Dosing Lugols iodine

[Scott Campbell]

1. It does not appear to be growth limiting in tests I have carefully done or in the scientific literature:

Chemistry And The Aquarium: Iodine In Reef Tanks 2: Effects On Macroalgae Growth ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/april2003/chem.htm

here in very recent scientific literature:

Iodine balance, growth and biochemical composition of three marine microalgae cultured under various inorganic iodine concentrations
https://link.springer.com/article/10.1007/s00227-016-2884-0

"However, the role of iodine in the metabolism of marine microalgae has hardly been studied and whether iodine is an element required for growth is not known."

"We conclude that although iodine is clearly taken up from the external medium and it is transformed by microalgae, it seems unlikely that it is an element required for growth or that it plays an essential role in microalgal metabolism."


2. There are many reasons why iodine might be taken up and not actually be useful in a reef tank. One is if the function is to make the macrolagae unpalatable to herbivorous fish. That could even be a disadvantage if you want fish to keep it under control.

Examples in the scientific literature:

Ecology of chemical defenses of algae against the herbivorous snail, Littorina littorea, in the New England rocky intertidal community
https://darchive.mblwhoilibrary.org/handle/1912/1688

"The release into seawater of volatile hydrocarbons and halomethanes from benthic algae and seagrass was measured to examine the possible role of these compounds as antiherbivore compounds. Bioassays indicated that CH2I2, a compound released into seawater from C. fragile, inhibited feeding of L. littorea."

3. A second reason it may be taken up and not useful is that the organism is actually intending to take up and use the much more common bromide ion into organic molecules, but since iodide looks a lot like bromide and is more chemically active, it gets taken up by accident. Since it is present at far, far lower concentrations (0.06 ppm) than bromide (67 ppm), even a tiny bit of accidental incorporation can use up all available iodide

Thank you for the thoughtful and rather convincing response.

The abstract only mentions phytoplankton as likely not being limited by an absence of iodine. But even in this study one species of microalgae was still limited - "In cultures of Phaeodactylum tricornutum, growth was inhibited at 2500 µM IO3−. " I imagine growth of macroalgae might be more involved and complex. Also wondering if iodine's similarity to bromide might make it difficult to completely limit algae access to iodine. Would the door not swing both ways and allow bromide to jump in as an iodine substitute in an iodine starved environment?

That said - it seems plausible to me that iodine might be used up as an antiherbivore agent in a manner that does not really benefit the aquarium. Or that iodine is being used up as bromide. But it also seems plausible that iodine might actually be useful in a manner that is not immediately obvious - at least at normal seawater concentrations. I'm not sure what the downside would be to maintaining natural seawater concentrations given the small cost.

 

[Randy Holmes-Farley Verified Community Expert]

Thank you for the thoughtful and rather convincing response.

The abstract only mentions phytoplankton as likely not being limited by an absence of iodine. But even in this study one species of microalgae was still limited - "In cultures of Phaeodactylum tricornutum, growth was inhibited at 2500 µM IO3−. " I imagine growth of macroalgae might be more involved and complex. Also wondering if iodine's similarity to bromide might make it difficult to completely limit algae access to iodine. Would the door not swing both ways and allow bromide to jump in as an iodine substitute in an iodine starved environment?

That means too much iodate is hurting them, not that they want more. That 2500 uM iodate way more than NSW, which has about 0.3 uM iodate.

 

[Scott Campbell]

That means too much iodate is hurting them, not that they want more. That 2500 uM iodate way more than NSW, which has about 0.3 uM iodate.

Ah - that makes sense!