Production of microbial protein from brown seaweed and spruce wood and its use as a novel feed ingredient in aquaculture

With the world’s rapidly expanding population, it is necessary to provide sustainable and nutritious food. Aquaculture is the world’s fastest growing food production sector and carries with it some major tradeoffs and constraints. It is facing a major sustainability challenge as it is heavily dependent upon marine-derived feedstocks such as fishmeal. With the inevitable increase in the price of fishmeal, declining supply and rising demand, more emphasis has been given to alternative feed sources. The partial replacement of fishmeal with plant based protein sources in the aquaculture has been steadily increasing, however, many of these ingredients can be used as human food directly. Furthermore, the anti-nutritional factors in many plant ingredients can have negative effects in carnivorous fish such as salmonids. Large efforts have been made to develop the technology to produce alternative protein sources by using unicellular microorganisms such as microalgae, yeasts, fungi or bacteria. In this thesis, we have produced microbial protein from Norwegian bioresources such as brown seaweed and Norway spruce that could partially replace fishmeal in the Atlantic salmon diets. The work included characterization of the feedstocks, enzymatic saccharification of seaweed, fermentation for microbial protein production and fish feed trials. This study is based on five research papers: A detailed characterization of Saccharina latissima biomass is presented in the Paper I. The paper describes the biomass production and chemical composition of S. latissima cultivated at different depths and harvested at different time points. The enzymatic saccharification process of S. latissima by using a blend of cellulases and an alginate lyase are described in Paper II. It was shown that the inclusion of alginate lyases improved the saccharification yield of the seaweed, particularly at high solid loading. The carbohydrate content and the enzymatic saccharification of the brown seaweeds Macrocystis pyrifera from Chile and Saccharina latissima from Norway was compared in Paper III. For both seaweeds, recombinant alginate lyases and oligoalginate lyases in combination with cellulases gave higher sugar release than using cellulases only. However, for saccharification of pretreated seaweed only cellulases were needed to achieve high sugar release, indicating that the pretreatment partially hydrolysed the alginate. Moreover, it was shown that seaweed hydrolysate could be used as a growth medium for the yeast Candida utilis.  

The cultivation of microbial yeast (C. utilis) from enzymatic hydrolysates of brown seaweed and spruce at different fermentation scales was studied in Paper IV. The yeast product quality in terms of amino acids composition, and mineral content were also studied. A feeding experiment with Atlantic salmon showed that the yeast biomass could partly replace a fishmeal diet, without affecting the growth, but with sub-optimal nutrient digestibility. The nutrient digestibility of C. utilis cultivated from three different carbon and nutrient sources: 1) a blend of woody hydrolysate and molasses, 2) spend sulphite liquor, and 3) a blend of brown seaweed and woody hydrolysate, and the impact of the different yeast biomasses on faecal mineral excretion was evaluated in Paper V. Inclusion of 30 % yeast cultivated on seaweed and spruce hydrolysates resulted in reduced digestibility of protein compared to both the fishmeal-diet and the two other yeast-based diets. In conclusion, this study demonstrated a proof-of-concept for utilization of brown seaweed and wood biomass for the production of microbial protein ingredients for the aquaculture sector.