In the aquatic environment, the effects of micropollutants to photosynthetic microorganisms are influenced by multiple biotic and abiotic factors. However, the combined effects of environmental stressors and specie-specie interaction on micropollutant effects are understudied. The present doctoral work aims to get an insight on the processes governing the combined action of light and copper micropollutants on phytoplankton. The research approach included studies with the green alga Chlamydomonas reinhardtii and cyanobacterium Synechocystis sp. at molecular, cellular and population level. The results revealed that the mechanisms of combined action of copper and light toward phytoplankton are complex, and might change depending on (i) light intensity and spectral composition, (ii) copper form (dissolved-nanoparticulate), (iii) exposure sequence and duration, (iv) phytoplankton species and their interaction. The outcomes of this work are expected to help reduce the uncertainties in lab-to-field extrapolation of toxicity data to perform multiple stressor risk assessment of micropollutants.