The present disclosure provides a method and use of green and blue-green algae for treating oil sands process-affected water (OSPW), to biodegrade naphthenic acids in the OSPW. The method comprises obtaining irradiated OSPW, combining the irradiated OSPW with blue-green algae or green algae to form a mixture, and applying light to the mixture. The irradiated OSPW may be OSPW that was irradiated with gamma or electron-beam irradiation. Light and heat may be applied to enhance the method and use.