| Summary: | The microbial community of recirculating aquaculture systems (RAS) is essential for the maintenance of water quality and disease management. Microbiome modulation, through the promotion of higher diversity and an increase in antagonistic microorganisms, can render an aquaculture system more resilient against disease. In line with this concept, this work aimed to 1) isolate and characterize fungi from the rearing water and biofilter of a fish RAS for potential application as microbiome modulators in the aquaculture sector and 2) assess the potential modulating effects of heat-killed biomass of Pseudoalteromonas spp. (HKP) on RAS bacterioplankton communities. In the first chapter, our microbial isolation efforts resulted in 18 fungal isolates all belonging to the Ascomycota and Basidiomycota phyla, with Pseudotaeniolina globosa being the most prevalent (11/18 isolates). In addition, three of the isolates were identified as Vishniacozyma carnescens. Other organisms found include the Ascomycota species Candida labiduridarum and the Basidiomycota species Dioszegia hungarica, Tilletiopsis lilacina, and Cystobasidium slooffiae. These species may produce secondary metabolites with biotechnological potential, such as antimicrobial and antifungal compounds, and carotenoids, and further research is needed to explore their valorisation in the aquaculture sector. In the second chapter of this thesis, the evaluation of the modulating effects of HKP showed that the strain HKP-SubTr2 had the greatest potential for RAS bacterioplankton modulation. The addition of HKP-SubTr2 (assumed to produce prodigiosin pigments, with possible antagonistic activity) to the rearing water was shown to have a clear modulating effect on aquaculture bacterioplankton communities. HKP-SubTr2 treatment significantly enriched the relative abundance of orders Bacteriovoracales, Chitinophagales, and Oceanospirillales in comparison with untreated control and treatment with unpigmented bacteria Escherichia coli HK-DH5α. In addition, the treatment significantly lowered the relative abundance of Rhodobacterales. No significant differences were found among the tested water quality parameters between the different treatments and the control. These findings indicated that the use of heat-killed microbial biomass may be an interesting strategy for aquaculture bacterioplankton modulation. However, further studies are necessary to investigate its potential effect on fish health and water quality during aquaculture production.
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