Molybdenum dioxide

Toxicity to other aquatic organisms

Description of key information

Relevant and reliable data that could be used for the determination of a PNEC for the freshwater environment - using the statistical extrapolation method - were retrieved for two different species: the snail Lymnaeus stagnalis and the frog Xenopus laevis. Test results were obtained in experiments that followed international accepted guidance, and were based on measured levels of dissolved molybdenum. Values were 221.3 mg Mo/L and 115.9 mg Mo/L, respectively.

Relevant and reliable data that could be used for the determination of a PNEC for the marine environment - using the statistical extraploation method - were retrieved for four different species: the oyster Crassostrea gigas, the sand dollar Dendraster excentricus, the sea urchin Strongylocentrotus purpuratus and the mussel Mytilus edulis. Test results were obtained in experiments that followed international accepted guidelines, and were based on measured levels of dissolved molybdenum. Values were 1174 mg Mo/L, 233.6 mg Mo/L, 325.8 mg Mo/L and 4.4 mg Mo/L, respectively.

Additional information

Freshwater compartment

For the snail Lymnaea stagnalis, the 28d-EC10 value of 221.3 mg Mo/L is retained for the derivation of a PNEC of molybdenum. As mentioned previously, EC10 values are preferred over NOEC-values as the latter are test-dependent values. In this specific case the 28d-EC10 of 221.3 mg Mo/L is slightly higher than the 28d-NOEC of 200 mg Mo/L for this endpoint. The latter concentration level, however, only caused 5-6% effect on growth, i. e., well below the effect level of 10% that is considered relevant for chronic toxicity. The 28d-LOEC is almost a factor of 2 higher than the 28d-EC10.

 For the frog Xenopus laevis, the 4d-EC10 value of 115.9 mg Mo/L (effect parameter: malformation) is retained for the derivation of a molybdenum PNEC. This 4d-EC10 is almost a factor of 5 higher than the 4d-NOEC for this effect parameter. It should be noted, however, that the 8% malformations associated with this NOEC was also found at an exposure level of 87.3 mg Mo/L. The lowest actual concentration that resulted in more than 10% effect was 177 mg Mo/L, i. e., a factor of 1.5 higher than the derived 4d-EC10. The low NOEC is the result of the 10% effect found at an exposure concentration of 48.7 mg Mo/L and the low inter-replicate variation, resulting in a significant effect compared to the control. Taking into account that 8% and 12% effect was observed at concentration levels of 87.3 mg Mo/L and 177 mg Mo/L, respectively, a 4d-EC10 of 115.9 mg Mo/L for X. laevis is considered relevant and reliable.

For the toad Gastrophryne carolinensis, the reported NOEC or EC10 values were categorized as unreliable upon evaluation. Therefore, this species is not taken into account in the effect assessment of molybdenum for the aquatic compartment.

 

Marine compartment

For the oyster Crassostrea gigas (Aquasense, 2009) an 48h-EC10 value of 1174 mg Mo/L (endpoint: larval development) is retained for the derivation of a predicted no-effect concentration for molybdenum in the marine compartment. The 72h-NOEC value for this endpoint was 1100 mg Mo/L. The EC10 value, however, is preferred over the NOEC value as the latter is a test design-dependent value.

For the sand dollar Dendraster excentricus a 48h-EC10 value of 233.6 mg Mo/L (endpoint: proportion of normal developed larvae) is retained for the derivation of a predicted no-effect concentration for molybdenum in the marine compartment. Comparable (though slightly higher) values were noted for other endpoints, e.g., number of normal larvae (254.5, 292.1 mg Mo/L). The 72h-NOEC value for these endpoints was 217.9 mg Mo/L (exception: mortality: 438.4 mg Mo/L). The EC10 value, however, is preferred over the NOEC value as the latter is a test design-dependent value.

For the sea urchin Strongylocentrotus purpuratus (Parametrix, 2008) the 94h-EC10 value of 325.8 mg Mo/L is retained for the derivation of a PNEC for molybdenum in the marine environment (endpoint: combined normal proportion). A comparable, though slightly higher 94h-EC10 value of 483 mg Mo/L was determined for the endpoint ‘normal proportion’. NOEC values for these endpoints were 80.2 -264.5 mg Mo/L. EC10 values, however, are preferred over NOEC values as the latter are test design-dependent values.

For the mussel Mytilus edulis (Morgan et al, 1986) an 48h-EC10 value of 4.4 mg Mo/L (endpoint: larval development) is retained for the derivation of a PNEC for molybdenum in the marine compartment.