Selection of Functional Bacteria from the Biofilter of a Freshwater Recirculating Aquaculture System for Microbial Control

The use of Recirculating Aquaculture Systems (RASs) in fish farms reduces water consumption and the emission of polluted effluents into the environment. The microbial community in a RAS biofilter is crucial for effective water purification. This study aims to investigate the biocontrol activity of two bacterial strains obtained from a RAS biofilter. Several heterotrophic strains were isolated and selected based on their metabolic profiles and antagonistic effects against bacterial pathogens. Two selected candidates, namely Serratia oryzae (R5) and Bacillus thuringiensis (R28), had a phenotypic profile potentially suitable for conditions commonly found in freshwater RAS biofilters, as well as an in vitro antagonistic effect against several pathogens. S. oryzae and B. thuringiensis were tested in a small-scale simulated biofilter for their effectiveness in controlling the growth of the Escherichia coli. Both strains successfully colonized the biofilter systems, but only S. oryzae maintained lower E. coli cell densities. Therefore, S. oryzae could be an interesting strain for its future applicability as an enrichment culture and biocontrol agent. Further studies could test the strain in situ for using as an environmentally friendly and biosecure alternative for improving biofiltration processes and preventing disease in RAS.