Although the release of biofilm particles into the water by biofilters is acknowledged to influence the suspended water microbiota in recirculating aquaculture system (RAS), several studies have described significant differences between the biofilm communities on biofilters and those in the water (Schreier et al. 2010; Bakke et al. 2017, Vadstein et al. 2018). The bacteria present suspended in the RAS water may interact with fish mucosal surfaces (Bugten et al. 2022), pointing to the necessity of maintaining optimal microbial water quality to mitigate the proliferation of opportunistic and pathogenic bacteria. Despite recognizing the importance of these interactions, the role of the biofilter’s biofilm microbiota in structuring the microbiota suspended in the water and associated with the fish is not well comprehended. Consequently, this study aimed to assess the influence of the biofilter’s biofilm microbiota on the microbiota associated with the water and fish.
Two identical laboratory-scaled RAS were operated under the same conditions, differing only in the biofilm carriers utilized in their biofilters. These carriers were obtained from two commercial RAS situated in different geographical regions of Norway (Trøndelag and Sunnhordland) and represented distinct biofilm communities. After 30 days of RAS operation, the biofilm carriers were exchanged between the systems and operated for another 10 days. The water, fish, and biofilter microbiota were characterized by Illumina sequencing of 16S rDNA amplicons. Before swapping the biofilters, we found that the water microbiota differed significantly between the two RAS, indicating that the biofilm microbiota of the biofilters influenced the water microbiota. After swapping the biofilters, the water microbiota became more similar to the newly introduced biofilter’s microbiota, thus supporting the finding that the biofilter microbiota influenced the microbiota suspended in the water (Figure 1). Notably, the skin and gut microbiota of the fry differed significantly from both biofilter and water microbiota. A noticeable difference in skin microbiota was observed between the two RAS after the initial 30 days of operation, suggesting a potential influence of biofilters’ biofilm microbiota mediated by the water microbiota. Thus, we demonstrated that the biofilter is an important determinant for system’s microbiota in RAS.