A STABLE ISOTOPE TECHNIQUE TO DETERMINE ZINC PATHWAYS AND AQUATIC TOXICITY IN THE AQUATIC ENVIRONMENT

It is difficult to assess uptake and ecological impacts of zinc contamination in aquatic ecosystems because zinc (Zn) is fairly ubiquitous in many natural environments and biological tissues. This work presents a Zn stable isotope technique that can be used to quantify Zn uptake kinetics within the tissues of aquatic organisms. Aquatic organisms are known to retain and accumulate dissolved heavy metals from their environment, with levels of uptake dependent on both exposure concentration and duration. In freshwater fish, the gill is widely considered to be the primary site of acute toxicity for many metals, and it has been proposed that accumulated metal can be used as an indicator of toxicity. In the study of aquatic Zn toxicity in fish, the measurement of Zn accumulation on gill tissue is complicated by inherent difficulties in distinguishing between endogenous Zn and Zn accumulated during experimental exposures. The radioisotope ⁶⁵Zn has been used in several studies as a tracer to quantify Zn kinetics at the gill; however, there are diverse factors limiting the widespread employment of this technique. Our research uses commercially-available enriched stable isotopes of Zn as a promising alternative method for the quantitative evaluation of Zn uptake. Juvenile rainbow trout (approximately 5 grams) were exposed to a range of ⁶⁷Zn-enriched waters. At sampling times of 0.5, 1.5, 3.0, 24, 48, and 72 hours, select fish were harvested, and fish gills were removed and separated from gill rakers within 24 hours of fish death. Gill tissue was digested via closed-vessel microwave digestion, and resultant samples were analyzed for total Zn and ⁶⁷Zn / ⁶⁶Zn and ⁶⁷Zn / ⁶⁴Zn isotopic ratios via Dynamic Reaction Cell™-Inductively Coupled Plasma-Mass Spectrometry. Significant differences were observed in accumulated gill Zn levels at the different exposure levels and exposure durations. Our experiments demonstrate that Zn stable isotopes can be used to distinguish between endogenous Zn and Zn accumulated during experiment exposures. This approach will allow for more in-depth studies of the kinetics and pathways of Zn uptake by aquatic organisms in contaminated systems.