Abstract
The increasing demand for sustainable food can partly be solved by the production of aquatic species in recirculating aquaculture systems (RAS). In the Norwegian aquaculture industry, Atlantic salmon (Salmo salar) is often hatched and grown in freshwater RAS until the fish develops into smolt, by which the smolt is transferred to net pens in the sea. Lately, a popular strategy is to keep the smolt in RAS with brackish water for an increased period, allowing the fish to grow bigger before it is transferred to sea. This cuts production time in the sea, thereby reducing problems related to sea lice. In RAS, the cultivation water is always treated to remove organic matter, ammonia, and carbon dioxide. Oxygen is added to the cultivation water prior to the rearing units. This allows an increased recirculation of the rearing water; however, some of the water needs to be exchanged daily to cope with the accumulation of nitrate. Nitrogen (N) is fixed industrially from the atmosphere into fertilizer, which is particularly energy demanding and carbon dioxide intensive. Similarly, phosphate (P) is introduced to the RAS with the fish feed, which is a limited resource obtained by mining. It is estimated that the phosphorus mines will be exhausted within the next 50–100 years, which highlights the importance of scavenging valuable nutrients such as N and P from the RAS water. This master’s thesis is part of the Wasteless project, a collaboration between the Norwegian university of science and technology (NTNU), Nofitech AS, Sintef Ocean AS, and Hardingsmolt AS. The aim of the project is to cultivate a microbial biofilm community containing microalgae with the RAS water as cultivation medium, and in that way bioremediate carbon, N, and P from the RAS water. The nutrients can be recovered into microbial biomass which can be harvested and potentially used as an ingredient in fish feed or for other purposes. In this master’s thesis, four lab scale experiments and one pilot demonstration at the RAS facility Hardingsmolt AS were performed to cultivate microbial biofilm with RAS water as cultivation medium. The master’s thesis investigated different selection parameters: Light, silica, nutrients, and harvest rate, to see whether they influenced the productivity and species composition of the microbial biofilm. The prokaryote and eukaryote components of the biofilm were investigated through 16- and 18S ribosomal RNA amplicon sequencing. The prokaryote part of the biofilm was analyzed through identification of the hypervariable region (V) 3 and 4 in DNBSEQ sequencing, whereas the eukaryote part of the biofilm was analyzed through investigation of the V7 in DNBSEQ sequencing, and the whole ribosomal DNA operon (V1-9) with Oxford nanopore technologies sequencing.
The major findings were that the productivity of the microbial communities was influenced by the selection parameters tested; however, that the species composition was not. The results showed that light was necessary to cultivate a microbial biofilm containing microalgae. The productivity was significantly increased when silica was added, as well as when the nutrient load of the RAS water was high. It was demonstrated that the biofilm should be harvested every 12th day to obtain the highest productivity and biomass yield. The characterization of the prokaryote part of the community showed that the biofilms, the initial RAS water, and the nitrifying biofilter were dominated by heterotrophic bacteria. The most abundant taxa were Alphaproteobacteria, followed by Gammaproteobacteria and Flavobacteriia. The eukaryote part of the biofilms contained a variety of species; however, two diatom microalgae species were highly dominating: Phaeodactylum tricornutum and Nitzschia sp. Diatoms can produce a repertoire of fatty acids, among others the valuable fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which highlights the potential of the microbial biomass as a sustainable ingredient in fish feed.