Influence of waterborne cations on zinc uptake and toxicity in rainbow trout,Oncorhynchus mykiss

Abstract

The effects of waterborne cations on65Zn uptake, Zn toxicity, and relationships with Ca uptake were examined in juvenile rainbow trout, Oncorhynchus mykiss, in soft water. Whole-body Zn uptake (waterborne [Zn] = 100 μg·L-1= 1.5 μM) was greatly reduced by a variety of cations. This reduction was directly related to the concentration of positive charges, regardless of which ion carried that charge. Thus, 1.0 mM Na+, K+, NH4+, and N-methyl-D-glucamine+and 0.5 mM Mg2+(divalent) reduced Zn uptake to a similar extent (approx. 50%), indicating a relatively nonspecific competition for anionic sites on the gill. Ca2+was an exception and was more potent at reducing Zn uptake, likely because only Ca2+would also compete for absorption. Although Na+and Mg2+were able to markedly reduce Zn uptake, they had no effect on Zn toxicity (measured with 96-h LC50tests), a result paralleled by their inability to restore Ca2+uptake that was inhibited by Zn. In contrast, Ca2+reduced Zn toxicity and restored Ca2+uptake. These results partially dissociate Zn uptake from Zn toxicity, implicate disturbed Ca2+uptake as the toxic mechanism, and have profound implications for water quality criteria where Ca2+and Mg2+(the two "hardness" cations) are traditionally considered to be equally protective.