The effect of dissolved organic matter (DOM) on sodium transport and nitrogenous waste excretion of the freshwater cladoceran (Daphnia magna) at circumneutral and low pH Hassan A. Al-Reasi a, ⁎, Usman Yusuf a , D. Scott Smith b , Chris M. Wood a a Department of Biology, McMaster University, Hamilton, ON, Canada L8S 4K1 b Department of Chemistry, Wilfrid Laurier University, Waterloo, ON, Canada N2L 3C5 abstract article info Article history: Received 26 June 2013 Received in revised form 29 August 2013 Accepted 29 August 2013 Available online 10 September 2013 Keywords: Dissolved organic matter Daphnia magna Na + metabolism Ammonia and urea excretion Dissolved organic matter (DOM), a heterogeneous substance found in all natural waters, has many documented abiotic roles, but recently, several possible direct influences of DOM on organism physiology have been reported. However, most studies have been carried out with a limited number of natural DOM isolates or were restricted to the use of commercial or artificial humic substances. We therefore employed three previously characterized, chemically-distinct natural DOMs, as well as a commercially available humic acid (Aldrich, AHA), at circumneutral (7–8) and acidic pH (~5), to examine DOM effects on whole-body Na + concentration, unidirec- tional influx and efflux rates of Na + , and ammonia and urea excretion rates in Daphnia magna. Whole-body Na + concentration, Na + influx, and Na + efflux rates were all unaffected regardless of pH, suggesting no influence of the various natural DOMs on active uptake and passive diffusion of Na + in this organism. Ammonia and urea excretion rates were both increased by low pH. Ammonia excretion rates were reduced at circumneutral pH by the most highly colored, allochthonous DOM, and at low pH by all three natural DOMs, as well as by the commer- cial AHA. Urea excretion rates were not influenced by the presence of the various DOMs in circumneutral solu- tions, but were attenuated by the presence of two allochthonous DOM sources (isolated from Bannister Lake and Luther Marsh) at acidic pH. The observed reductions may be attributed partially to the higher buffering ca- pacities of natural DOM sources, as well as their ability to interact with biological membranes as estimated by a new measure calculated from their acid–base titration characteristics, the Proton Binding Index (PBI). © 2013 Elsevier Inc. All rights reserved. 1. Introduction Dissolved organic matter (DOM) is a complex group of molecules produced during the decomposition of lignin-rich plant materials and the decay of dead organic biomass in a poorly-understood process known as humification (Ertel et al., 1984; Hatcher and Spiker, 1988). In freshwater ecosystems, DOM molecules are ubiquitous and their mass (usually ≥50% dissolved organic carbon or DOC as a heteroge- neous mixture of humic and fulvic acids) exceeds that of living organ- isms (Thurman, 1985; Thomas, 1997). The source of DOM in the ecosystem can be allochthonous (i.e. terrigenous - organic matter pro- duced on land and then washed into the water body), autochthonous (organic matter generated within the water column by microrganisms such as algae and bacteria), or of mixed autochthonous and allochtho- nous origin (McKnight et al., 2001). Depending on their concentrations and origins, DOM molecules are responsible for the yellow to brown color of surface water; allochthonous DOMs tend to be darker in color (Schwartz et al., 2004). The heterogeneous nature of various DOM sources also reflects their variability in chemical structure and composi- tion, characteristics which can be probed using the absorbance and fluo- rescence spectroscopy and titration (e.g. Al-Reasi et al., 2013). Several direct interactions of DOM with freshwater organisms have been documented recently (Steinberg et al., 2006). For example, DOM molecules have been shown to accumulate on cell membranes (Campbell et al., 1997) with impacts on their permeability (Vigneault et al., 2000), especially under conditions of low water pH coupled with higher DOC concentrations. Even at circumneutral pH, the pres- ence of added DOM induced a more negative transepithelial potential in trout gills, and the effects were greater with darker, more allochtho- nous DOMs (Galvez et al., 2009). Some studies have indicated that DOM molecules have the potential to induce toxicity (e.g. Meems et al., 2004; Timofeyev et al., 2004; Matsuo et al., 2006; Steinberg et al., 2006). However, other investigations have found that organisms in soft acidic waters experience higher survival (Hargeby and Petersen, 1988) and improved growth (Barth and Wilson, 2010) in the presence of DOMs, and are protected against negative changes in ionoregulation (Gonzalez et al., 1998, 2002; Wood et al., 2003; Matsuo et al., 2004). DOMs may also facilitate increased ammonia excretion at low pH, so Comparative Biochemistry and Physiology, Part C 158 (2013) 207–215 ⁎ Corresponding author at: Department of Biology, Sultan Qaboos University, P.O. Box: 36 Al-Khod, Postal Code: 123 Muscat, Oman. Tel.: +968 24141448; fax: +968 24413415. E-mail address: alreasi@squ.edu.om (H.A. Al-Reasi). 1532-0456/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.cbpc.2013.08.004 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part C journal homepage: www.elsevier.com/locate/cbpc