Introduction
While biofilm formation in biofilters has been extensively studied, microbiota attached to other surfaces within recirculating aquaculture system (RAS), such as tank walls, remain largely unexplored. This is particularly relevant, as sloughed biofilm can contribute to microbial dispersal throughout the system via water recirculation. Gaining insights into the factors influencing biofilm development is therefore crucial for managing system microbial dynamics and potential pathogen reservoirs. To this end, this study investigated the effects of system type and diet on the temporal dynamics of tank biofilm over an eight-week period.
Materials & Methods
Three mariculture systems with different water refreshment rates were examined: a flow-through system (FTS), a high refreshment (HR-RAS), and a low refreshment RAS (LR-RAS). Each system consisted of four tanks evenly stocked with juvenile gilthead seabream (Sparus aurata). To assess the effect of diet, fish in each tank were fed restrictively one of two aquafeeds in duplicate: a diet rich in non-starch polysaccharides (NSP-DIET) or a starch-rich diet (S-DIET). Water quality and biofilm development were analyzed at the tank level at Week 0, 2, 5, and 8. Water samples were filtered at 0.22 μm immediately after collection, and biofilm samples were obtained by swabbing tank walls. Water physicochemical parameters were assessed using standard analytical methods. Microbial abundance was measured via quantitative polymerase chain reaction (qPCR). Microbial community composition was determined through full-length 16S rRNA gene sequencing using the ONT MinION device; sequences were taxonomically assigned using the SILVA v148.1 database and further analyzed with the R microeco package.
Results & Discussion
By the end of the trial, tank biofilm in the LR-RAS showed the highest microbial abundance and a trend toward higher organic carbon levels, indicating that nutrient accumulation promotes biofilm growth. Alpha diversity also varied most in the LR-RAS, with a short-term decline linked to the NSP-DIET. This contrasted with trends observed in the tank water—where alpha diversity fluctuated more under higher water refreshment rates—suggesting that biofilm and water microbial dynamics are likely independent of each other. Beta diversity in tank biofilm shifted over time across all systems, with each maintaining distinct prokaryotic communities throughout. Despite these shifts, the relative abundance of dominant taxa remained stable and included no known gilthead seabream pathogens (Fig1A), even though potentially pathogenic bacteria (e.g. Vibrio, Coxiella) were detected in water from the two RAS. Differences among systems were partly explained by physicochemical water quality parameters, with organic carbon and nitrogen levels strongly shaping biofilm prokaryotic communities in the two RAS. In the LR-RAS, diet also influenced biofilm community structures (Fig1B). Nonetheless, it did not promote the presence of common sulphate-reducing (i.e. Desulphobacteroda) or off-flavor producing (e.g. Oscillatoria perornata, Anabaena spp., and Pseudanabaena) bacteria, which remained at low relative abundances in all systems, despite their frequent presence in previous biofilm studies (Houle et al., 2011; Rojas-Tirado et al., 2021). Overall, the present findings highlight that environmental conditions are the primary drivers of biofilm development in aquaculture systems, while diet plays a secondary but notable role under low water exchange.
Funding
This work was supported by the TNA program (PID: 25209 and 25195) within H2020 AQUAEXCEL3.0 project (871108) to E. Syropoulou for accessing IATS-CSIC facilities.
References
Houle, S., Schrader, K. K., Le François, N. R., Comeau, Y., Kharoune, M., Summerfelt, S. T., Savoie, A., & Vandenberg, G. W. (2011). Geosmin causes off-flavour in arctic charr in recirculating aquaculture systems. Aquaculture Research, 42(3), 360–365. https://doi.org/10.1111/j.1365-2109.2010.02630.x
Rojas-Tirado, P., Aalto, S. L., Åtland, Å., & Letelier-Gordo, C. (2021). Biofilters are potential hotspots for H2S production in brackish and marine water RAS. Aquaculture, 536. https://doi.org/10.1016/j.aquaculture.2021.736490