Biological conversion of agricultural residues into microbial proteins for aquaculture using PHA-producing mixed microbial cultures

The current trend of increasing human population means that alternative protein sources need to be sought to avoid malnutrition. Fish is the most efficient protein-rich food, but aquaculture relies on fishmeal, whose production is environmentally unsustainable. In this study we evaluated the nutritional value of bacterial biomass produced by the biological conversion of zootechnical residues, for its use as feed in aquaculture. A bioreactor was fed with the fluid produced by the fermentation of zootechnical residues. Polyhydroxyalkanoates-producing microbial cultures were used to produce a biomass rich in proteins (61.8% of dry biomass) and amino acids, with glutamic (12.2% of total protein) and aspartic acid (8.7%) and the essential amino acids lysine (6.6%) and leucine (5.3%) being the most abundant. Feeding trials performed on Zebrafish revealed that the microbial proteins produced are a suitable alternative to the commercially available feed, with fish showing survival rates (74%) comparable to the control feed (82%), and even better results when enriched with oils and polyhydroxyalkanoates (96%). The results showed that it is possible to valorise agricultural residues into fish feed via biological conversion by polyhydroxyalkanoates-producing bacteria, while eliminating waste and producing biogas at the same time.