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The effects of acute waterborne exposure to sublethal concentrations of molybdenum on the stress response in rainbow trout, Oncorhynchus mykiss

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Juvenile and fingerling rainbow trout were held for 96 hours in water with added sodium molybdate at up to 1000 mg/L molybdenum. Gill, plasma, liver and muscle took up molybdenum, but none of the physiological or cellular stress markers measured changed. The authors conclude that the provincial water-quality guideline for protecting aquatic life is highly protective for this species.

Key numbers

  • Exposure concentrations: 0 (background about 0.0010 mg/L), 2, 20 and 1000 mg/L molybdenum for 96 h; 2 mg/L is the British Columbia aquatic-life guideline and 1000 mg/L about half the 96 h LC50 estimate for local fish (Methods, p. 3).
  • Control juvenile tissue molybdenum: gill 6.0 ± 3.46, plasma 10.5 ± 2.02, white muscle 46.5 ± 1.44, liver 227 ± 2.02 ng/g dry weight, mean ± SEM, n = 6 (Results, p. 6).
  • After 96 h at 20 mg/L: gill and plasma molybdenum rose about 3,000-fold and 800-fold; liver 23-fold; white muscle by 38% (Results, p. 6).
  • No change in plasma cortisol, blood glucose, hematocrit, heat shock proteins (hsp70, hsp90) or metallothionein at any concentration tested (Abstract, p. 1; Results, pp. 7 to 12).
  • Freshwater background: unimpacted waters rarely exceed 0.02 mg/L; waters receiving mine discharge reported at 1.4 to 100 mg/L (Introduction, p. 1; secondary).
  • Reported 96 h LC50 values in freshwater fish range from more than 50 to more than 10,000 mg/L as sodium molybdate (Introduction, p. 2; secondary).

Methods (brief)

Fish were exposed in static-renewal chambers to sodium molybdate dihydrate, with water quality monitored every 8 h. Tissue molybdenum was measured by ICP-MS after nitric acid digestion of dried tissue (juveniles only). Stress hormones, glucose and hematocrit were measured in blood; stress proteins by Western blot.

Evidence fitness

Supports statements about acute aquatic toxicity and tissue uptake of molybdenum from water in one fish species. It does not support human toxicity, human dietary exposure or chronic-effect conclusions. Muscle values are from a laboratory exposure, not from fish sold as food.

Limitations

Acute 96 h exposures only, in one species and two life stages; small group sizes. Funded by Canadian public research grants; the authors declare no competing interests.

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