Dipotassium phosphonate

Toxicity to aquatic algae and cyanobacteria

Description of key information

EC10 72h (grow rate) = 27.5 mg/l

EC10 72h (yeld) = 20.3 mg/l

EC50 72h (grow rate) = 137.5 mg/l

EC50 72h (yeld) = 101.3 mg/l

Key value for chemical safety assessment

EC50 for freshwater algae:

137.5 mg/L

EC10 or NOEC for freshwater algae:

27.5 mg/L

Additional information

The influence of the test item Potassium phosphonate (KH₂PO₃/K₂HPO₃) on the growth of the green algal species Pseudokirchneriella subcapitata, formerly known as Selenastrum capricornutum, was investigated in a 72-hour limit test according to the OECD Guideline No. 201, 2011. (Tediosi E., 2013)

For this purpose, exponentially growing test algae were exposed for 72 hours to an aqueous test medium containing the test item at only nominal concentration of 200 mg/L as active ingredient Potassium phosphonate, under defined conditions.

Besides the concentration,a negative control without the test item was also prepared to check the acceptability of the algal inoculum.

The actual test concentration of phosphite were analytically measured at the beginning of the test (before and after having inoculated the test flasks) and at the end of the test.

The analytical recovery of phosphite concentrationin the freshly prepared solutionwas 92.4 % (before inoculum) and 109.9 % (after inoculum) of the nominal concentration.

After 72 hours of exposure,the analytical recovery remained unchanged, being 108.8 % of the nominal phosphite concentration.

Additionally, also a not inoculated sample was analysed after 72 hours and the analytical recovery was 109.3 %.

The analytical recoveries were referred to the phosphite nominal content (corrected for the exact weighed amount).

As recommended by OECD Guidance Document on testing of difficult substances No. 23 (2000), the biological results were referred to the nominal concentration of Potassium phosphonate (KH₂PO₃/K₂HPO₃), since the analytical recoveries of phosphite were in the range 80 % - 120 %.

Algal cell density was measured every 24 hours taking aliquots from each test concentration replicate and from negative control, diluting them in a NaCl 9 g/L solution and reading with an electronic particle counter.

At the end of the test, cell density in the negative control increased on average by a factor of 293. This value complies with the validity criterion of the test, according to OECD guideline No. 201, 2011, which indicates a minimum increase factor of 16.

The negative control also met the other validity criterion, with a coefficient of variation of daily growth rates at 72 hours of 25.9 % and a coefficient of variation of average growth in replicates negative control cultures during the test period of 1.1 %.

For the tested concentration, 200 mg a.i./L, a significant biological effect was showed in comparison with negative control for both the endpoints, yield and growth rate, after 72 hours of exposure.

 

The 72-hour EC₁₀, EC₂₀, EC₅₀(with their confidence limits) and the NOEC, calculated by statistical analysis, expressed in terms of the nominal Potassium phosphonate concentration, are reported in the following table:

 

Endpoint	0 - 72 h EC10	0 - 72 h EC20	0 - 72 h EC50	0 - 72 h NOEC
Growth rate	27.5 (26.7 – 28.5)*	55.0 (53.3 – 57.0)*	137.5 (133.3 – 142.4)*	< 200**
Yield	20.3 (20.2 – 20.3)*	40.5 (40.4 – 40.6)*	101.3 (101.1 – 101.6)*	< 200***

*Linear interpolation method.

**Equal variance t method.

***Unequal variance t method.