Aquatic Toxicology 72 (2005) 161–175 Influence of water chemistry and natural organic matter on active and passive uptake of inorganic mercury by gills of rainbow trout (Oncorhynchus mykiss) Joel Klinck, Michael Dunbar, Stephanie Brown, Joel Nichols, Anna Winter, Christopher Hughes, Richard C. Playle ∗ Department of Biology, Wilfrid Laurier University, Waterloo, Ont., Canada, N2L 3C5 Received 21 June 2004; received in revised form 21 October 2004; accepted 27 November 2004 Abstract To distinguish physiologically regulated uptake from passive uptake of inorganic Hg in fish, rainbow trout (Oncorhynchus mykiss) were exposed to inorganic Hg (0.5, 1, or 2 M total Hg) in ion-poor water with various treatments. Addition of ions to the water (mM concentrations of Ca, K, Cl) did not consistently alter Hg accumulation by trout gills, although there was a trend to higher Hg accumulation at higher ion concentrations. The apical Ca channel blockers Verapamil and lanthanum also did not consistently affect Hg accumulation by trout gills. Pre-treatment of trout with the Na channel blocker Phenamil decreased Hg uptake by about half. These results suggest a combination of physiologically regulated and passive uptake of Hg by trout gills. Strong complexing agents of Hg (EDTA, NTA, ethylenediamine, cysteine) decreased Hg-binding by trout gills in a dose-dependent manner. From these data, a conditional equilibrium binding constant for Hg to the gills was estimated as log K Hg–gill = 18.0, representing very strong binding of Hg to the gills. This value is a first step in creating a biotic ligand model (BLM) for inorganic Hg and fish. Natural organic matter (2–10 mg C/L) also decreased Hg-binding by trout gills, although mM concentrations of Na, K, and Cl interfered with this effect. At low concentrations of these ions, natural organic matter samples isolated from various sources bound Hg to similar degrees, as judged by Hg accumulation by trout gills. A conditional binding constant to natural organic matter (NOM) was estimated as log K Hg–NOM = 18.0 with about 0.5 mol binding sites per mg C, representing strong binding of Hg to NOM. © 2004 Elsevier B.V. All rights reserved. Keywords: Inorganic mercury; Fish gills; Natural organic matter; Physiology; Biotic ligand model ∗ Corresponding author. Tel.: +1 519 884 0710x3407; fax: +1 519 746 0677. E-mail address: rplayle@wlu.ca (R.C. Playle). 1. Introduction Mercury (Hg) is a biologically non-essential, toxic metal that is widely found in the aquatic environment, even in remote ecosystems, although typically at con- 0166-445X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2004.11.013