randy holmes-farley
Advanced Reefer
- Location
- Arlington, MA
There has been some suggestion on Reef Central that mangroves may deplete magnesium from reef tanks because they have a magnesium/sodium transporter that helps them stay in salt balance. Others suggested that the salt is then excreted on the leaves (which IME, is true, some salt is excreted).
So to test the hypothesis I collected two salt grains from a mangrove leaf, dried it out, and tested it for magnesium. The details are given below for those interested. Note that this is but a single test, and tests by others would help substantiate this tentative result. For that matter, the resuilts are probably available in the literature, but it's faster to run the test than to try to look the answer up.
The result is that this salt is not enriched in magnesium. In fact, I find it greatly depleted in magnesium relative to natural seawater. Thus, if this result is true, this cannot be a magnesium depletion mechanism in reef tanks.
Here's exactly how I did the test:
I looked over the two mangroves in my refugium. Many of the older leaves had a very thin coating of salt on them, which looked shiny/wet in places as the humidity is high in my basement refugium area. On one leaf I saw two larger chunks (about the size of a tick!) that could be collected.
I put them both into a single scintillation vial, and dried them in high vacuum at 45 deg C for 24 h. The resulting weight was 0.010 g (i.e., not much).
I dissolved the grains in 5.83 mL of DI water, which is the size of the collection tube of the Hach hardness test kit. The grains readily and apparently completely dissolved. I let the sample sit for several hours.
I then tested the sample with the Hach Total Hardness and Calcium test kit #1457-01. The sample was pink before any EDTA titrant was added, but turned completely blue after only a single drop of titrant. Thus, the concentration of Mg++ in the tested sample was less than 20 ppm of CaCO3 equivalent.
So how much magnesium might have been there?
20 ppm CaCO3 equivalent = 20 mg/L CaCO3 = 0.02 g/L CaCO3
= 0.00020 moles/L CaCO3 equivalent
= 1.2 x 10-6 moles/5.83 mL
= 0.000028 g Mg++ maximum
So the initial 0.01 g sample was less than 0.28% magnesium by weight.
Of the salts in seawater, magnesium comprises about 3.7% of the total by weight.
Consequently, this salt sample was depleted in magnesium relative to seawater. If this test is correct, excretion from leaves cannot be a significant sink for magnesium in reef tanks.
Even if this sample were pure MgCl2, it would require more than a thousand such mangroves to deplete the magnesium by 1% in my reef tank.
So to test the hypothesis I collected two salt grains from a mangrove leaf, dried it out, and tested it for magnesium. The details are given below for those interested. Note that this is but a single test, and tests by others would help substantiate this tentative result. For that matter, the resuilts are probably available in the literature, but it's faster to run the test than to try to look the answer up.
The result is that this salt is not enriched in magnesium. In fact, I find it greatly depleted in magnesium relative to natural seawater. Thus, if this result is true, this cannot be a magnesium depletion mechanism in reef tanks.
Here's exactly how I did the test:
I looked over the two mangroves in my refugium. Many of the older leaves had a very thin coating of salt on them, which looked shiny/wet in places as the humidity is high in my basement refugium area. On one leaf I saw two larger chunks (about the size of a tick!) that could be collected.
I put them both into a single scintillation vial, and dried them in high vacuum at 45 deg C for 24 h. The resulting weight was 0.010 g (i.e., not much).
I dissolved the grains in 5.83 mL of DI water, which is the size of the collection tube of the Hach hardness test kit. The grains readily and apparently completely dissolved. I let the sample sit for several hours.
I then tested the sample with the Hach Total Hardness and Calcium test kit #1457-01. The sample was pink before any EDTA titrant was added, but turned completely blue after only a single drop of titrant. Thus, the concentration of Mg++ in the tested sample was less than 20 ppm of CaCO3 equivalent.
So how much magnesium might have been there?
20 ppm CaCO3 equivalent = 20 mg/L CaCO3 = 0.02 g/L CaCO3
= 0.00020 moles/L CaCO3 equivalent
= 1.2 x 10-6 moles/5.83 mL
= 0.000028 g Mg++ maximum
So the initial 0.01 g sample was less than 0.28% magnesium by weight.
Of the salts in seawater, magnesium comprises about 3.7% of the total by weight.
Consequently, this salt sample was depleted in magnesium relative to seawater. If this test is correct, excretion from leaves cannot be a significant sink for magnesium in reef tanks.
Even if this sample were pure MgCl2, it would require more than a thousand such mangroves to deplete the magnesium by 1% in my reef tank.
