Introduction
Microbes inhabit all the compartments of recirculating aquaculture systems (RAS), being suspended in water phase and forming biofilms on the surfaces. The majority of these microbes is considered to be harmless for the fish, and some of them are even beneficial, having an important role in the nitrifying biofilters. However, uncontrolled growth of microbial biomass can promote the abundance of harmful microbes, and increases the need for aeration and degassing. To maintain sufficient water quality and to hinder microbial blooms, water treatment methods to remove microbes and other organic matter are currently searched and developed for RAS. Two potential methods are foam fractionation and ozonation, the first removing organic matter, while the latter destroys and inactivates microbes directly. In this study, we examined the effect of foam fractionation and/or ozonation on water quality and on microbial communities in water and biofilter biofilms in replicated freshwater RAS with rainbow trout.
Materials and methods
The experiment was conducted in 12 replicated, 0.8 m3 pilot-scale freshwater RAS stocked with 8.05 ± 0.03 kg juvenile rainbow trout (Oncorhynchus mykiss) and with final feed loading of 1.66 kg feed m-3 make-up water. Four treatments were applied in triplicate: 1) three control RAS, 2) three RAS with foam fractionator (ff), 3) three RAS with ozone (oz), 4) and three RAS with ozone and foam fractionator (oz + ff). The trial lasted eight weeks. Water quality characteristics, and microbial activity and abundance were measured every week. The composition of microbial communities in water was analysed at beginning of the experiment (week 0) and at weeks 1, 3, and 7 and the composition of communities in biofilter biofilms at week 7 using next generation sequencing with IonTorrent PGM targeted on 16S rRNA gene V4 region. The detailed description of the experimental setup and sample analysis is given in de Jesus Gregersen et al. (2021) and Aalto et al. (2022).
Results and discussion
Both ozonation and foam fractionation improved the system water quality by decreasing the biological and chemical oxygen demand and the amount of microparticles in water (Table 1), and this effect was pronounced when these two water treatments were combined. The same trend was observed with microbial activity, which decreased by 10-fold when applying both ozonation and foam fractionation as compared to the control systems. However, foam fractionation affected neither the microbial community composition (Fig. 1A) nor the abundance of microbes (data not shown), suggesting that the lower microbial activity observed under foam fractionation was due to the lower amount of organic matter being available for heterotrophic microbes. In constrast, ozone posed a strong selection pressure by attacking the microbes directly, having a significant effect on the microbial community composition already one week after application (Fig. 1). Moreover, the microbial abundance was significant lower in ozonated units (oz, oz + ff) than in foam fractionation or control units (data not shown). While the biofilter biofilm communities were more resistant to the water treatment than the communities suspended in the system water, we found comammox Nitrospira, carrying out the complete nitrification (Daims et al., 2015), to be more abundant in the foam fractionation unit biofilters. This together with the significantly lower nitrite concentration found in the foam fractionation treatment (Table 1) suggests that by removing organic matter, foam fractionation promotes more stable nitrification in the biofilters by decreasing the activity of heterotrophs, which allows a higher abundance of nitrite oxidizers but also a shift in the nitrifying community from canonical two-step process into the complete nitrification. Altogether, the results indicate that although these two water treatment methods have similar outcomes on physico-chemical water quality and microbial activity, their underlying mechanisms are different, potentially leading to different outcomes under the long-term application.
References
Aalto, S.L., Syropoulou, E., de Jesus Gregersen, K.J., Tiirola, M., Pedersen, P.B., Pedersen, L.F., 2022. Microbiome response to foam fractionation and ozonation in RAS. Aquaculture 550. https://doi.org/10.1016/j.aquaculture.2021.737846
Daims, H., Lebedeva, E. V., Pjevac, P., Han, P., Herbold, C., Albertsen, M., Jehmlich, N., Palatinszky, M., Vierheilig, J., Bulaev, A., Kirkegaard, R.H., Von Bergen, M., Rattei, T., Bendinger, B., Nielsen, P.H., Wagner, M., 2015. Complete nitrification by Nitrospira bacteria. Nature 528, 504–509. https://doi.org/10.1038/nature16461
de Jesus Gregersen, K.J., Pedersen, L.F., Pedersen, P.B., Syropoulou, E., Dalsgaard, J., 2021. Foam fractionation and ozonation in freshwater recirculation aquaculture systems. Aquac. Eng. 95, 1–8. https://doi.org/10.1016/j.aquaeng.2021.102195