Plants subjected to stress imposed by their environment often accumulate misfolded proteins, that would be cytotoxic if not properly removed by the ubiquitin-proteasome pathway (UPP). During severe stress, the UPP becomes impaired, but the mechanisms that damage it are not well understood in plants. In this study, the effects of mild and severe stress selenium on the UPP were analyzed using the unicellular green algae Chlamydomonas. Mild selenium stress increased proteasome activity. However, inhibition of the UPP caused by severe selenium stress was associated with reactive oxygen species, including mitochondrial superoxide. Additionally, this is the first time that proteasome activity has been reported in lower plants.