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
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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