Silver

Toxicity to aquatic plants other than algae

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic plants other than algae

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Substance considered to fall within the scope of the read-across 'Silver metal: Justification of a read-across approach for environmental information requirements' (document attached in IUCLID section 13).

Reason / purpose for cross-reference:

read-across source

Duration:

7 d

Dose descriptor:

EC10

Effect conc.:

14 µg/L

95% CI:

>= 7 - <= 29

Nominal / measured:

meas. (geom. mean)

Conc. based on:

element (dissolved fraction)

Basis for effect:

frond number

Duration:

7 d

Dose descriptor:

EC10

Effect conc.:

5.2 µg/L

95% CI:

>= 3.2 - <= 8.5

Nominal / measured:

meas. (geom. mean)

Conc. based on:

element (dissolved fraction)

Basis for effect:

frond area

Key result

Duration:

7 d

Dose descriptor:

EC10

Effect conc.:

1.4 µg/L

95% CI:

>= 0.4 - <= 4.2

Nominal / measured:

meas. (geom. mean)

Conc. based on:

element (dissolved fraction)

Basis for effect:

total root length

Duration:

7 d

Dose descriptor:

EC10

Effect conc.:

19 µg/L

95% CI:

>= 3.9 - <= 91.9

Nominal / measured:

meas. (geom. mean)

Conc. based on:

element (dissolved fraction)

Basis for effect:

dry weight

Reported statistics and error estimates:

Effect concentrations were calculated based on relative responses (expressed relative to the mean control response of the respective experiment). The EC10, EC20, and EC50 values and corresponding confidence intervals were determined based on a log-logistic concentration response model with 2 parameters using Statistica software.
The NOECs and LOECs were calculated with the Williams (1971) test, after evaluation of the data for adherence to the underlying assumptions of normality and homogeneity of variances.

Overview of average^a growth rate in the different exposure treatments of the 7d-Lemna minor tests for the growth rate endpoints (r_n& r_a), root length and dry weight.

Nominal Ag	Dissolved Ag in fresh solutions^b (µg/L)	Dissolved Ag in old solutions^b (µg/L)	Geometric mean dissolved Ag	Growth rate (frond number) rn	Growth rate (frond area) ra	Root length	Dry weight
(µg/L)			(µg/L)	(d^-1)	(d^-1)	(mm)	(mg)
0	<0.035	<0.035	<0.035	0.40±0.02	0.30±0.02	9.8±1.8	5.2±1.0
0.32	0.18±0.01	0.09±0.01^c	0.13	0.41±0.01	0.31±0.01	10.8±0.9	6.4±0.2
1	0.59±0.04	0.30±0.04	0.42	0.40±0.003	0.30±0.004	10.2±1.0	6.4±0.6
3.2	1.8±0.2	0.97±0.07	1.30	0.41±0.02	0.30±0.01	9.2±0.6	6.3±0.8
10	6.0±1.6	3.4±0.5	4.31	0.36±0.02	0.28±0.02	8.2±0.7	5.4±0.8
32	28±4	14±5	18.3	0.36±0.002	0.24±0.02	5.2±0.5	4.8±0.3
100	84±6	48±13	60.4	0.31±0.01	0.20±0.01	3.8±0.4	3.3±0.3
320	292±17	206±27	243	0.27±0.02	0.14±0.02	3.7±0.7	2.3±0.3
(1000)d^c	351±9	333±55	336	(0.29±0.01)	(0.17±0.004)	(3.4±0.6)	(3.1±0.3)

^aAverage of all replicates ± standard deviation is reported.

^bAverage of three samples± standard deviation is reported.

^cBelow limit of quantification of ICP-MS (Limit of quantification 0.12 µg/L)

^dThe responses of the 1000 µg nominal Ag/L treatment were not taken into account for concentration response fitting, as measurements of actual silver concentrations indicates that silver precipitation likely occurred in the exposure solution.

Above table shows that all endpoints showed a clear concentration–response behavior with increasing Ag dosing levels.

The concentration response of the dry weight endpoint showed a significant hormesis effect. This effect was not observed for any of the other endpoints. The corresponding effect concentrations are reported in the table below.

Effect concentrations (expressed as the geometric mean of the measured dissolved Ag concentrations in fresh and old solutions) of ionic silver (Ag) to the aquatic species Lemna minor.

Endpoint	EC10 (µg Ag/L)	EC20 (µg Ag/L)	EC50 (µg Ag/L)	NOEC (µg Ag/L)	LOEC (µg Ag/L)
Growth rate (frond number)	14 (7-29)	62 (42-92)	769* (381-1550)	1.3 (-1.8±5.4)	4.3 (9.9±5.0)
Growth rate (frond area)	5.2 (3.2-8.5)	18 (13-25)	159 (124-205)	1.3 (0.4±3.5)	4.3 (9.5±5.3)
Root length	1.4 (0.4-4.2)	4.8 (2.2-10.5)	42 (25.1-68.9)	4.31 (16.1±7.5)	18.3 (47.5±5.4)
Dry weight	19.0 (3.9-91.9)	41.8 (14.5-120.4)	162 (78-336)	18.3 (6.6±4.9)	60.4 (35.8±4.9)

EC_xvalues were calculated using a log-logistic concentration response model with 2 parameters.

NOEC and LOECs were calculated using the Williams-test. The average growth rate inhibition ± standard deviation (%) relative to the control at the NOEC or LOEC are reported between brackets.

^*Extrapolated outside the tested concentration range (geometric mean of the highest silver treatment used for concentration response fitting was 243 µg/L).

Validity criteria fulfilled:

yes

Remarks:

Average doubling time in the control treatments: 1.75 ± 0.10 d. Average control growth rate: 0.40 ± 0.02 d–1 (rn) and 0.30 ± 0.02 d–1 (ra). Average end root length in control: 9.8 ± 1.8 mm, average control dry weight : 5.2 ± 1.0 mg.

Conclusions:

Based on the ECx values, root length was the most sensitive endpoint after 7d exposure of L. minor to silver nitrate.. The EC10 and EC50 values for root length were 1.4 and 41.6 µg dissolved Ag/L, respectively. The growth rate (rn and ra) had the lowest NOEC and LOECs: 1.3 (NOEC) and 4.3 (LOEC) µg dissolved Ag/L. The intratreatment variation was generally higher for the root length and dry weight endpoints.

Executive summary:

In a 7 day study with the aquatic plant Lemna minor exposed to silver nitrate, the EC10 for the most sensitive endpoint (root length) was 1.4  µg dissolved Ag/L.

This is a guideline study considered suitable for use as a key study for this endpoint.

Description of key information

Read across from ionic silver

Plus supporting published data from 2 studies included in the REACH dossier as Endpoint Study Records with various sizes of nanoparticles, showing that nanosilver is equally or less toxic than ionic silver

Additional information

Summary of available data for uncoated and coated nanomaterials

Reliable and relevant data on the toxicity of nanosilver to aquatic plants are available from two studies (Gubbins et al. 2011 and Oukarroum et al. 2013). Gubbins et al. (2011) reported 14 day EC10 (dry weight) values for Lemna minor of 9.71 and 8.14 µg/L for citrate capped nanomaterials with mean particle size distribution of 29.2 and 93.52 nm (determined by TEM), respectively. A comparative exposure to ionic silver (silver nitrate) performed in the same study resulted in a 14 day EC10 (dry weight) value of 6.01 µg/L, which is comparable to the 7 day EC10 for Lemna minor of 6.0 µg/L ionic silver reported by Naumann et al. (2007), used as key data in the current silver REACH CSR. Based on these data, citrate coated nanosilver particles, irrespective of size, would appear to be of similar toxicity as ionic silver to Lemna minor, despite the differences in exposure duration.

The Oukarroum et al. (2013) macrophyte study reports a seven day EC10 (growth rate) of 3.6 µg/L for an uncoated spherical silver particle with a raw nominal particle size of 50 nm. This EC10 is marginally (< factor of 2) lower than the EC10 results reported for ionic silver by both Naumann et al. (2007) and in the Gubbins et al. (2011) study. However, as Oukarroum et al. (2013) do not report the results of a comparative test with ionic silver, these data in isolation are not sufficient to suggest that nanosilver should be considered more toxic to macrophytes than ionic silver.

These data support the precautionary read-across of properties from ionic silver to nanosilver in the freshwater compartment. At present, as the available algal dataset is small, no conclusions can be made regarding the influence of particle size or coating material on the resulting toxicity of nanosilver to aquatic plants.