Strontium

Toxicity to aquatic algae and cyanobacteria

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Test conducted under GLP according to standard OECD guidelines

Justification for type of information:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).
Due to the buffering capacity of most environmental systems, it may reasonable be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation.

The solubility of strontium is not greatly affected by the presence of most inorganic anions as there is little tendency for strontium to form complexes with inorganic ligands (Krupka et al. 1999. EPA 402-R-99-004B and references therein). Free Sr2+ cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high pHs. In solutions with a pH below 4.5, the Sr2+ ion is dominant. Under more neutral conditions (pH 5 to 7.5), SrSO4 forms. Strontium carbonate controls strontium concentrations in solutions only under highly alkaline conditions. Further, dissolved strontium forms only weak aqueous complexes with chloride and nitrate (Salminen et al. 2015 and references therein, Krupka et al. 1999. EPA 402-R-99-004B). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as strontium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, strontium does not form strong complexes with fulvic or humic acids based on the assumption that strontium would exhibit a similar (low) stability with organic ligands as calcium and that strontium could not effectively compete with calcium for exchange sites because calcium would be present at much greater concentrations (Krupka et al. 1999. EPA 402-R-99-004B). In sum, strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015). Thus, it may further be assumed that the behaviour of the dissociated strontium ions in the environment determine the fate of strontium upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.Therefore, the assessment of the ecotoxicity of strontium is based on elemental strontium concentrations. Read-across of ecotoxicity data available for soluble strontium substances is applied since the strontium ions determine the ecotoxicological potential of strontium.

Qualifier:

according to guideline

Guideline:

OECD Guideline 201 (Alga, Growth Inhibition Test)

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

Samples for possible analysis were taken from all test concentrations and the control:- frequency: at t=0, t=24h and t=72h- volume: 4.8 mL- storage: samples were stored in a freezer until analysisAt the end of the exposure period, the replicates with algae were pooled at each concentration before sampling

Details on test solutions:

Pre-culture medium: M2; according to OECD 201 Guideline, formulated using MilliQ tap water and subsequently passed over activated carbon and ion-exchange cartridges

Test organisms (species):

Raphidocelis subcapitata (previous names: Pseudokirchneriella subcapitata, Selenastrum capricornutum)

Details on test organisms:

In-house laboratory culture; strain NIVA CHL 1Stock culture: Algae stock cultures were started by inoculating growrth medium with algal cells from a pure culture on agar. The suspensions were continuously aerated and exposed to light in a climate room at a temperature of 21-24 degrees Celcius. Pre-culture: 4 days before the start of the test, cells from the algal stock culture were inoculated in culture medium at a cell density of 10,000 cells/mL. The pre-culture was maintained under the same conditions as used in the test. Te cell density was measured immediately before use.

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

72 h

Hardness:

test medium (m2): 24 mg/L as CaCO3

pH:

test medium (M2): 8.1 +/- 0.2

Nominal and measured concentrations:

nominal: 0.1, 1, 10 and 100 mg/L of strontium nitrateMeasured 100 mg/L: 102.5 mg/L strontium nitrate (t=0)Measured 100 mg/L: 104 mg/L strontium nitrate (t=24h)Measured 100 mg/L: 107.5 mg/L strontium nitrate (t=72h)Measured 100 mg/L (mean): 104.7 mg/L strontium nitrate, i.e., 43.3 mg Sr/L

Details on test conditions:

Vigorous shaking was sufficient to completely dissolve the test substance in mediumTest vessels: 100 mL; all glass - vessels were distributed at random in the incubator Test volume per replicate: 50 mLControl: test medium without test substance or other additives No of replicates for control and highest test concentration: 6No. of replicates for remaining test concentrations: 31 or 2 replicates of each test concentration without algae1 replicate of each concentration for sampling purposesCell concentration at start: 10,000 cells/mL Illumination: continuously using TLD-lamps of the type 'Cool-white' of 30 Watt, with a light intensity within the range off 66 to 77 uE/m2/s Algal cells were kept in suspension by continuous shaking- pH was measured at the beginnig and end of the test period, and was not supposed to deviate by more tan 1.5 units during the test - at the end of the final test microscopic observations were performed on the limit and one lower concenration to verify a normal and healthy appearance of the inoculum algae and to observe for any abnormal appearance of algae. - Cells were counted using a microscope and a counting chamber at the beginning of the test; thereafter cell densities were determined by spectrophotometric measurement of samples at 720 nm using a Varian Cary 50 single beam spectrophotometer with a cuvette (pathlength : 10 mm). Quantification of cell density was based on a calibration curve.

Reference substance (positive control):

yes

Remarks:

potassium dichromate

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 43.3 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

element

Basis for effect:

growth rate

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 104.7 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

growth rate

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 43.3 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

element

Basis for effect:

growth rate

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 104.7 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

growth rate

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 43.3 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

element

Basis for effect:

biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 104.7 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 43.3 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

element

Basis for effect:

biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 104.7 mg/L

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

biomass

Details on results:

No significant differences were recorded between the values for growth rate or yield at the limit concentration, i.e., 100 mg/L when compared to the control group. The growth and yield in lower test concentrations were stimulated rather than reduced/inhibited by the test substance. Microscopic observations at the end of the test revealed a normal and healthy appearance of the exposed cells when compared to the control. Compliance with the Quality criteria regarding mainenance of actual concentrations was demonstrated by running a test vessel at the highest substance concentrartion but without algae, and samples for analysis were taken at the start, after 24 hours of exposure and at the end of the test period. Test results were in compliance with all validity criteria (minimum increase in cell density, criteria on variation (growt rates, section-by-section specific growth rates) were not exceeded.

Results with reference substance (positive control):

The 72h-EC50 for growth reduction was 1.2 mg/L (95%CL: 0.77-2.0 mg/L).The historical range of this value lie between 0.82 and 2.3 mg/L.

Reported statistics and error estimates:

Recorded effects were not significant (< 10%); EC50 values could not be calculated because the test substance proved to be non-toxic within the tested concentration range.For determination of the NOEC and the EC50 the approaches recommended in the OECD guideline 201 are used. An effect was considered to be significant if statistical analysis of the data obtained for the test concentrations compared with those obtained in the negative control revealed significant reduction of growth rate or yield inhibition (ANOVA, Tukey test, Bonferroni t-test, TOXSTAT Release 3.5, 1996, D.D. Gulley et al).

Percentage reduction of growth rate (total test period)

Nominal Concentration Strontium nitrate (mg/L	Mean growth rate - u (0 -72h)	Mean growth rate - reduction (%)
Control	0.05924
0.10	0.06675	- 12.7
1.0	0.06704	-13.2
10	0.06628	-11.9
100	0.05931	-0.1

Validity criteria fulfilled:

yes

Conclusions:

No statistically significant differences were found between the control and any test item concentration up to and including 100 mg test item/L. The results of the study demonstrate that the EC50 for Pseudokirchneriella subcapitata is >100 mg test item/L. The no-observed effect concentration (NOEC) is ≥104.5 mg/L strontium nitrate (based on measured concentrations). Considering the measured concentrations, the EC50 for Pseudokirchneriella subcapitata is > 43.3 mg Sr/L (measured in the highest concentration at test end). The no-observed effect concentration (NOEC) is ≥ 43.3 mg/L Sr.These data can be used for hazard and risk assessment purposes (classification; PNEC-derivation)

Description of key information

One reliable acute/chronic toxicity study (Klimisch 1, GLP) for an algal species  -the green alga Pseudokirchneriella subcapitata - has been identified. Based on measured Sr-level in the water column, an unbound 72h-ErC50 of > 43.3 mg Sr/L and a 72h-NOErC of >= 43.3 mg Sr/L were reported by Tobor-Kaplon (2010), using  Sr(NO3)2 as test substance.

Additional information

One reliable acute/chronic toxicity study (Klimisch 1, GLP) for an algal species - the freshwater green alga Pseudokirchneriella subcapitata - has been identified. No effect (growth rate) was noted at the highest (nominal) test concentration of 100 mg test material/L. Based on measured Sr-levels, this effect levels translates to 43.3 mg Sr/L. Therefore, based on measured Sr-levels in solution, the ErC₅₀ and NOEC amount to >43.3 mg Sr/L and >=43.3 mg Sr/L, respectively.