Production and use of microalgae biomass for aquaculture feeds

For a sustainable aquacolture industry, current levels of fish meals/oils in aquafeeds need to be drastically reduced and mostly replaced by vegetable counterparts. Microalgae show large potential as alternatives to fish-based feeds for aquaculture due to their high content of good-quality protein, vitamins, minerals and long-chian PUFAs. However, the high production cost of microalgae biomass still prevents its commercial exploitation as raw material for aquafeeds. Further R&D efforts are needed to achieve the ambitious goal of producing algal biomass at costs competitive with those of fish meal (€ 1-2 kg-1). Aim of this work was to develop an economically sound cultivation process for outdoor mass production of marine microalgae thanks to low-cost cultivation systems and reduced operational costs (mixing, cooling and fertilizers). Several microalgae species were tested in laboratory for their growth characteristics, stress resistance, biochemical composition and "in vitro" digestibility. Hence, among the most promising species, the marine Tetraselmis suecica was cultivated outdoors in "Green Wall Panel" photobioreactors (WO2004/074423; WO 2011/013104), evaluating its performance in terms of productivity and energy balance. Tetraselmis has shown a potential for producing about 60 tons of dry biomass ha-1 year-1, of which 30 tons may be protein. Since this microalga is also robust and versatile and shows a well balanced nutritional compostion, it seems feasible to produce biomass at relatively low costs. In feeding experiments with seabass, diets including graded levels of dried T. suecica, Isochrysis sp. (T-ISO) or a blend of the two marine microalgae showed similar growth performance and feed utilisation compared to the control diets. The effects of the dietary microalgae inclusion on fillet proximate and fatty acid composition were also considered. Techniques of algal cell disruption are under test to increase microalgae digestibility and will be discussed.