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Toxicity to aquatic algae and cyanobacteria

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Description of key information

In a 72h algal growth inhibition study according to GLP and OECD guideline 201, exposure of algae (Pseudokirchnerella subcapitata ) did not cause any adverse effects.
The NOEC (toxicity) was estimated to be 97.1 mg DAP/L (no inhibition of algal growth observed in study) The NOEC for stimulation was established to be 3.57 mg DAP/L. No adverse inhibitory effects were observed. There was some growth stimulation at 6.25 mg/L and above. This was consistent with the known stimulatory effect of phosphate compounds. The reported NOEC reflects this stimulatory response and is not considered to be an adverse effect.
Read across from diammonium phosphate to magnesium ammonium phosphate is considered justified based on following background:
Since both the magnesium ammonium phosphate and diammonium phosphate dissociate to their respective ammonium, (magnesium) and phosphate ions, it is considered acceptable to approach the assessment of magnesium ammonium phosphate based on the individual components.
Since Diammonium phosphate is very soluble in water and therefore much more bioavailable (588 g/l; The Merck Index“, 14th Edition, M. J. O’Neil (Editor), Merck Research Laboratories, Division of Merck & Co., Inc., Whitehouse Station, NY, USA (2006)) read across to the only slightly soluble magnesium ammonium phosphate (0.1 g/l) is considered as worst-case assumption. The only difference between the two inorganic salts is the replacement of one ammonia ion and one hydrogen ion by magnesium.
Magnesium from magnesium ammonium phosphate is not assumed to pose an additional risk for acute ecotoxicity since it is an essential mineral and ubiquitous present in food. Magnesium is the eighth most abundant element, constitutes about 2% af the Earth's crust by weight and is an essential element in biological systems, present in every cell type and organism.
The most common source of magnesium in groundwater is through the erosion of rocks, such as dolomite and minerals, such as magnesit.

Key value for chemical safety assessment

EC50 for freshwater algae:
97.1 mg/L
EC10 or NOEC for freshwater algae:
97.1 mg/L

Additional information

The study was performed according to GLP and OECD guideline 201.

The measured NOEC value of 97.1 mg/l is read across to magnesium ammonium phosphate. No further testing is required.