Stress-dependent shifts in genotoxic sensitivity of zooplankton.

The ocean is increasingly exposed to multiple stressors, including rising temperatures and metal pollution. Zooplankton are central components of the marine food web and understanding lethal responses and underpinning molecular mechanisms across species from different habitats is essential for predicting ecosystem resilience. This study compares two zooplankton species from an Arctic fjord, the near surface-dwelling Acartia longiremis and the lipid-rich, deeper-dwelling Calanus finmarchicus, for their molecular responses to acute and sub-acute temperature and copper (Cu) stress. Endpoints were mortality, DNA damage measured with the fast micromethod assay, and expression of selected genes. Elevated temperature (11 °C) and Cu (20 µg/L), separately and in combination, resulted in species-specific responses. The individual stressors caused greater mortality in A. longiremis than in C. finmarchicus. Significant temperature-induced DNA damage was detected in C. finmarchicus but not A. longiremis. Genotoxicity of the combined stress probably reflects DNA strand breaks and cross-links, and transcriptomic changes indicated oxidative stress and impaired DNA defence and repair mechanisms. A. longiremis was more sensitive to both Cu and temperature at acute levels, while C. finmarchicus was more susceptible to sublethal genotoxic effects. These findings demonstrate that zooplankton with distinct habitat preferences within the same Arctic ecosystem can exhibit different stress sensitivities, response patterns, and defence mechanisms. Accounting for species-specific variation is therefore essential when predicting ecosystem responses to climate change and pollution.