Micro-algae constitute powerful unicellular factories with enormous potential for mitigation of miscellaneous pollutants from effluent gases and waste waters. Especially their outstanding capacity for photosynthetic CO₂ fixation underlines their potential for diminishing current ecological problems. Together with these contributions to beneficial environmental development, various micro-algae accumulate high-value marketable products as cell constituents. Low cell densities and moderate growth rates typical for algal cultivation are known as the major obstacles in industrial manufacturing of algal products: Here, high cell densities are required to obtain reasonable volumetric productivities. This raises the challenges for the process engineering design and novel nutrient supply regimes as needed for efficient algal cultivation. For some algal strains, mixotrophic cultivation by providing organic substrates together with CO₂ results in increased biomass concentration in a first cultivation step. For this purpose, numerous organic waste streams can be applied as substrate. In a second step, active algal biomass accumulates desired products via CO₂ fixation, e.g. from industrial effluent gases. This can be realized by two-stage, continuously operated photo-bioreactor systems. After optimized product recovery, the value-added conversion of residual algal biomass for energy recovery e.g. in biogas plants, constitutes another challenge.