Osmoregulatory Physiology in Aquatic Insects: Implications for Major Ion Toxicity in a Saltier World

Scheibener, Shane

ABSRACT: The salinization of freshwater ecosystems has emerged as an important ecological issue. Several anthropogenic causes of salinization (e.g. surface coal mining, hydro-fracking, road de-icing, irrigation of arid lands, etc.) are associated with biodiversity losses in freshwater ecosystems. As freshwater systems are increasingly threatened by salinization it is important to understand fundamental aspects of aquatic insect physiology (e.g. osmoregulatory processes) that contribute to insect responses to these stressors. These osmoregulatory processes are also important determinants of trace metal exposure and toxicity. I compared the uptake dynamics of Na in two species thought to differ in their sensitivity to salinity – the tolerant caddisfly Hydropsyche betteni and a more sensitive mayfly Maccaffertium sp. Across a range of Na concentrations encompassing the vast majority of North American freshwater ecosystems (0.06 – 15.22 mM), both species appeared to tightly regulate their whole body sodium concentrations (at ~ 47 ±1.8 μmol/g wet weight) over 7 days. However, at the highest Na concentration (15.22 mM), Na uptake rates in a related species, Hydropsyche sparna (419.1 nM Na g-1 hr-1 wet weight), appeared close to saturation while Na uptake rates in Maccaffertium sp. were considerably faster (715 nM Na g-1 hr-1 wet weight) and unlikely close to saturation. Thus, ion turnover rates (and their associated energetic costs) may play a role in determining sensitivity differences across species. A comparison of Na uptake rates (at 0.57 mM Na) across 9 species representing 4 major orders (Ephemeroptera, Plecoptera, Trichoptera and Diptera) demonstrated large physiological differences across species after accounting for body weight. Faster Na uptake rates were associated with species described as being sensitive to salinization in field studies. The metals silver (Ag) and copper (Cu), known to be antagonistic to Na uptake in other aquatic taxa did not generally exhibit this effect in aquatic insects. Ag only reduced Na uptake at extremely high concentrations, while Cu generally stimulated Na uptake in aquatic insects, rather than suppressing it. This finding may explain why aquatic insects appear to be tolerant to these metals in traditional aqueous toxicity tests. As no information about sulfate uptake dynamics in stream insects was available in the literature, I characterized SO4 uptake kinetics in 5 mayflies and assessed the influence of different ions on SO4 uptake. When SO4 was held constant and Na was increased, a dramatic decrease in SO4 transport was observed. Experiments with other salts demonstrated that Na (and not Cl or HCO3) is antagonistic to SO4 transport. This is a novel discovery that has not been observed in other aquatic taxa. Based on this observation, I tested the hypothesis that increasing Na would protect against SO4 induced toxicity in a Na-dependent manner. Increasing Na from 0.7 to 10.9 mM improved 96-hr survivorship associated with 20.8 mM SO4 from 44% to 73% in a concentration dependent manner. However, when Na reached 21.8 mM, survivorship decreased to 16%, suggesting that other interactive effects of major ions caused toxicity under those conditions. Thus, the combination of elevated sulfate and low sodium commonly observed in streams affected by mountaintop coal mining has the potential to cause toxicity in sensitive aquatic insects. Overall, these results highlight the need to better understand fundamental physiological processes in this ecologically important faunal group.