Old Yellow Enzymes (OYEs) were discovered in the 1930s and have been the subject of much research ever since. This is because these biocatalysts — coloured yellow by an auxiliary molecule — are capable of performing reactions that are very valuable for the chemical industry, such as producing drug precursors or fragrances. Even though OYEs are found in many organisms, their natural role for these organisms has hardly been understood so far — possibly because the research focus has been on their biotechnological application. Researchers have now shown that an OYE of the unicellular green alga Chlamydomonas reinhardtii is vital for this plant microorganism to protect itself from photooxidative stress.