Pathogen contamination in the environment is inevitable with the rapid development of intensive aquaculture. Therefore, alternative ecofriendly biological strategies to control pathogenic bacteria are required. However, our aim was to investigate the ability of oysters (Crassostrea gigas) to filter the important opportunistic pathogen, Aeromonas salmonicida (strain C4), using a green fluorescent protein tag (GFP) in the Atlantic salmon (Salmo salar) farming wastewater. Hence, A. salmonicida removal efficiency and ingestion rate were detected in two different oyster stages (larvae and adults). To evaluate the practical performance of oysters as A. salmonicida biofilter, adult oysters were applied to an integrated constructed wetlands system (ICWS) and their long-term C4-GFP removal efficiency was recorded for 60 days. Overall, our results clearly indicated that oysters had substantial A. salmonicida removal ability via their ingestion process when observed under a fluorescent microscope. Approximately 88-95% of C4-GFP was removed by oyster larvae at an ingestion rate of 6.4 × 103-6.2 × 105 CFU/h·ind, while 79-92% of C4-GFP was removed by adult oysters at an ingestion rate of 2.1 × 104-3.1 × 106 CFU/h·ind. Furthermore, 57.9 ± 17.2% of C4-GFP removal efficiency was achieved when oysters were applied to ICWS. We, therefore, concluded that using oysters as a biofilter represents an effective alternative for removing A. salmonicida from aquaculture wastewater. However, the fate of oysters after ingesting the pathogenic bacteria, acting as a potential reservoir or vector for pathogens, is still debatable. This research provides the basis for the application of oysters as a biofilter to remove pathogens from aquaculture wastewater in industrialized production.
Keywords: Aeromonas salmonicida; Bacterial control; Biofilter; Oysters; Wastewater treatments.