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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The lowest valid value for acute and chronic toxicity to freshwater algae is >36.6 and 2.11 mg Sb/L for Pseudokirchneriella subcapitata (Heijerick and Vangheluwe, 2004). No valid studies with marine algae were identified for antimony.

Key value for chemical safety assessment

Additional information

There are two reliable studies that report on the effect of antimony on algal growth (Heijerick and Vangheluwe, 2004; LISEC, 2001).

In the study by Heijerick and Vangheluwe (2004), exponentially growing cultures of the unicellular green alga Pseudokirchneriella subcapitata were exposed to various concentrations of trivalent antimony (SbCl3) for a period of 72 h. This study was performed with three replicates, seven concentrations (range: 1.22 – 36.6 mg Sb/L) and a control (<0.002 mg Sb/L), using growth as an endpoint, resulting in the same NOEC value, 2.11 mg Sb/L, for both biomass growth (EbCx) and growth rate (ErCx), with the latter being the preferred parameter according to the revised ISO 8692 guideline. The resulting NOEC for algae is thus 2.11 mg Sb/L, with a corresponding LOEC of 4.00 mg Sb/L.

In the study by LISEC (2001) exponentially growing cultures of the unicellular green algae Pseudokirchneriella subcapitata were exposed to various concentrations of trivalent antimony (Sb2O3) for a period of 72 h. This study was performed with three replicates, six concentrations (range: 0.074 – 2.4 mg Sb/L) and a control, using growth as an endpoint, and resulted in two NOEC values, 0.323 and 0.396 mg Sb/L. The former was calculated for growth (biomass), and the latter was calculated for growth rate, which is the preferred parameter according to the revised ISO 8692 guideline. The resulting NOEC for algae is thus 0.396 mg Sb/L, with a corresponding LOEC of 1.32 mg Sb/L. However, this NOEC value will not be taken forward as the key endpoint from this study. Instead the highest concentration tested, with an inhibition of growth rate of 3 % will be used. The reason for using the highest tested concentration of 2.4 mg Sb/L, instead of the reported NOEC or the LOEC (EC3), is that it is not totally clear whether the true beginning of a real dose-response curve is observed, since the highest concentration tested only resulted in an inhibition of 3 % using the recommended endpoint of growth rate. This study has an unusually low variation between the replicates, and a very low effect, even at the highest concentration tested, as shown in the table below. It should also be noted that – where possible – an EC10 is preferred over a NOEC; for this particular study, the EC10 would be >2.4 mg Sb/L as the effects percentage at thic concentration level was well below 10%.

Table: Inhibition of the growth rate of Pseudokirchneriella subcapitata due t oexposure to Sb2O3 (Lisec, 2001)

Measured concentration

mg Sb/L

 0  0.074

0.156

0.323 

0.396

1.32 

2.4 

 Growth rate inhibition

(%)

 --

 0.8

0.8

0.1 

 1

2.3 

 Relative Standard Deviation

(%)

 2.1

2.2 

1.8

 1.1

1.8 

3.5 

1.2 

A confirmatory test would most likely result in a higher NOEC due to the normally much larger variation. A review of data from 41 algal tests indicates that the ErC10 on average corresponds reasonably well with the NOEC (Heitmann and Staveley, 2003). The choice of the highest concentration tested as the key concentration from this study may therefore still be considered as protective, since the inhibition at this concentration is only 3 %. As a result of the low effect at the highest concentration, i. e. 3 %, no EC50 could be determined.

The study by LISEC (1994) on Selenastrum capricornutum (now known as Pseudokirchneriella subcapitata) is considered as unreliable, despite measured concentrations being reported. The reasons are that (i) the reported effect concentrations were not based on the dissolved concentrations but on nominal concentrations, as the total measured concentrations, which consisted of “whole media” (dissolved and dispersed amount of test material) were within 10% of the nominal concentrations, (ii) the measured concentrations in the filtrate differed substantially between the samples taken at the start (0 h) and the end (72 h) of the experiment (with higher concentrations at the end of the experiment). In addition, the EC50 based on growth rate is extrapolated since only a 16 % inhibition in growth rate was observed in the highest test concentration, which exceeds the maximum solubility of Sb2O3.

Nam et al (2009) and Duester et al (2011) reported on the toxicity of antimony potassium tartrate to the green alga P.subcapitata. The generated EC50 values of 206 mg Sb/L and >369 mg Sb/L cannot be considered reliable for the hazard assessment of antimony and antimony compounds; the used test substance is deemed unsuitable as dissolved antimony forms a complex with tartrate, and therefore only a part of the total amount of antimony will be present as “free” antimony; the exact concentration of free antimony can only be estimated via speciation modeling. The reported EC50 values therefore represent the toxicity of the dissolved Sb-tartrate complex at equilibrium, and not the toxicity of the Sb-ion. In addition, the endpoint that was considered by Duester et al (2011) was inhibition of the photosystem II. This endpoint is not considered relevant for the hazard assessment of substances. Duester et al (2011) also evaluated the adverse effect of a pentavalent antimony substance (potassium hexahydroxoantimonate) on the inhibition of photosystem II in P.subcapitata. As this parameter is not considered relevant, the unbounded EC50 of 336 mg Sb/L is not used for the hazard assessment of antimony and antimony compounds. It should also be noted that none of these value are critical, i.e., they do not represent the lowest adverse concentration that has been identified for this species.