A system and method of generating electricity from the salinization of freshwater is provided. In one embodiment, the diffusion of cations and anions from saline to freshwater is rapidly alternated in order to generate electrical power in the form of alternating current. To create pathways for the rapidly alternating diffusion of cations and anions, rhodopsins (light-activated ion channels and pumps) are expressed in bacteria that are growing as a biofilm on a membrane that separates the saline and freshwater. Illumination of the biofilm with blue light permits cation diffusion through cation-permeable channelrhodopsins. Illumination of the biofilm with yellow light permits diffusion of anions through halorhodopsins.
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