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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Data that were retrieved, suggest that molybdenum bioconcentration through the food chain 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 33 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. Limited information on transfer of Mo through the food chain indicates that molybdenum does not biomagnify in aquatic food chains.  

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Additional information

Data that were retrieved suggest that molybdenum bioconcentration through the food chain is negligible: whole body internal concentrations generally remain below 1 mg/kg dw at background concentration levels in water. At highly contaminated sites, and up to several mg Mo/L, whole body internal concentrations generally remain below 10 mg/kg dw. Recently, Stubblefield (2010) and Regoli et al (2012) investigated the bioconcentration of molybdenum in rainbow trout that were exposed to 11.1 mg Mo/L for a 60 day period, and molybdenum concentrations in both filet whole body concentrations at steady state were indeed below 1 mg Mo/kg (average filet: <0.2 mg Mo/kg; average whole body: 0.52 mg Mo/kg, corrected for growth dilution).

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 90thpercentile of Mo-concentration in whole body samples was 3.00 mg/kg wet weight.

Also available are Mo-levels in fish organs. Regardless of the exposure concentration, Mo-levels in livers of fish ranged from <0.019 to 14.9 mg/kg wet weight (n=15), with a median value of 0.45 mg/kg wet weight, and a 90thpercentile of 1.71 mg/kg wet weight. Mo-levels in muscle samples were markedly lower, i.e., ranging from 0.01 to 7.59 mg/kg wet weight (n=17), with a median value of 0.1 mg/kg wet weight, and a 90thpercentile of 0.54 mg/kg wet weight. Median Mo-level in the muscle tissue is one order of magnitude lower than the median Mo-level in liver samples.

Median levels for other organs like bone (n=4), brain (n=4), intestines (n=4), kidneys (n=4), skin (n=4), spleen (n=5) and stomach (n=4), were 0.185, 0.028, 0.064, 0.1184, 0.071, 0.092 and 0.041 mg/kg wet weight, respectively. For the gill only three data points were identified, ranging from 0.016 to 5.94 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.