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Toxicity to aquatic algae and cyanobacteria

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Description of key information

NOEC for freshwater algae = 44.8 mg/L.  

Key value for chemical safety assessment

EC50 for freshwater algae:
73 mg/L
EC10 or NOEC for freshwater algae:
44.8 mg/L

Additional information

The ASA Category comprises the following 5 aromatic sulphonic acids:

 TSA, Toluene-4-sulphonic acid (EC 203-180-0, CAS 104-15-4)

XSA, (Xylenes and 4-ethylbenzene) sulphonic acid (EC 701-247-3, CAS -) Former EC 246-839-8

CSA, p-cumene sulphonic acid (EC 240-210-1, CAS 16066-35-6)

BSA, Benzene sulphonic acid (EC 202-638-7, CAS 98-11-3)

HBSA, 4-hydroxybenzensulphonic acid (EC No. 202-691-6, CAS No. 98-67-9)

The short-term toxicity on aquatic invertebrates was assessed with available studies on ASA and related salt (Hydrotropes category). Hydrotropes are completely dissociated in water and could be used in read across for the related acid form. The growth inhibition in aquatic plants has been tested on BSA and HBSA.

There is a 2005 good quality "key study" for a 99.8% pure BSA (CAS No. 98-11-3). It followed OECD Guideline 201 and was done under GLP with analytical confirmation and a reference substance. The 72h-EC50 for Selenastrum capricornutum was 70 mg/L active ingredient based on biomass inhibition. The 72h-EC50 was 73 mg/L for growth rate inhibition. The reported 72h-NOEC was 44.8 mg/L. Both values are based on measured test substance. It appears the pH of the test substance was responsible for the observed toxicity. The report indicates that the observed toxicity may very well have been due to the acidity of the test substance. The pH of the test medium at the two highest test concentrations (100 and 220 mg/L) was in the range of 5 and 3, respectively. The test conditions were confirmed to be valid and the measured concentrations were in the range of 90-110% of the nominal concentrations for the 5 exposures.

In a 2020 study, HBSA was tested for the growth inhibition of the freshwater green alga Pseudokirchneriella subcapitata after 72 hours with the following effect values (based on the nominal test item concentration): the EC50-values for inhibition of growth rate (ErC50) and yield (EyC50) after 72 hours were 128 mg/L and 101 mg test item/L, respectively, corresponding to the active substance concentrations 84.2 mg/L and 66.8 mg a.s./L, respectively. The NOEC-values for inhibition of growth rate and yield after 72 hours were 75.0 and 9.38 mg/L, respectively, corresponding to 49.4 mg a.s./L and 6.18 mg a.s./L, respectively.

There are four tests related to algal toxicity studies for the hydrotropes on STS, SCS, SXS, CaXS (i. e., the salt form of the aromatic sulphonic acids) and all studies indicate toxicity in the same range as for the acid form.

One study is a 1993 USEPA Guideline study - EPA OTS 797.1500 - Algal Toxicity performed on Sodium Xylene sulphonates.. The study was not conducted with GLP requirements but considerable details are available. The freshwater Selenastrum sp. was exposed to a range of nominal concentrations of the test substance, sodium xylene sulphonate (CAS No 1300-72-7). The 96-hour EC50 was 230 mg/L active substance based on cell density (with a 95% CL of 47-1200 mg/L). There was an apparent dose response and the 96-hour NOEC was 31 mg/L. The test substance was observed to be algistatic but not algicidal at the highest concentration, 1000 mg/L active substance.

The 2nd study is a 1994 EPA Guideline study (EPA OTS 797.1500) of calcium xylene sulphonate (CAS No 28088-63-3) conducted under GLP requirements and with analytical confirmation reports a 96h-EC50 of 758 mg/L active substance (with a 95% confidence limit of 592-924 mg/L) and a NOEC of 240 mg/L based on growth rate of Selenastrum sp.

The 3rd study is a 1995 EU Guideline study (EU C.3 Algal Inhibition Test) of sodium toluene sulphonate (CAS No 657-84-1 that is similar to CAS No 12068-03-3) conducted under GLP requirements and with analytical confirmation reports a 72h-EbC50 of 150 mg/L active substance based on cell density and a 72h-ErC50 of 310 mg/L based on specific growth rate of Scenedesmus sp. The 72h-NOEC is 40 mg/L active substance based on biomass.

The 4th study was perfomed, in 2020 on SCS, in GLP, according to OECD 201. Growth (ErC50) and yield (EyC50) after 72 hours were > 252 mg/L, corresponding to > 100 mg a.s/l.

The study on BSA has been selected as representative for the category, since it is a good quality study, performed on a very pure substance. The two studies performed on acids have very similar results and the lower has been taken as conservative representative value. Results on the salt forms of STS, SCS and SXS demonstrated that the acid form is conservative towards the aquatic plants, being the EC50 values of Hydrotropes in general higher. Based on the tests results, both ASA and Hydrotropes substances do not have any concern for the aquatic plants.