- Joined
- May 22, 2016
- Messages
- 6,547
- Reaction score
- 10,107
Just for fun, here's a made-up argument for why maintaining iodine levels might be useful in reef tank water even for keeping organisms with no known need for iodine directly. In short, iodine might be important as a redox-active element that can influence the form (oxidized or reduced) of other biologically important traces.
I'm out of my depth to know how far the argument makes sense and where it doesn't. So I'm not sure how much of the science fiction is science and how much is fiction.
So don't take this as me advocating Iodine dosing, I don't know if I believe my story or not.
Iodine in the surface ocean water is often in the ballpark of 0.060 ppm, with 20-30% of that as the more reduced form, iodide or I- and the other 70-80% as the more oxidized form, iodate or IO3-. There are other things that can exist in oxidized and reduced forms, Nitrogen, Sulfur, Fe, Mn, Cu species for instance, but in oxygenic surface water those are usually overwhelmingly in the most oxidized stable form - NO3, SO4, Fe3+ etc. And the metals exist in much lower quantities than iodine. Also, iodine is quite reactive - the two forms I- and IO3- are quite redox-active with a few other important seawater components - namely the trace metals themselves.
So iodine occupies an important (?) (unique ??) niche - it is the highest concentration element that has a significant portion in two different oxidation states and those two states are quite reactive with other biologically required trace elements.
My first interpretation of this was by Redox to pH analogy. Buffers exist as a mix of two forms to react with excess acids or bases and keep the pH stable. So the analogy goes that iodine acts as a reservoir of oxidized and reduced forms in oxygenic surface water to react with excess oxidizers and reducers to "buffer" the ORP of the water, and thus keep other stuff from getting totally oxidized or totally reduced.
There isn't much evidence of that mechanism - I don't find much discussion of I- reducing say Fe3+ to Fe2+ in seawater. But the other half seems well established.
Review on the physical chemistry of iodine transformations in the oceans
This paper is dense, but Fig 4 shows that reduced Fe or Mn, would react with oxidized IO3- and get oxidized. Fig 3 shows that NO2 similarly would get oxidized by IO3- as well.
"Comparing Figures 3, 4 indicates that the Mn2+ and NO−2 reactions with iodine species have a similar range of ΔlogK reaction values [favorable at seawater pH] whereas the Fe2+ reactions with iodine species are more favorable (higher ΔlogK reaction values)."
"Thus, there is no thermodynamic inhibition to IO3- reduction to I- by Fe2+, and this abiotic reaction at a pH of 7 was reported to be 92% complete after 2 hours using initial concentrations of 2 mM Fe2+ and 0.1 mM IO3-"
So rather than "buffering" oxidized and reduced species, it looks like IO3- might act as a reservoir of oxidizer capable of oxidizing biologically important trace metals (and maybe nitrite too - if the amounts are small).
I wonder if this is part of why Fauna Marin's ICP database finds strong correlation (say it with me - "...is not causation") between low iodine and nuisance algae (dino) issues.
I'm out of my depth to know how far the argument makes sense and where it doesn't. So I'm not sure how much of the science fiction is science and how much is fiction.
So don't take this as me advocating Iodine dosing, I don't know if I believe my story or not. Iodine in the surface ocean water is often in the ballpark of 0.060 ppm, with 20-30% of that as the more reduced form, iodide or I- and the other 70-80% as the more oxidized form, iodate or IO3-. There are other things that can exist in oxidized and reduced forms, Nitrogen, Sulfur, Fe, Mn, Cu species for instance, but in oxygenic surface water those are usually overwhelmingly in the most oxidized stable form - NO3, SO4, Fe3+ etc. And the metals exist in much lower quantities than iodine. Also, iodine is quite reactive - the two forms I- and IO3- are quite redox-active with a few other important seawater components - namely the trace metals themselves.
So iodine occupies an important (?) (unique ??) niche - it is the highest concentration element that has a significant portion in two different oxidation states and those two states are quite reactive with other biologically required trace elements.
My first interpretation of this was by Redox to pH analogy. Buffers exist as a mix of two forms to react with excess acids or bases and keep the pH stable. So the analogy goes that iodine acts as a reservoir of oxidized and reduced forms in oxygenic surface water to react with excess oxidizers and reducers to "buffer" the ORP of the water, and thus keep other stuff from getting totally oxidized or totally reduced.
There isn't much evidence of that mechanism - I don't find much discussion of I- reducing say Fe3+ to Fe2+ in seawater. But the other half seems well established.
Review on the physical chemistry of iodine transformations in the oceans
This paper is dense, but Fig 4 shows that reduced Fe or Mn, would react with oxidized IO3- and get oxidized. Fig 3 shows that NO2 similarly would get oxidized by IO3- as well.
"Comparing Figures 3, 4 indicates that the Mn2+ and NO−2 reactions with iodine species have a similar range of ΔlogK reaction values [favorable at seawater pH] whereas the Fe2+ reactions with iodine species are more favorable (higher ΔlogK reaction values)."
"Thus, there is no thermodynamic inhibition to IO3- reduction to I- by Fe2+, and this abiotic reaction at a pH of 7 was reported to be 92% complete after 2 hours using initial concentrations of 2 mM Fe2+ and 0.1 mM IO3-"
So rather than "buffering" oxidized and reduced species, it looks like IO3- might act as a reservoir of oxidizer capable of oxidizing biologically important trace metals (and maybe nitrite too - if the amounts are small).
I wonder if this is part of why Fauna Marin's ICP database finds strong correlation (say it with me - "...is not causation") between low iodine and nuisance algae (dino) issues.
