We investigated the combined effects of mercury (HgCl(2)) and acute heat on survival of the springtail Folsomia candida. The springtails were exposed to a range of aqueous concentrations (0-48 mg Hg(2+)/L) of HgCl(2) for 24 h. Subsequently, the same individuals were exposed to a range of high temperatures, from 20 to 35.5 degrees C. We found a highly significant synergistic interaction between effects of mercury and heat, with a reduced tolerance to heat after exposure to sublethal concentrations of mercury. Further, the heat hardening ability of F. candida was studied at sublethal concentrations of mercury. F. candida was able to heat harden (exposure to a mild heat treatment increasing survival of subsequent severe heat); however, when the springtails experienced a previous exposure to as little as 1 mg Hg(2+)/L, heat hardening failed to improve survival of heat shock at 34.5 degrees C, even though this was much lower than concentrations affecting survival without heat stress. Mild heat stress is known to induce the heat shock protein, HSP70, and real-time quantitative PCR confirmed that pre-acclimation to 32 degrees C did indeed cause >5-fold up-regulation of HSP70 expression. This up-regulation was not affected by previous exposure to 1 mg Hg(2+)/L.