PARENT SESSION

TA3 Metals in the Environment: Aquatic Biological Perspectives
254 Portland Ballroom
8:00 AM - 12:00 PM, Tuesday

() Metal tolerance in Daphnia pulex.

Folt, C.¹, Glaholt, S.¹, Chen, C.¹, Hamilton, J.², Shaw, J.¹, ¹ Dartmouth College, Hanover, NH, USA² Dartmouth Medical School, Hanover, NH, USA

ABSTRACT- Selective pressures from pollution often result in acquisition of tolerance, which can manifest in individuals/clones as physiological modifications (acclimation) and over time restructure natural populations via genetic adaptation. However, little attention has been given to understanding mechanisms and costs associated with tolerance. Therefore, investigations were conducted to explore generational effects of exposure history on acute and chronic responses of Daphnia pulex to metals. For these studies, three metal-acclimated populations of D. pulex were developed by continuous exposure to cadmium concentrations ranging from 0.25 to 2.5 g/L for up to 30 generations (>1 yr) in the laboratory. Acute (48-h) toxicity tests revealed that cadmium acclimated Daphnia exhibit increased tolerance (higher LC50) following exposure to cadmium, zinc, and silver but were not cross-tolerant to arsenic. These studies suggest that altered uptake is not responsible for metal tolerant phenotypes, since cadmium/zinc and silver are accumulated by different pathways. Rather they hint at a role of the metal binding protein, metallothionein (MT) since these metals are known potent inducers of MT, whereas, arsenic is not known to stimulate MT expression. Chronic studies (21-d) revealed that in the presence of cadmium (1 and 2.5 g/L), metal acclimated populations had greater mass, cumulative reproduction, reproduction per adult, and rates of net reproductive output (R₀) compared to naïve populations that were not pre-exposed to elevated concentrations of metals. However, these patterns were reversed in control conditions as naïve outperformed metal-acclimated Daphnia, suggesting there is a physiological cost associated with tolerance. Multi-generation tests revealed that metal tolerance was rapidly lost (≤ 2 generations) following removal from metal. Collectively, these results demonstrate the importance of considering tolerance when evaluating effects of chemical stressors on aquatic organisms. Future work will focus on defining adaptive limits of metal acclimation and identifying the molecular pathways responsible for increased metal tolerance.

Key words: metal, Daphnia pulex, tolerance, acclimation