Light makes light work of fatty acids Photosynthetic organisms are notable for their ability to capture light energy and use it to power biosynthesis. Some algae have gone a step beyond photosynthesis and can use light to initiate enzymatic photodecarboxylation of fatty acids, producing long-chain hydrocarbons. To understand this transformation, Sorigué et al. brought to bear an array of structural, computational, and spectroscopic techniques and fully characterized the catalytic cycle of the enzyme. These experiments are consistent with a mechanism starting with electron transfer from the fatty acid to a photoexcited oxidized flavin cofactor. Decarboxylation yields an alkyl radical, which is then reduced by back electron transfer and protonation rather than hydrogen atom transfer. The wealth of experimental data explains how algae harness light energy to produce alka(e)nes and provides an appealing model system for understanding enzyme-catalyzed photochemistry more generally. Science , this issue p. eabd5687