Brew12
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Thanks, that helps! I'll need to digest this for a bit but I may have more questions.I think the ratio between orthophosphate on one side and total phosphate (orthoposphate + polyphosphates + organic phosphates) tells something about how quickly phosphate circles in the system. The higher the proportion of polyphosphates + organic phosphates the faster the phosphates are cycling and the higher the proportion of phosphate that is always cycling in organisms.
When thinking about metabolism and cycling of phosphates you must forget the N cycle. Phosphate and N are completely different. Phosphate gets neither reduced nor oxidized. Phosphate forms bonds with sugars like ribose or desoxyribose to form RNA and DNA. In organisms phosphate is bound to other phosphates with polyphosphate bonds, for example ADP + P --> ATP. This is the way organisms store and transport energy to make use of the stored energy in other reactions. In metabolism there is always some orthophosphate that is split from ATP and not yet bound again to ADP. So it may leak from the cell. That is why there must be some phosphate in the water to compensate for the leakage with new uptake. In contrast there is never much free ammonium in the metabolism. Ammonium at once gets bound to glutamate (amino acid with one amino group) to form glutamin (AA with two amino groups) and from then on it is always bound to organic acids to form amino acids or to enzyms during transfer of the amino group until ammonium is excreted through the gills, after for example the organic acid of the aminoacid has gotten oxidized in catabolism to make use of the stored energy.
Particulate phosphates from the fish feces are for example calciumphosphate from fish bone particles in dry foods or from crustacean exoskeltons in frozen food. Also incompletely digested algae and other feedstuff may form particulate phosphate. Bacteria and microalgae with their phosphate stores are also particulate phosphate.
The predominance of these other forms of phosphate in some systems with low orthophosphate concentrations is an important difference to, and in my eyes a good argument against simple orthophosphate dosage.
I think mat formation in cyanobacteria is a strategy to cover, protect and make use of phosphate stores in substrate and rocks.