Iodide oxidation by a marine bacterium

The Genus Iodidimonas: From Its Discovery to Potential Applications

The genus Iodidimonas was recently proposed in the class Alphaproteobacteria. Iodidimonas strains are aerobic, mesophilic, neutrophilic, moderately halophilic, and chemo-organotrophic. They were first discovered in natural gas brine water containing a very high level of iodide (I−). They exhibited a unique phenotypic feature of iodide oxidation to form molecular iodine (I2). Iodidimonas was also enriched and isolated from surface seawater supplemented with iodide, and it is clearer now that their common habitats are those enriched with iodide. In such environments, Iodidimonas species seem to attack microbial competitors with the toxic form I2 to occupy their ecological niche. The iodide-oxidizing enzyme (IOX) purified from the Iodidimonas sp. strain Q-1 exhibited high catalytic efficiency for iodide and consisted of at least two proteins IoxA and IoxC. IoxA is a putative multicopper oxidase with four conserved copper-binding regions but is phylogenetically distinct from other bacterial multicopper oxidases. The IOX/iodide system could be used as a novel enzyme-based antimicrobial system which can efficiently kill Bacillus spores. Furthermore, the IOX/iodide system can be applied to the decolorization of recalcitrant dyes, where iodide may function as a novel inorganic natural redox mediator.

Iodidimonas gelatinilytica sp. nov., aerobic iodide-oxidizing bacteria isolated from brine water and surface seawater

Chemo-organotrophic iodide (I^-)-oxidizing bacterial strains Hi-2^T and Mie-1 were isolated from iodide-rich natural gas brine water in Chiba and surface seawater in Mie, Japan, respectively. Cells of strains Hi-2^T and Mie-1 were aerobic, Gram-negative and rod-shaped (0.3-0.5 µm width and 1.2-4.4 µm in length). Two isolates grew optimally at 30 °C, pH 7.5 and with 3% NaCl (w/v). Iodide oxidation to form molecular iodine (I₂) was a biochemically unique trait for strains Hi-2^T and Mie-1. The major cellular fatty acids are C_18:1ω7c, C_16:1ω5c and C_18:1 2-OH. A phylogenetic analysis based on the 16S rRNA gene sequence revealed that strains Hi-2^T and Mie-1 were located near Iodidimonas muriae C-3^T with 99.2% sequence similarity. The calculated digital DNA-DNA hybridization (dDDH) value of 65.7-65.9% between the two isolates and I. muriae C-3^T was lower than the threshold of 70%, which was used for prokaryotic species delineation. Strains Hi-2^T and Mie-1 differed in the hydrolysis of aesculin, the hydrolysis of gelatin and the major cellular fatty acids composition from I. muriae C-3^T. Considering these biochemical properties, the major cellular fatty acids composition and dDDH value, a novel species is proposed for strains Hi-2^T (= JCM 17844^T = LMG 28661^T) and Mie-1 (= JCM 17845 = LMG 28662), to be named Iodidimonas gelatinilytica.

Bioleaching of Gold from Sulfidic Gold Ore Concentrate and Electronic Waste by Roseovarius tolerans and Roseovarius mucosus

Gold bioleaching mediated by iodide oxidizing bacteria (IOB) has been proposed as a sustainable alternative to conventional technologies such as cyanidation. This study evaluated the ability of two IOB sourced from a commercial culture collection, Roseovarius (R.) tolerans DSM 11457^T and R. mucosus DSM 17069^T, to bioleach gold from electronic waste (e-waste) (1030 ppm gold) and sulfidic gold ore concentrate (45 ppm gold) using one-step, two-step and spent medium leaching at 1% pulp density over 10 days. Two-step bioleaching of ore concentrate resulted in the highest gold leaching yields (approximately ~100% and 34% for R. tolerans and R. mucosus, respectively), followed by spent medium leaching and one-step leaching. The yields remained low for e-waste with both strains (maximum 0.93% and 1.6% for R. tolerans and R. mucosus, respectively) and decreased over time, likely due to the instability of the solubilized gold at relatively low redox potentials (<300 mV vs. Ag/AgCl). Another limiting factor may be the partial inhibition of bacterial growth in the presence of the ore concentrate and e-waste. Therefore, future studies should evaluate the pre-treatment of the ore concentrate and e-waste to remove inhibitory and oxidant consuming compounds before bioleaching with IOB to optimize leaching yields.

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.

Gold Dissolution from Ore with Iodide-Oxidising Bacteria

Gold leaching from ore using iodide-iodine mixtures is an alternative to gold cyanidation. This study evaluated the ability of iodide-oxidising bacteria to solubilise gold from ore that was mainly composed of gold, pyrite, galena, and chalcopyrite. Eight bacterial strains were successfully isolated from brine. Those strains were incubated in a liquid culture medium containing ore with a gold content of 0.26 wt.% and pulp density of 3.3 w/v% to evaluate their abilities to mediate the dissolution of gold. The gold was solubilised completely within 30 days of incubation in the iodine-iodide lixiviant solution generated by three bacterial strains. One strain, in particular, completed the dissolution of gold within 5 days of incubation and was identified as a member of the genus Roseovarius. Thus, the possibility of bacterial gold leaching using iodide-oxidising bacteria was successfully demonstrated. Bioleaching gold with iodide would likely be more environmentally sustainable than traditional cyanide leaching. Further research is required to evaluate the techno-economic feasibility of this approach.

Distribution and fate of 129I in the seabed sediment off Fukushima

In this study, seabed sediment was collected from 26 stations located within 160 km from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) during the 2 years which followed the FDNPP accident of March 2011 and the concentrations of ¹²⁹I and ¹³⁷Cs were measured. By comparing the distribution of these two radionuclides with respect to their different geochemical behaviors in the environment, the transport of accident-derived radionuclides near the seafloor is discussed. The concentration of ¹²⁹I in seabed sediment recovered from offshore Fukushima in 2011 ranged between 0.02 and 0.45 mBq kg^-1, with ¹²⁹I/¹³⁷Cs activity ratios of (1.9 ± 0.5) × 10^-6 Bq Bq^-1. The initial deposition of ¹²⁹I to the seafloor in the study area was 0.36 ± 0.13 GBq, and the general distribution of sedimentary ¹²⁹I was established within 6 months after the accident. Although iodine is a biophilic element, the accident-derived ¹²⁹I negligibly affects the benthic ecosystem. Until October 2013, a slight increase in activity of ¹²⁹I in the surface sediment along the shelf-edge region (bottom depth: 200-400 m) was observed, despite that such a trend was not observed for ¹³⁷Cs. The preferential increase of the ¹²⁹I concentrations in the shelf-edge sediments was presumed to be affected by the re-deposition in the shelf-edge sediments of ¹²⁹I desorbed from the contaminated coastal sediment. The results obtained from this study indicate that ¹²⁹I/¹³⁷Cs in marine particles is a useful indicator for tracking the secondary transport of accident-derived materials, particularly biophilic radionuclides, from the coast to offshore areas.

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.

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 10(6) CFU/ml Escherichia coli, Pseudomonas aeruginosa, and Bacillus pumilus were exposed to various concentrations of molecular iodine (I(2)), 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).

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 microM 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 10(2) to 10(5) colony-forming units (CFU) mL(-1). However, IOB were not detected in natural seawaters and terrestrial soils (fewer than 10 CFU mL(-1) and 10(2) 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 10(3) to 10(5) CFU mL(-1). 16S rDNA sequencing and phylogenetic analyses showed that the IOB strains are divided into two groups within the alpha-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.

Production of free and organic iodine byRoseovariusspp

Two strains of iodine-producing bacteria were isolated from marine samples. 16S rRNA gene sequences indicated the strains were most closely related to Roseovarius tolerans, and phylogenetic analysis indicated both belong to the same genus. 5 mM iodide inhibited the growth of strain 2S5-2 almost completely, and of strain S6V slightly. Both strains produced free iodine and organic iodine from iodide. CH2I2, CHI3 and CH2ClI were the main organic iodines produced by strain 2S5-2, and CHI3 and CH2I2 by strain S6V. Experiments using cells and spent media suggested that the organic iodines were produced from the compounds released or contained in the media and cells were necessary for the considerable production of CH2I2 and CH2ClI, though CHI3 was produced by spent media with H2O2 or free iodine.