Iodine soil dynamics and methods of measurement: a review

A review of iodine in plants with biofortification: Uptake, accumulation, transportation, function, and toxicity

Iodine deficiency can cause thyroid disease, a serious health problem that has been affecting humans since several years. The biofortification of plants with iodine is an effective strategy for regulating iodine content in humans. In addition, radioiodine released into the atmosphere may contaminate terrestrial ecosystem along with dry or wet deposition and its accumulation in plants may cause exposure risks to humans via food chain. Recent progress in understanding the mechanisms related to iodine uptake, elementary speciation, dynamic transportation, nutritional role, and toxicity in plants is reviewed here. First, we introduced the iodine cycle in a marine-atmosphere-land system. The content and speciation of iodine in plants under natural conditions and biofortification backgrounds were also analyzed. We then discussed the mechanisms of iodine uptake and efflux by plants. The promotion or inhibition effects of iodine on plant growth were also investigated. Finally, the participation of radioiodine in plant growth and its safety risks along the food chain were evaluated. Furthermore, future challenges and opportunities for understanding the participation of iodine in plants have been outlined.

Mass-spectrometric determination of iodine-129 using O2-CO2 mixed-gas reaction in inductively coupled plasma tandem quadrupole mass spectrometry

This paper presents a mass-spectrometric method for determining the radionuclide iodine-129 (¹²⁹I) from the significant amount of interference in inductively coupled plasma tandem quadrupole mass spectrometry (ICP-MS/MS) using a dynamic reaction cell passing a mixture gas of O₂ and CO₂. Thus far, mass spectrometry analysis of trace amounts of ¹²⁹I has been hampered by the presence of xenon-129 (¹²⁹Xe) and the formation of polyatomic ions from excess amounts of stable isotope ¹²⁷I. In this study, flowing a mixture gas of O₂ and CO₂ into the dynamic reaction cell (Q2) successfully removed both ¹²⁹Xe interference and polyatomic interference (¹²⁷IH₂) in the analysis of ¹²⁹I in ICP-MS/MS. The resulting ratio of (background noise of m/z 129)/¹²⁷I was 4.6 × 10^-10 ± 3.3 × 10^-10, which enables the analysis of 10 mBq/L of ¹²⁹I in the presence of 100 mg/L of stable ¹²⁷I without chemical separation. The detection limit of this method was 0.73 mBq/L (= 0.11 ng/L) with an APEX-Q sample inlet desolvation device. For demonstration purposes, spike and recovery analysis of rainwater was performed, and good agreement between the spiked and recovered amounts was achieved.

Multiple geochemical factors may cause iodine and selenium deficiency in Gilgit-Baltistan, Pakistan

Deficiencies of the micronutrients iodine and selenium are particularly prevalent where populations consume local agricultural produce grown on soils with low iodine and selenium availability. This study focussed on such an area, Gilgit-Baltistan in Pakistan, through a geochemical survey of iodine and selenium fractionation and speciation in irrigation water and arable soil. Iodine and selenium concentrations in water ranged from 0.01-1.79 µg L-1 to 0.016-2.09 µg L-1, respectively, which are smaller than levels reported in similar mountainous areas in other parts of the world. Iodate and selenate were the dominant inorganic species in all water samples. Average concentrations of iodine and selenium in soil were 685 µg kg-1 and 209 µg kg-1, respectively, much lower than global averages of 2600 and 400 µg kg-1, respectively. The 'reactive' fractions ('soluble' and 'adsorbed') of iodine and selenium accounted for < 7% and < 5% of their total concentrations in soil. More than 90% of reactive iodine was organic; iodide was the main inorganic species. By contrast, 66.9 and 39.7% of 'soluble' and 'adsorbed' selenium, respectively, were present as organic species; inorganic selenium was mainly selenite. Very low distribution coefficients (kd = adsorbed/soluble; L kg-1) for iodine (1.07) and selenium (1.27) suggested minimal buffering of available iodine and selenium against leaching losses and plant uptake. These geochemical characteristics suggest low availability of iodine and selenium in Gilgit-Baltistan, which may be reflected in locally grown crops. However, further investigation is required to ascertain the status of iodine and selenium in the Gilgit-Baltistan food supply and population.

Short-Term Iodine Dynamics in Soil Solution

Assessing the reactions of iodine (I) in soil is critical to evaluate radioiodine exposure and understand soil-to-crop transfer rates. Our mechanistic understanding has been constrained by method limitations in assessing the dynamic interactions of iodine between soil solution and soil solid phase over short periods (hours). We use microdialysis to passively extract soil solution spiked with radioiodine (¹²⁹I^- and ¹²⁹IO₃^-) to monitor short-term (≤40 h) in situ fixation and speciation changes. We observed greater instantaneous adsorption of ¹²⁹IO₃^- compared to ¹²⁹I^- in all soils and the complete reduction of ¹²⁹IO₃^- to ¹²⁹I^- within 5 h of addition. Loss of ¹²⁹I from solution was extremely rapid; the average half-lives of ¹²⁹I^- and ¹²⁹IO₃^- in soil solution were 4.06 and 10.03 h, respectively. We detected the presence of soluble organically bound iodine (org-¹²⁹I) with a low molecular weight (MW) range (0.5-5 kDa) in all soils and a slower (20-40 h) time-dependent formation of larger MW org-I compounds (12-18 kDa) in some samples. This study highlights the very short window of immediate availability in which I from rainfall or irrigation remains in soil solution and available to crops, thus presenting significant challenges to phytofortification strategies in soil-based production systems.

Iodine uptake, storage and translocation mechanisms in spinach (Spinacia oleracea L.)

Iodine is an essential micronutrient for human health; phytofortification is a means of improving humans' nutritional iodine status. However, knowledge of iodine uptake and translocation in plants remains limited. In this paper, plant uptake mechanisms were assessed in short-term experiments (24 h) using labelled radioisotopes; the speciation of iodine present in apoplastic and symplastic root solutions was determined by (HPLC)-ICP-QQQ-MS. Iodine storage was investigated in spinach (Spinacia oleracea L.) treated with I^- and IO₃^-. Finally, translocation through the phloem to younger leaves was also investigated using a radioiodine (¹²⁹I^-) label. During uptake, spinach roots demonstrated the ability to reduce IO₃^- to I^-. Once absorbed, iodine was present as org-I or I^- with significantly greater concentrations in the apoplast than the symplast. Plants were shown to absorb similar concentrations of iodine applied as I^- or IO₃^-, via the roots, grown in an inert growth substrate. We found that whilst leaves were capable of absorbing radioactively labelled iodine applied to a single leaf, less than 2% was transferred through the phloem to younger leaves. In this paper, we show that iodine uptake is predominantly passive (approximately two-thirds of total uptake); however, I^- can be absorbed actively through the symplast. Spinach leaves can absorb iodine via foliar fertilisation, but translocation is severely limited. As such, foliar application is unlikely to significantly increase the iodine content, via phloem translocation, of fruits, grains or tubers.

SPECIATION OF IODINE IN SOIL SOLUTION IN FOREST AND GRASSLAND SOILS IN ROKKASHO, JAPAN

The behaviour of I in soil depends on its chemical form in soil solution. Stable I (127I) in the soil solution under actual soil conditions was investigated as a natural analogue of long-lived radioiodine (129I). Soil samples were collected at 5-cm depth intervals down to 20 cm from forests and grasslands in Rokkasho, where the Japanese first commercial nuclear fuel reprocessing plant is located, and the soil solution was extracted by centrifugation. Almost half of total I in the soil solution was iodide, and the other half was dissolved organic I (DOI), with iodate under the detection limit. The proportion of DOI in total I at 0-5 cm depth was larger than the proportions at 5-20 cm depth. The concentration of DOI was positively correlated with that of DOC in the soil solution, suggesting that the behaviour of DOI in the surface soil is affected by labile organic matter dynamics.

Environmental Parameters Affecting the Concentration of Iodine in New Zealand Pasture

Iodine (I) is an essential trace element commonly deficient in agricultural systems. Whereas there is much information on I in food crops, there is a lacuna of knowledge on the environmental factors that affect pasture I concentrations. We aimed to identify the most important environmental factors affecting the concentration of I in New Zealand pastures, and the consequences to agricultural systems. Soil and pastoral samples were collected throughout the country and analyzed for I and other elements. The soils contained 1.1 to 86 mg I kg, with 0.005 to 1.4 mg kg in the pasture. In 26% of pastures, I concentrations were insufficient for sheep nutrition, whereas 87% contained insufficient I for cattle nutrition. Pasture I concentrations were negatively correlated with the distance from the sea, and the concentration of oxalate-extractable amorphous Al, Fe, and Si oxides, which immobilize soil I. Soil organic C and clay increased I retention in soil but did not significantly affect pasture I concentrations. Future work should investigate how soil properties affect pasture I uptake in inland areas.

African savanna elephants (Loxodonta africana) as an example of a herbivore making movement choices based on nutritional needs

BACKGROUND

The increasing human population and global intensification of agriculture have had a major impact on the world's natural ecosystems and caused devastating effects on populations of mega-herbivores such as the African savanna elephants, through habitat reduction and fragmentation and increased human-animal conflict. Animals with vast home ranges are forced into increasingly smaller geographical areas, often restricted by fencing or encroaching anthropogenic activities, resulting in huge pressures on these areas to meet the animals' resource needs. This can present a nutritional challenge and cause animals to adapt their movement patterns to meet their dietary needs for specific minerals, potentially causing human-animal conflict. The aim of this review is to consolidate understanding of nutritional drivers for animal movement, especially that of African savanna elephants and focus the direction of future research. Peer reviewed literature available was generally geographically specific and studies conducted on isolated populations of individual species. African savanna elephants have the capacity to extensively alter the landscape and have been more greatly studied than other herbivores, making them a good example species to use for this review. Alongside this, their movement choices, potentially linked with nutritional drivers could be applicable to a range of other species. Relevant case study examples of other herbivores moving based on nutritional needs are discussed.

METHODS

Three databases were searched in this review: Scopus, Web of Science and Google Scholar, using identified search terms. Inclusion and exclusion criteria were determined and applied as required. Additional grey literature was reviewed as appropriate.

RESULTS

Initial searches yielded 1,870 records prior to application of inclusion and exclusion criteria. A less detailed review of grey literature, and additional peer-reviewed literature which did not meet the inclusion criteria but was deemed relevant by the authors was also conducted to ensure thorough coverage of the subject.

DISCUSSION

A review of peer reviewed literature was undertaken to examine nutritional drivers for African elephant movement, exploring documented examples from free-ranging African savanna elephants and, where relevant, other herbivore species. This could help inform prediction or mitigation of human-elephant conflict, potentially when animals move according to nutritional needs, and related drivers for this movement. In addition, appropriate grey literature was included to capture current research.