In response to environmental stress cells produce reactive oxygen species (ROS), which can cause molecular damage in the form of DNA breaks and modification to lipids and proteins. On the other hand, ROS are important in cell signaling to induce differential gene expression, metabolic changes and thus, adaptation to different environmental conditions. The hypothesisof this projectis thattheROS responseanditsmodulation by transition metals areacommon denominator of different environmental challenges, including radiation and microgravity during spaceflight, directly applicabletothe unicellular green algaChlamydomonas reinhardtiigrown in spaceflight experiments. The goal of this thesis was to characterize the response of C. reinhardtiito stress by examining biochemical and gene expression changes using heat shock. The heat shocked (42 °C) C. reinhardtiiexhibited an increase in ROS in both inside the celland in the supernatant, paralleled by a selective regulation of ion transporters, specific for copper. These results provide new insights about the cellular response to stress and its possible mediation during long-term environmental pressureand provide a basis for understanding the evolution of multicellularity.