Microalgae for Sustainable Energy Production?

Abstract

Carbon-neutral liquid fuels are doubtlessly needed for future sustainable transport. Biodiesel produced from oil crops, animal waste, and used cooking oil potentially is an alternative for mineral oils used preferably today, but the quantities available are by far too small to satisfy the needs of a future market. Microalgae constitute powerful unicellular factories with enormous potential for mitigation of miscellaneous pollutants from effluent gases and waste waters. Most of all their outstanding capacity for photosynthetic CO2 fixation underlines their high potential for diminishing current ecological problems. Together with these contributions to beneficial environmental development, various microalgae accumulate high concentrations of oils and even hydrocarbons (30–80 % of cell dry mass formed) beside other high-value marketable products (e.g., polyunsaturated fatty acids or pigments like astaxanthin) as cell constituents. Low cell densities and moderate growth rates typical for algal cultivation are known as the major obstacles toward a broad market penetration of microalgal products: Here, high cell densities are required to obtain reasonable volumetric productivities. For some microalgal strains, mixotrophic cultivation by providing organic carbon 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, the fresh catalytically active algal biomass accumulates desired products via CO2 fixation, e.g., from industrial effluent gases, as the sole carbon source. This can be realized by two-stage, continuously operated closed photo-bioreactor systems. After cell harvest and optimized product recovery, the value-added conversion of residual algal biomass for generation of sustainable energy sources, e.g., in biogas plants, constitutes another challenge.