This study investigated the photostimulation of organic degradation with Scenedesmus quadricauda microalga, which is capable of assimilating organic carbon as a carbon source under mixo- and hetero-trophic growth conditions. The assimilability of carbonaceous organics in the microalga was significantly inhibited for polymeric organic compounds (a mixture of humic substances and polysaccharides) due to their recalcitrant characteristics when compared to using glucose. However, continuous illumination (150 mu mol photons M-2 s(-1)) under non -aerated conditions stimulated the microalga to increase the degradability of the organic mixture. The subsequent algal cultivation in the light using aeration with 10% (v/v) CO2 achieved a significant assimilation of the carbonaceous organics along with nitrate, which was comparable to that observed when using glucose as a carbon source. This is the first time that the photostimulation of S. quadriccunla has been shown to induce the degradation of humic-like substances that are typically resistant to microbial decomposition.