MICROALGAE PRODUCTION RECOVERING CARBON AND NUTRIENTS FROM AGRO-INDUSTRIAL WASTES

Abstract In the last decades several companies worldwide pain significant interest on microalgae production biotechnology as these microorganisms are able to produce high added value compounds such as lipids and polyunsaturated fatty acids, proteins and essential amino acids, antioxidants and pigments that could be used in several sectors, i.e. food, feed, green chemistry, cosmetical, nutraceutical and pharmaceutical industries and bio-energy field. Nowadays, autotrophic cultivation is the main modality in which microalgae are industrially produced, but its success is limited by factors such as light availability, carbon dioxide supply and high production costs. The main challenge to overcome however is the development of high efficiency strategies for the large-scale production of microalgae at low costs. One possibility to overcome these limits is to exploit the ability of some microalgae species to use organic substrates as a carbon source, i.e. Mixotrophy. Mixotrophy is a metabolic pattern in which microalgae drive both autotrophy and heterotrophy, thus utilizing organic carbon sources as substrates of growth, improving the productivity of the system. Agro-industrial wastes and wastewaters are rich in nutrients and have been widely considered as a potential nutrient source for the cultivation of microalgae. Integration with wastewater treatment is a possible synergy for algal production, where algae may grow on sewages as a water and nutrient source, reducing the total production costs. In Lombardy Region the Agro-food system is one of the most advanced in the whole European Union, and each year it produces huge amount of agro-industrial waste stream. This thesis was focused on the cultivation of some microalgal strains on nutrient rich wastes and is structured in four main chapters. After a general introduction in which are highlighted the possibilities to produce microalgae biomass using agro-industrial wastes through mixotrophic metabolism, in Chapter 2 are shown results of the cultivation of Chlorella sp. and Phaedoctylum tricornutum on livestock wastes, showing the ability to growth and the good depuration performances obtained. The results demonstrated that the microalgal growth was comparable to that attained with standard mineral medium. In Chapter 3, were analyzed the possibilities to growth Chlorella sp. and Nannochloropsis salina on carbon-rich wastes (cheese whey, wine lees, glycerol) sampled from different agro-food activities in Lombardy. Evaluation of the different mixotrophic performances were compared with autotrophy (control) and a macromolecular analysis of the obtained biomasses were performer showing very good results in terms of biomass quantity and quality produced under mixotrophic cultivation. In addition, a Life Cycle Assessment analysis was performed to evaluate the environmental impacts of cultivation of Chlorella sp. grown both autotrophically and mixotrophically. Chapter 4 is devoted to the conclusions where is underline that the microalgae cultivation under mixotrophic conditions could be a viable way to reduce the total production cost of microalgae biomass, increasing the productivity and the quality of the final biomass. On the other hand is very difficult to maintain pure culture without any external contamination, and so lead this production process at industrial scale probably remain far remove from reality.