LAUSR

Iodine (I) is one of the least abundant elements on Earth’s surface; soils have only about 3 mg kg−1 of total I (Mohiuddin et al., 2019). However, this value can be higher in places close to the coast and lower in areas with slight marine influence (Fuge and Johnson, 2015). The marine environment is rich in this element, having about 60 μg L−1 and being the largest I reservoir on the planet (Wong, 1991). Regarding availability of I in soils, a small amount of it is present in the soil solution, with the major fraction being associated with the solid phase, i.e., organic matter and clay minerals, as well as iron (Fe) and aluminum (Al) oxides (Fuge and Johnson, 1986). Some substrate characteristics, such as mineral/organic composition, pH, texture, and redox conditions, limit I mobility and, thus, its absorption by plants (Gonzali et al., 2017). Consequently, knowing the distribution of I worldwide is key for a better understanding of its importance in living beings, from microorganisms to humans, and in plants. Iodine is an essential element for animals, being involved in regulation of growth, development, and metabolism (Blasco et al., 2008), as it is required for the synthesis of thyroid hormones (thyroxine and triiodothyronine) (Landini et al., 2012). According to Dai et al. (2004), I deficiency in humans can cause a series of diseases and health problems, such as goiter, cretinism, reduced intellectual capacity, spontaneous abortions in pregnant women, congenital defects in fetuses, and deaths in babies at birth. Iodine’s bioavailability in food is considered high (∼99%) (Weng et al., 2013). However, some factors, such as food preparation and storage, among others, can affect the bioavailability of I in the human body, causing its deficiency (Gonzali et al., 2017). In the marine environment, algae (especially brown algae) and phytoplankton are I hyperaccumulators, helping to convert iodate (IO3) into iodide (I), the most absorbable form for terrestrial plants (Chance et al., 2007). The importance of I in plants has not yet been fully explained, but application of I in plant species has provided greater accumulation of the element in edible parts of lettuce (Lactuca sativa), spinach (Spinacia oleracea), and curly endive (Cichorium endivia L. var. crispum Hegi) (Zhu et al., 2003; Weng et al., 2008a; Blasco et al., 2013; Smoleń et al., 2016; Sabatino et al., 2021), as well as rice (Oryza sativa), wheat (Triticum aestivum), and maize (Zea mays) (Cakmak et al., 2017), because this process of biofortification is an affordable way to avoid I deficiency in human populations (Blasco et al., 2008, 2013; Prom-u-thai et al., 2020), especially when I is applied as potassium iodate (KIO3) (Cakmak et al., 2017). Plants, algae, and phytoplankton are also capable of volatilizing I in the form of iodomethane (also known as methyl iodide, CH3I), and this reaction is catalyzed by enzymes with methyltransferase activity dependent on S-adenosyl-L-methionine (Itoh et al., 2009). Volatilization is possibly associated with defense function while also serving to aid in global I cycle (Fuge and Johnson, 2015; Gonzali et al., 2017). However, emissions, both terrestrial and marine,