Humic Substances Influence Sodium Metabolism in the Freshwater CrustaceanDaphnia magna

Abstract

Humic substances are ubiquitous components of natural waters with important roles in alleviating metal toxicity to aquatic organisms. Recent literature reports suggest that humic substances may also exert direct influences on biota. This study investigated the influence of two commercially available humic substances on sodium metabolism in Daphnia magna, a hyperregulating freshwater crustacean. Environmentally realistic levels of Suwannee River natural organic matter (SRN) and Aldrich humic acid (AHA) significantly enhanced sodium transport. This effect was described as an uncompetitive stimulation of sodium influx, as characterised by an increased maximal sodium transport rate (Jmax), accompanied by a decreased uptake affinity (increased Km). SRN exposure also significantly promoted the unidirectional loss of sodium from the daphnids to the water, an effect not observed in the presence of AHA. A 24-h preexposure to AHA before influx measurement had no effect on AHA-induced stimulation of sodium influx. Conversely, 24-h preexposure to SRN resulted in influx values that returned to control (humic-free) levels. Whole-body sodium levels reduced by SRN exposure were also restored to control levels following 24-h SRN preexposure. The significance and potential mechanisms of these actions are discussed, and the toxicological implications of these findings are assessed.

The influence of dissolved organic matter (DOM) on sodium regulation and nitrogenous waste excretion in the zebrafish (Danio rerio)

“…A recent study has shown that highly allochthonous DOM from the Amazonian Rio Negro protects zebrafish against disturbances in ionoregulation and ammonia excretion at extremely low pH (Duarte et al, 2016). While some direct effects of DOM appeared to be more pronounced at low pH (Hargeby and Petersen, 1988;Campbell et al, 1997;Vigneault et al, 2000), interactions with aquatic organisms have also been observed at circumneutral pH, including beneficial effects on ionoregulation, particularly by allochthonous DOM sources (Matsuo et al, 2004;Glover et al, 2005;Glover and Wood, 2005a;Galvez et al, 2009). It therefore seems possible that DOM molecules may interact directly with epithelial ion transporters, yet it is not clear how DOM mediates the actions mechanistically.…”

Assessing the sublethal effects of in-river concentrations of parameters contributing to cumulative effects in the athabasca river basin using a fathead minnow bioassay

“…Chem. 32,2013There was an increase in TOC in the 100% treatment of the dissolved sulphate experiment to 6.90 mg/L. While this was also based on a single measurement, unlike the spike seen in the 12.5% treatment in the dissolved sodium, this is still within the averaged range seen in the mouth-water treatments and could therefore reasonably be considered representative of average conditions for this experiment.…”

“…There was no significant difference among treatments in the mouth experiment for cumulative eggs/female/day; however 32,2013there was a significant increase in both total eggs/female/day and mean eggs/female/day (Figs. 1c and 2c) in the 50% treatment compared to the 0% control.…”

“…Chem. 32,2013percentage of change from control was seen in the 50% treatment. It is unclear from the present study what factors might be attributed to these different changes.…”

“…This happens when humic substances complex with biologically active ions, such as free Ca2 þ , from the water. Free Ca2þ is an important constituent of epithelial tight junctions, and reductions in this ion to very low levels are known to cause a general increase in diffusive ion losses, as well as selectively enhance the permeability of the gills to Naþ relative to Cl-, resulting in hyperpolarization [31,32].…”