[Randy Holmes-Farley]

There has been some suggestion on RC 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.

[icemark]

You are not including the magnesium and salts used by the actual plants metabolism though, as part of the sink.

However, I do agree with your results of your hypothesis, that a considerable number of plants would make only a minimal drop in the magnesium levels.

[Randy Holmes-Farley]

I agree that there are small sinks for magnesium in tissue, whether it be macroalgae, fish, or mangroves. But these are trivial with respect to the amount of magnesium in a marine tank. The concentration of magnesium inside of living tissue is typically lower than in seawater. So making more tissue does not create much of a sink for magnesium.

[ranaman]

Hate to tell you this but leaves have alot of Mg in them. Just as our hemoglobin is built around an atom of Fe. All plants build chlorophyll around an atom of Mg. Mg is considered to be a plant macro nutrient so I would believe a mangrove brightly lit might cause a drop in Mg levels. Alot of plants will turn darker green with Mg supplements.

[Randy Holmes-Farley]

ranaman:

I don't doubt that tissues contain a lot of magnesium. Are you suggesting that mangroves contain more than macroalgae that also contain chlorophyll?

IMO, chlorophyll can't be a big sink for magnesium in mangroves growing in reef tanks.

A typical chlorphyll molecule is about 2.7% magnesium by weight. Even if all of the leaves in my mangroves were 50% chlorophyll (a ridiculous number, the real number is likely orders of magnitude lower), they could only account for a miniscule percentage of the magnesium in my tank. I'd estimate the leaves to weigh something like 50 g, they'd hold 0.7 g of magnesium. Since my tank holds over 700 g of magnesium, that doesn't seem like a big sink to me.

[ranaman]

I did a little bit of searching and found on average plant leaves contain on average 0.2% magnesium based on dry weight. If Iestimate the dry weight of your 50g leaf to be about 20g that would be about 4g Mg, 10g about 2g Mg. From my reading magnesium is not that toxic to plants though at high doses it might stunt growth due to imbalances with K and Ca ratios. So plants may uptake more than they need. This is what is called luxury consumption.
Thinking about it with growing stony corals and coralline algae, they might be bigger sinks making carbonates really starts some thoughts about a tanks biomass. The big difference would be that hobbyist don't allow mangrove leaves to drop into tanks and decay. Also the salt deposits on leaves are washed back into the ocean by rain.
Don't know how macro algae compare to mangroves but they do have far more water in them.

[Randy Holmes-Farley]

ranaman:

0.2% of 20 g is 0.04 g, not 4 g. So the magnesium potential in the leaves is 100 times lower than you quote.

It would take 175 such leaves to drop my magnesium by 1%, and if the leaves took up significant sodium and other ions, they may not actually drop magnesium at all, but simply lower salinity as the use up ions.

[ranaman]

DAMN THOSE DECMIAL POINTS! Just took my restricted use pesticide test and hope I didn't make a stupid mistake like that on the calibration questions. 2% is alot different than 0.2% point taken.