Biorecovery of nutrient waste as protein in freshwater macroalgae

Over 150 million tonnes of protein are consumed annually by domesticated animals and this demand is expected to double by 2050. Freshwater macroalgae are a group of organisms that could contribute significantly to these future protein requirements as they can be cultured on-site, utilising the nutrient waste water from animal production. The aims of this study were to investigate the relationship between nitrogen supply, biomass productivity and the quantity and quality of protein in the freshwater macroalga, Oedogonium, cultured in situ in the waste water from an intensive freshwater fish farm. The dry weight (DW) productivity of Oedogonium ranged between 23.9 and 35.7 g·m^−2·day^−1, whilst on an ash free basis the rate of productivity ranged between 17.1 and 23.6 g·m^−2·day. These productivities are the highest documented for freshwater macroalgae. The protein content (sum of amino acids) of this biomass increased linearly with increasing nitrogen content of the biomass from a minimum of 3.96 g·100 g^−1 DW when the internal nitrogen content was 0.86%, to a maximum of 18.07 g·100 g^−1 DW when the nitrogen content was 4.16%. The quality of the protein in Oedogonium was high, with the essential amino acids accounting for 43.1–43.8% of the total amino acids. Methionine accounted for between 1.6 and 1.9%, and lysine 6.8 and 7.3% of this protein, with the proportion of each slightly increasing as the internal nitrogen content of the biomass decreased. The quantity and quality of protein in the Oedogonium biomass in this study are equivalent to, or higher than, many terrestrial crops that are currently used as a protein source in animal feeds. As such, integrating the production of Oedogonium into the waste management of intensive animal production will provide a mechanism to recover nutrients which, firstly, delivers a novel source of protein for the agricultural sector and, secondly, contributes to the environmental sustainability of intensive animal production through bioremediation