We investigated the physiological effects of dissolved organic matter (DOM) on sodium (Na +) transport in juvenile Oncorhynchus mykiss (∼2.5 g) in the presence and absence of simultaneous acute exposure to copper (Cu 2+; 0, 70, and 300 μg l -1). Trout were acclimated in either hardwater (∼1000 μM Ca 2+) or softwater (∼100 μM Ca 2+), and DOM was tested at approximately 8 mg C l -1 using a natural (NOM) and a commercial (AHA) source. Ion transport was evaluated based on kinetics estimates (maximum Na + uptake rates, J max; substrate affinity, K m) and unidirectional flux measurements (J in, J out, J net). J max was higher and unidirectional flux rates were greater in softwater-acclimated trout. Fish exposed to DOM alone in hardwater exhibited an increased Na + transport capacity indicated by both the kinetics (67% higher J max for AHA) and J in measurements (153% higher for AHA and 125% higher for NOM). In softwater, the effects of DOM alone on kinetic parameters and unidirectional flux rates were negligible. Cu 2+ affected Na + uptake by a mixed-type inhibition (both non-competitive and competitive). In hardwater, only K m was increased (i.e., affinity decreased), whereas in softwater, K m was increased and J max was decreased, with more marked effects at the higher Cu 2+ level. In hardwater, the stimulatory effect of AHA on J max persisted even in the presence of 300 μg l -1 Cu 2+, whereas both AHA and NOM prevented the increase in K m caused by Cu 2+; these effects were reflected in J in measurements. In softwater, AHA helped to protect against the increased K m caused by high Cu 2+, but there was no protection against the inhibition of J max. Unidirectional flux measurements indicated that in softwater, Cu 2+ inhibited J in at 70 μg l -1, whereas at 300 μg l -1 Cu 2+, J out was also stimulated. Fish were more affected by Cu 2+ in softwater, as indicated by the inability to control diffusive losses of Na + and a reduced ability to take up Na +, but in the presence of DOM, losses were better controlled at the end of 6 h exposure. We conclude that DOM has direct effects on the gills, as well as protecting fish against acute Cu 2+ toxicity. This occurs because DOM complexes Cu 2+, and because it acts on the transport and permeability properties of the gills. These effects differ depending on both water hardness and the nature of the DOM source. © 2004 Elsevier B.V. All rights reserved.
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