Phosphate - Food for thought

[ZaneTer]

Hello everyone

We have long debated phosphate in the home aquarium and questioned why some people have low residual PO4 levels yet maintain healthy and vibrant corals. Many of us believe that this could be due to high bacterial levels in the water being a food source for the corals.

My question is has anyone ever analysed a sample of seawater, including all organics within the sample and sent it off for ICP analysis. The sample must not have been through a centrifuge or filter before undergoing analysis. If this has been done can anyone share the results?
Hopefully a study will show two sets of results, one for the total phosphate including that which is bound up in bacteria and another to show the residual level present in the water excluding bacterial sources.

I am hoping to get a better understanding of how our corals uptake phosphate from the water.

Thanks
Zane

 

[NY_Caveman]

Hello everyone

We have long debated phosphate in the home aquarium and questioned why some people have low residual PO4 levels yet maintain healthy and vibrant corals. Many of us believe that this could be due to high bacterial levels in the water being a food source for the corals.

My question is has anyone ever analysed a sample of seawater, including all organics within the sample and sent it off for ICP analysis. The sample must not have been through a centrifuge or filter before undergoing analysis. If this has been done can anyone share the results?
Hopefully a study will show two sets of results, one for the total phosphate including that which is bound up in bacteria and another to show the residual level present in the water excluding bacterial sources.

I am hoping to get a better understanding of how our corals uptake phosphate from the water.

Thanks
Zane

Cool idea. Someone must have done this or similar.

 

[Randy Holmes-Farley Verified Community Expert]

Hello everyone

We have long debated phosphate in the home aquarium and questioned why some people have low residual PO4 levels yet maintain healthy and vibrant corals. Many of us believe that this could be due to high bacterial levels in the water being a food source for the corals.

My question is has anyone ever analysed a sample of seawater, including all organics within the sample and sent it off for ICP analysis. The sample must not have been through a centrifuge or filter before undergoing analysis. If this has been done can anyone share the results?
Hopefully a study will show two sets of results, one for the total phosphate including that which is bound up in bacteria and another to show the residual level present in the water excluding bacterial sources.

I am hoping to get a better understanding of how our corals uptake phosphate from the water.

Thanks
Zane

I think the answer is complicated and hard to use, since even knowing the relative concentrations doesn't really say anything at all about the relative bioavailability to any given organism. That said, if you want to compare concentrations of organic phosphate to inorganic phosphate, I'd suggest comparing ICP (total P including both inorganic and organic P) and phosphate measured by an ordinary test kit.

As a very rough guide, however, Ken Feldman measured total organic carbon in a variety of reef tanks. He got as much as 5 ppm in an unskimmed tank and about 0.5 to 1 ppm in skimmed tanks.

https://www.advancedaquarist.com/2008/9/aafeature2

Using the admittedly inaccurate Redfield ratio to extrapolate to phosphate, we use a typical formula of (CH2O)106(NH3)16(H3PO4) .

Thus there is typically one P for eacgh 106 carbon atoms. Phosphate weighs more than carbon, so the weight ratio is about one ppm P for each 41 ppm carbon.

So we extrapolate that in the skimmed tanks Ken looked at (with 0.5 to 1 ppm organic C), there may be about 0.01 to 0.02 ppm phosphate. In his unskimmed tank, it would be higher, about 0.1 ppm.

 

[ZaneTer]

Thank you very much Randy

Is it possible to chemically break down a sample containing bacteria and such with an acid perhaps? If it is possible do you have any suggestions on what acid to use?
I am planning on sending two samples for ICP analysis. One with untreated tank water and a treated tank sample to see if there is any difference.

 

[Randy Holmes-Farley Verified Community Expert]

Yes, Hach has a total phosphate kit (PO-24) that digests organic P down to inorganic P. It is a pain to use, but should work fine.