Tricky EDTA question

[norskfisk]

Hi folks,

I am into the science part of this hobby and I have been experimenting with setting up a phytoplankton refugium for years now. I grow phytoplankton in a refugium, in the same water as the main display. I works, sort of, but particularly long term stability is hard. For example, now I have problems getting good growth in water that has been reused for 10 weeks, no matter how much fertilizer I add. The water has been filtered well with GAC, and a bit of ozone, along the way. Right now I am investigating if the EDTA in the Guillard F/2 fertilizer I am using is causing issues. In total I have now added at least 4 times the amount of F/2 fertilizer that it would take to get the whole water volume to F/2 concentration.

Is it so that EDTA stays in the water even after the metal it was chelated to is taken up? Can it potentially chelate a new ion, recycling itself? Is it stable so that it is only removed very slowly by GAC, ozone and biological processes? Could it make life gradually harder (as the concentration rises) for microorganisms when it comes to taking up metals? At the moment the potential concentration could be more than 50 micromol per liter, if nothing has been lost.

 

[Randy Holmes-Farley Verified Community Expert]

OK, I think I understand the question: does EDTA keep rising and limit metal bioavailability more and more as time goes by with more EDTA addition because the availability of some metals is lost when bound to EDTA (like copper).

As best I can tell is that all of these things you mention can happen, but to what extent one dominates over the others, I do not know.

My understanding is that with some complexes, such as EDTA iron, the complex photodegrades and makes the iron become bioavailable (Spotte covers this in "Captive Seawater Fishes"). That suggest that the accumulation of EDTA may be limited by this degradation. Ozone also can degrade EDTA, but more slowly when tightly complexed to a metal than when free EDTA/sodium.

Some organisms can take up EDTA metal complexes:
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422003000600020
"However, it has been recently demonstrated that the bacterial strain DSM 9103 (located in the Rhizobium-Agrobacterium branch), degrades EDTA as a sole carbon source and it is able to perform the cellular uptake of the metallic complex EDTA-Ca (II), "