperoxidase activity of T.ISO. The increase of iodide uptake by T.ISO upon Fe(III) addition suggests the nonenzymatic oxidation of iodide by Fe(III) in a redox reaction and subsequent influx of HOI. This is the first report on the mechanism of iodide uptake in a marine microalga.     

Hydrogen Peroxide-Dependent Uptake of Iodine by Marine Flavobacteriaceae Bacterium Strain C-21

The cells of the marine bacterium strain C-21, which is phylogenetically closely related to Arenibacter troitsensis, accumulate iodine in the presence of glucose and iodide (I⁻). In this study, the detailed mechanism of iodine uptake by C-21 was determined using a radioactive iodide tracer, ¹²⁵I⁻. In addition to glucose, oxygen and calcium ions were also required for the uptake of iodine. The uptake was not inhibited or was only partially inhibited by various metabolic inhibitors, whereas reducing agents and catalase strongly inhibited the uptake. When exogenous glucose oxidase was added to the cell suspension, enhanced uptake of iodine was observed. The uptake occurred even in the absence of glucose and oxygen if hydrogen peroxide was added to the cell suspension. Significant activity of glucose oxidase was found in the crude extracts of C-21, and it was located mainly in the membrane fraction. These findings indicate that hydrogen peroxide produced by glucose oxidase plays a key role in the uptake of iodine. Furthermore, enzymatic oxidation of iodide strongly stimulated iodine uptake in the absence of glucose. Based on these results, the mechanism was considered to consist of oxidation of iodide to hypoiodous acid by hydrogen peroxide, followed by passive translocation of this uncharged iodine species across the cell membrane. Interestingly, such a mechanism of iodine uptake is similar to that observed in iodine-accumulating marine algae.     

Iodine uptake in Laminariales involves extracellular, haloperoxidase-mediated oxidation of iodide

Sporophytes of Laminaria digitata (L.) Lamour. were assayed for their content of accumulated iodine, which ranged from 0.4% of dry weight in adult plants up to 4.7% for young plantlets. Sporophyte tissue from Laminaria saccharina (L.) Lamour. and L. digitata took up iodide according to Michaelis-Menten kinetics. Hydrogen peroxide and various substances known to interfere with oxidative metabolism were shown to either inhibit or enhance the uptake of iodide, confirming that apoplastic oxidations play a key role in iodide uptake in Laminaria. Consistently, iodide uptake was triggered in L. saccharina protoplasts by incubation in the presence of hydrogen peroxide. Similarly, the uptake of iodide was enhanced in L. digitata gametophytes by addition of haloperoxidase, suggesting that this enzyme catalyses the oxidation of iodide by hydrogen peroxide and plays a key role in iodine uptake. Oxidative stress resulted in a marked efflux of the intracellular iodine. In both influx and efflux experiments, a marked proportion (10–30%) of the tracer was not accounted for, indicating volatilisation of iodine. The mechanism and possible functions of the accumulation of iodine by kelps are discussed. Received: 11 February 1998 / Accepted: 18 June 1998     

Radiotracer Experiments on Biological Volatilization of Organic Iodine from Coastal Seawaters

Biological volatilization of iodine from seawaters was studied using a radiotracer technique. Seawater samples were incubated aerobically in serum bottles with radioactive iodide tracer (¹²⁵I), and volatile organic and inorganic iodine were collected with activated charcoal and silver wool trap, respectively. Iodine was volatilized mainly as organic iodine, and inorganic iodine volatilization was not observed. Influence of light intensity on the volatilization was determined, but no significant differences were observed under light (70,000 lux) and dark conditions. The effect of the chemical form of iodine on the volatilization was determined, and the results suggested that volatilization preferentially occurs from iodide (I^?) but not from iodate (IO₃ ^?). Volatilization did not occur when the samples were autoclaved or filtered through a 0.22-μm pore size membrane filter. Incubation of the samples with antibiotics caused decreased volatilization. Conversely, enhanced volatilization was observed when the samples were incubated with yeast extract. Fifty-nine marine bacterial strains were then randomly isolated from marine environments, and their iodine-volatilizing capacities were determined. Among these, 19 strains exhibited significant capacities for volatilizing iodine. 16S ribosomal RNA gene comparisons indicated that these bacteria are members of Proteobacteria (α and γ subdivisions) and Cytophaga-Flexibacter-Bacteroides group. One of the strains, strain C-19, volatilized 1 to 2% of total iodine during cultivation, and the gaseous organic iodine was identified as methyl iodide (CH₃I). These results suggest that organic iodine volatilization from seawaters occurs biologically, and that marine bacteria participate in the process. Considering that volatile organic iodine emitted from the oceans causes atmospheric ozone destruction, biological iodine volatilization from seawater is of great importance. Our results also contribute to prediction of movement and diffusion of long-lived radioactive iodine (¹²⁹I) in the environment.