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Diss Factsheets

Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Description of key information

Silicon powder dissolves quite slowly to water and aquatic algae test results are not available for elemental silicon. Read across information is used instead, since valid silica/Si(OH)4 studies applying measured test concentrations are available (studies applying soluble silicate salts and pH adjusted to 6-8). Based on existing read across information from soluble silica tests and pure amorphous silica (fume) tests, silicon is not acutely toxic to algae. This would means that green algae NOEC and NOEC/EC50 values were > 100 mg/l if tested.

Key value for chemical safety assessment

EC50 for freshwater algae:
100 mg/L

Additional information

Aquatic algae, diatoms (Bacillariophycea) build up their skeletons by taking up large amounts of silicic acid from water and further synthetising amorphous silica skeletal structures. Silicon is therefore an essential element for diatoms and silicon is a major factor influencing (primary) algal production in many lakes and fresh water systems. Utilization of silica in the trophogenic zone of lakes by diatoms often reduces the surface water (epilimnetic) concentrations and includes, along with other factors, a seasonal succession of diatom species. When the concentration of silica is reduced below about 0.5 mg/L, many diatomic species cannot compete effectively with nonsiliceus algae, and their growth rates decline until silica concentration gets higher again. In deep lakes many of the dead diatom structures undergo partial dissolution before reaching the sediment (Wetzel 1983). Silicon is not an essential element for green algae.

Silicon, particulate or dissolved, is expected to be in the oxidized form in aquatic environments. Elemental silicon at a zero valency state may be present only inside of particles (subsurface) and when the surface is oxidized and covered by hydroxyl groups. The dissolved fraction is in the form of monosilicic acid.

Therefore results of tests carried out applying pure soluble silicates may be used for read-across purposes to make conclusions on toxicity of silicon to aquatic plants.