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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

Data that were retrieved, suggest that molybdenum bioconcentration is negligible: whole body internal concentrations remain below 1 mg/kg at concentration levels up to several mg/L. Reported whole-body BAFs vary more than 2 orders of magnitude, but there is a distinct inverse relationship between exposure concentration and BAF, i. e., decreasing BAFs with increasing Mo-levels in the water column.

Of all the 41 BCF/BAF reported all below 100 with the exception of one BAF measured for a mollusc exposed to background Mo water concentrations (BAF of 164). The data demonstrates that Mo, like other essential elements, shows homeostatic control of Mo by these organisms. The homeostatic control of Mo is observed to continue to function up to the milligramme range of exposure.

More detailed information can be found in the section “Additional information” and in the Background Document “Environmental fate properties of molybdenum”, which is attached in the technical dossier in IUCLID Section 13.

Key value for chemical safety assessment

Additional information

Aquatic bioaccumulation

Essential elements (EE’s) are different from other substances because organisms need uptake of EE’s to stay alive. All organisms have an internal regulation mechanism (homeostasis) to keep the internal concentration of EE’s within the limits that assure that all functions vital for life are satisfied. This means that, within the boundaries of the essentiality window, the internal concentration, is (nearly) independent of changes in external concentration. Nearly, because oligotrophic organisms usually show a slight increase in internal concentration if the exposure concentration rises. This increase is, however, much smaller than the increase in external concentration. In cases of higher trophic organisms sometimes the internal concentration is perfectly constant. The theoretical relationship is visually presented in the Background Document "Environmental fate properties of molybdenum". The BCF/BAF, will be very high at the deficiency area. Organisms will try to maximise their EE intake in this area in order to survive. Where regulation of the internal concentration takes place, the BCF/BAF strongly decreases with increasing external concentration. At the upper boundary of the essentiality window, the BCF/BAF reaches a minimum that remains almost constant at higher external concentrations.

EE’s do not show biomagnification, i.e. the internal concentration does hardly increase from autotrophic organisms up to organisms higher in the food chain. The reason is that autotrophic organisms concentrate EE’s up to a level required by organisms and no further increase in higher organisms is needed nor desired. The homeostasis mechanism of such higher organisms only has to stabilise the EE concentration in their food. In principle, all organisms of the food chain have about the same internal concentration (roughly within one order of magnitude). Therefore, secondary poisoning is not normal for EE’s.

 

The data specific for molybdenum

Internal concentrations of molybdenum in aquatic organisms

The current data set comprises thirty-seven values that represent whole body Mo-levels in fish (based on wet weight). Values are situated between 0.012 and 14.3 mg/kg wet weight, with a median Mo-concentration of 0.20 mg/kg wet weight. Four values below detection limit were not included as the detection limit of <0.5 mg/kg wet weight was greater than the median value. In addition, the measured minimum value < 3.0 for Salvenius namaycush (Tong, 1974) was also not included. Assuming that levels in these fish were equal to their detection limit, a median value of 0.31 mg/kg wet weight is obtained. The 90th percentile of Mo-concentration in whole body samples was 3.00 mg/kg wet weight.

Whole body Mo-levels in crustaceans and other invertebrates ranged from 0.035 to 0.455 mg mo/kg wet weight at background concentration levels in water. At a highly contaminated site, whole body internal concentrations ranged from 2.8 to 32 mg Mo/kg wet weight, although the Mo concentration in the water was not reported.

The data for Mo levels in other organisms (molluscs and phytoplankton) were also generally found below 1 mg Mo/kg wet weight.

Further to the data reviewed and presented here, Eisler (1989) made an exhaustive compilation of environmental concentrations of Mo, including the aquatic compartment. This review concluded also that Mo concentrations in algae, freshwater and marine fish and molluscs are all well below 10 mg/kg dry weight.

 

Bioconcentration/bioaccumulation Factors

Bioconcentration factors (BCFs) represent the uptake from the water only, whereas bioaccumulation factors (BAFs) also take other intake routes into account (i.e. via food ingestion). In practice – and especially when data are generated from field-collected organisms, it is not always possible to make a distinction between the BCF and BAF. Reported whole-body BCF/BAFs vary more than 2 orders of magnitude but, as theoretically predicted for essential elements, there is a distinct close relationship between exposure concentration and BCF/BAF, i.e., decreasing BCF/BAFs with increasing Mo-levels in the water column, showing homeostatic control of Mo by these organisms (see Background document). The homeostatic control of Mo is observed to continue to function up to and within the milligramme range of exposure.

From the data where BCF/BAF could be derived, it is noted that BCF/BAF range from 30.1 to 71.6 (average of 49) for fish exposed to background Mo concentrations in water (wet weight basis). The BCF/BAF range from 0.05 to 9.9 for fish exposed to waters contaminated with Mo (with levels of Mo up to the mg/L range). The lowest bioaccumulation factor of 0.05 was observed by Stubblefield (2010) who exposed rainbow trout (O.mykiss) for 60 days to a nominal molybdenum level that equalled the 2010-derived freshwater PNEC for this element (12.7 mg Mo/L).

BCF/BAF range from 76.2 to 97.6 (average 84) for crustaceans exposed to background Mo concentrations in water (wet weight basis). A BAF of 4.8 was derived for crustaceans exposed to waters contaminated with Mo. 

For molluscs, the BCF/BAF ranged from 77.3 to 164.3 (average 121) exposed at background Mo concentrations (wet weight basis). No BCF/BAF could be derived for molluscs exposed to Mo contaminated waters.

For phytoplankton a BCF of 20.4 was derived from exposure at background Mo concentrations.

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