Competition between protons and cadmium ions in the planktonic food chain leading to the phantom midge Chaoborus

Abstract

Cadmium concentrations in the phantom midge Chaoborus can be related to those of the free Cd ion, Cd2+, in lakewater provided that the competitive influence of H+ on Cd uptake sites is considered. Because this predator takes up its Cd from plankton, competition between H+ and Cd2+ ions could take place at several levels in the food chain to which this insect belongs. To identify at which trophic levels this occurs, we first measured H+-Cd−1 competition in the gut of a given Chaoborus species (Chaoborus americanus) by exposing it to constant Cd concentrations in water and food but at various ambient pH levels (4.5, 5.5, 6.5). There was no difference in the efficiency with which this predator assimilated Cd at the various pHs, suggesting that H+ swallowed by Chaoborus do not compete with Cd at uptake sites in its gut. pH-sensitive dyes showed that C. americanus is able to maintain its gut pH between 6.5 and 8.0, even when the pH of ambient water varies beyond this range (4.5-9.0). We then determined whether H+-Cd2+ competition is likely to take place on the prey of Chaoborus by measuring the importance of water as a Cd source for the calanoid copepod Diaptomus minutus. Copepods fed with Cd-rich green algae and exposed to either a high [Cd2+] (5 nmol L−1) or a low [Cd2+] (1.6 nmol L−1) accumulated a majority of their Cd from water, suggesting that H+ and Cd2+ ions are likely to compete at Cd uptake sites on these crustaceans. Last, we measured Cd accumulation by this copepod at various pHs. Copepods held at pH 4.8 accumulated less Cd than those at pH 5.5, suggesting that H+-Cd2+ competition occurs in this animal. A bioaccumulation model designed to take into account H+-Cd2+ competition was parameterized using our data for Cd accumulation by copepods at pH 4.8 and 5.5 and then used to predict measured responses at pH 6.4.