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Iodine - (Iodum) - I

Iodine occurs on Earth exclusively in the form of iodine compounds, primarily diluted in salt-water and freshwater. With an average molar concentration of 4.10-7 mol/l, iodine is the eighteenth most abundant element in sea water. Expressed in mass units, the iodine concentration ranges between 2,5.10-5 - 6,4.10-5 g/l (0,025 - 0,064 mg/l).


Version 2012-III

Iodine - (Iodum) - I

I

Iodine occurs on Earth exclusively in the form of iodine compounds, primarily diluted in salt-water and freshwater. With an average molar concentration of 4.10-7 mol/l, iodine is the eighteenth most abundant element in sea water. Expressed in mass units, the iodine concentration ranges between 2,5 - 6,4.10-5 g/l (0,025 - 0,064 mg/l).

Occurrence

Due to the presence of diluted oxygen, iodine should in theory occur in sea water exclusively in the iodate form (IO3-), and certain citations adhere to this thesis. In reality though, marine water also contains significant iodine quantities in the form of iodide (I-), especially at the surface waters.

A continuous iodide supply in marine waters is safeguarded on the one hand through the inlet of river water, where significant iodide quantities are available, and on the other hand through the reduction of iodates in hypoxic zones. The iodide oxidation to iodate hypoxic ocean zones is relatively slow and is no match for continuous iodide supply disposal. The geochemical cycle of iodine is completed in the oceans with the consequent iodine evaporation in the atmosphere in the form of methyl-iodide (CH3I) - a key process to maintain the global geochemical iodine cycle.

Biogenous role

Most organic iodine is present in seaweed and in fish meat. Just as for mammals, iodine plays an important biogenous role also for marine fish. It is part of the thyroid hormones, which influence nutrient metabolism, as well as the activity of the nerve and gonad system.

The effect of thyroxine on fish metamorphosis was investigated for instance on coral trout (Plectropomus leopardus). Juvenile fish (35 days old) exposed to the effect of thyroxine, had already completed metamorphosis in 3 days, in comparison to 13 days for the control group. The presence of thyroxine-like substances was confirmed in the polyp tissue of certain corals (e.g. Leptogorgia virgulata), proving in this manner the effect of thyroxine on the coral skeletons formation.

Summary

Occurrence in seawater:

Iodate (IO3-) a Iodide (I-)

Recommended concentration:

0,06 mg/l (0,04 – 0,08 mg/l)

Influence on:

Saltwater fish, invertebrates

Biogenous role:

Growth, propagation

Literature

Marine Geochemistry, Roy Chester, 2nd edition, Blackwell Science Ltd.
An Introduction to Marine Biochemistry, Susan M. Libes, John Wiley & Sons, Inc.
Effects of thyroxine and thiourea on the metamorphosis of coral trout grouper Plectropomus leopardus. DODY DHARMAWAN TRIJUNO, KENZO YOSEDA, JUN HIROKAWA, MASATOMO TAGAWA, MASARU TANAKA (2002). Fisheries Science 68 (2), 282–289.
Comparative Biochemistry and Physiology - Part A: Molecular & Integrative Physiology. Volume 129, Issue 4, July 2001, Pages 897-907

 
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