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As always, nice experiments. I need to try the sludge addition, but first...
A funny thing happened to my Spirulina culture when I added rinsed aragonite sand from the aquarium. It barely grew compared to the no sand control and only at the water line. @taricha reminded me about the adsorption of PO4 by aragonite. Sure enough, an experiment proved that 0.15 mL of sand in 5 mL of medium was sufficient to seriously deplete the medium of PO4. After adding fresh medium, the Spirulina growth increased but barely at all on the sand, just at the water line. I am in the process of repeating the sand experiment results with additional observations to help understand why there might be this effect. I am now wondering whether Spirulina adapts to being cultured differently then the Oscillatoria @taricha is culturing. I also wonder if Oscillatoria has different PO4 requirements. That means @taricha needs to measure the PO4 level in his cultures for me
Here are the results for PO4 adsorption and desorption of rinsed aragonite for 0.15 mL well rinsed sand. The last point (green dot) is where I treated the sand with fresh Instant Ocean acidified to pH 5 to see if more PO4 would desorb. Take away messages for me:
Aragonite sand is a PO4 sponge and could have depleted my culture medium of PO4
Only a portion of the adsorbed PO4 was desorbed
I cannot say how much PO4 was consumed by the organisms growing on the sand grain versus the formation of insoluble calcium phosphate.