Calcium molybdate

Toxicity to aquatic algae and cyanobacteria

Description of key information

Reliable data for freshwater algal species have been generated in different laboratories, using different strains of the same algal species. Effect levels based on growth rate were situated between 61.2 and 366.2 mg Mo/L. The geometric mean of data that were generated with the most sensitive strain was 74.3 mg Mo/L, and this value is put forward for the derivation of a freshwater PNEC in a weight of evidence approach. The geometric mean of EC50 values (acute toxicity) that were obtained with the most sensitive strain of P. subcapitata was 333.1 mg Mo/L.

For the marine environment, reliable growth-rate based EC10-levels have been identified for the microalga Dunaliella tertiolecta and the diatom Phaeodactylum tricornutum. The 72h-EC10 values of these species that are used for the determination of a marine PNEC, are 881 mg Mo/L and 169.9 mg Mo/L, respectively. Marine EC50-values exceeded the highest tested concentration and could not be quantified.

Additional information

Acute toxicity for freshwater algae:

The reliable ErC50-values that were determined by De Schamphelaere et al (2008) for the green alga P. subcapitata were situated between 295 and 1568.9 mg Mo/L. Rodriguez (2008) reported reliable ErC50s that ranged from 289.2 to >419.9 mg Mo/L for the same species. The acute data confirmed the findings of the chronic data, (see further), i. e., that the CIMM algal strain was the most sensitive strain. The geometric mean of the 4 CIMM-data points is 333.1 mg Mo/L and is considered as a reliable acute toxicity value for hazard assessment purposes (e. g., classification). No reliable EC50-values were reported for any other freshwater algal species.

 

Chronic toxicity for freshwater algae

For Pseudokirchneriella subcapitata, a 72h-ErC10 of 156 mg Mo/L was reported by ECTX (2007a) but this value was based on nominal exposure concentrations.

An algal exchange and testing program between Ghent University and CIMM (Centro de Investigación Minera y Metalúrgica) was conducted with the green alga Pseudokirchneriella subcapitata. Tests that were performed with the algal strain from CIMM resulted in four 72h-ErC50 values that were situated between 61.2 and 88.7 mg Mo/L, i. e. only a difference of a factor of 1.4 between the lowest and highest value. Such degree of variation can be considered as natural biological variation among ecotoxicity tests that are conducted with the same species and same test substance. These four effect levels also indicate that the origin of the test medium or the test facility had little or no influence on the outcome of the test. Comparable results (62.5 – 99.3 mg Mo/L) were also obtained with the UGhent strain tested at CIMM. The tests conducted with this strain at the University of Ghent, however, produced 72h-ErC10 levels that were ± a factor of 4 to 5 higher than the other six tests, but were in line with the test result that was generated by De Schamphelaere et al (2008a) at the same testing facility, and using the same strain.

Based on these findings, the following was concluded:

- Similar 72h-ErC10-values were produced by the two testing facilities and using test media from these two facilities, with the P. subcapitata strain originating from CIMM.

- All test results are in line (maximum difference of a factor of 2.5) with the outcome of the accepted test result that was generated by ECTX (2007a).

- Comparison of the research results given in Rodriguez (2008) and De Schamphelaere and Janssen (2008), however, indicates that the CIMM-algal strain may be more sensitive than the Ghent University strain. From a conservative point of view it was therefore decided that only the 72h-ErC10 values obtained with the former strain should be taken into account for the derivation of a NOEC for P. subcapitata, i. e., excluding the effect levels that were obtained with the UGhent strain. The geometric mean of these four values is proposed as the chronic no-effect level for this species, and is 74.3 mg Mo/L.

 It should be noted that the geometric mean of all relevant and reliable data would result in a geometric mean that is higher than the proposed chronic no-effect level of 74.3 mg Mo/L, and this value can therefore be considered as a more conservative value.

 

Acute toxicity for marine algae

The reliable 72h-ErC50-value that was determined by Aquasense (2009) the alga P. tricornutum was 356.9 mg Mo/L. Le Page and Hayfield (2010) reported and unbounded 72h-ErC50 of >938 mg Mo/L. Both marine EC50 -values were higher than the geomean value of 333.1 mg Mo/L for the freshwater green alga P. subcapitata that was put forward as a reference value for acute molybdenum toxicity to algae.

Chronic toxicity for marine algae

For the micro-alga Dunaliella tertiolecta the 72h-EC10 value of 881 mg Mo/L (endpoint: growth rate) is retained for the derivation of a predicted no-effect concentration for molybdenum (Le Page and Hayfield, 2010). For biomass, the 72h-EC10 was 513 mg Mo/L. Although this value is lower than the EC10for growth rate, the latter endpoint is considered more ecologically relevant, and is preferred over biomass in OECD guideline for algal testing. The 72h-NOEC value for both biomass and growth rate was 938 mg Mo/L, which is higher than both EC10 values.

For the diatom Phaeodactylum tricornutum the 72h-EC10 value of 169.9 mg Mo/L (endpoint: growth rate) is retained for the derivation of a predicted no-effect concentration for molybdenum. The 72h-NOEC value for this endpoint was 150 mg Mo/L. The EC10 value, however, is preferred over the NOEC value as the latter is a test design-dependent value.

Researchers at the North Austalian Marine Research Alliance conducted an OECD 201 algal growth inhibition test with the tropical marine microalga Isochrysis galbana. No adverse effects on growth rate were observed up to the highest concentration tested (9.5 mg Mo/L). Data could therefore not be used in an SSD-approach but are taken into account when assessing whether the derived PNECmarine is sufficiently protective for species that are not included in the SSD (reliable supportive information).