Investigations into Cd tolerance in Chironomus riparius: spatial patterns of Cd transport and sequestration

Abstract

Chironomus riparius are the least sensitive aquatic species in the U.S. EPA (2000) species sensitivity distribution (SSD) for cadmium (Cd). LC50 values over 25,000 times the Criterion Maximum Concentration suggest that chironomids have an extraordinary capacity to excrete or sequester cadmium as a means of increasing their overall tolerance to toxic metals. Using atomic absorption spectroscopy, we have shown saturable uptake of cadmium by whole chironomids and isolated guts. The transport of Cd from the gut lumen to the hemolymph exposes other tissues such as the nervous system and muscles to Cd. To quantify the movement of Cd2+ from the main point of entry, the digestive tract, we have identified regional differences of Cd transport along the gut using a Cd2+ -selective microelectrode in conjunction with the Scanning Ion Selective Electrode Technique (SIET). Cd2+ fluxes were determined in high mucosal or serosal Ca2+ to analyze the contribution of Ca2+ transporters to Cd2+ uptake. The major tissues responsible for elimination of Cd2+ from the hemolymph are the posterior midgut and the ileum. In addition, experiments using an isolated Malpighian tubule preparation (the Ramsay assay) have shown that the Malpighian tubules both sequester and secrete Cd2+. The tubules can secrete the entire hemolymph burden of Cd^2+ in~ 18 hours.