Introduction
Aquaculture is undergoing rapid development worldwide (FAO, 2020), but the sustainability of aquaculture systems is more and more questioned in view of the many changes taking place (climate, animal welfare, antibiotic resistance, etc.) (Sudarshan et al., 2021) . To reduce their dependence on the surrounding unstable environment, the Recirculating Aquaculture Systems (RAS) have been developed. Those systems allow to reuse 90-99% of water but also to create an optimal growth environment for fish (temperature, nutrients, etc.). The functioning of RAS is highly dependent on the equilibrium between fish and microbial communities (e.g.: biofilter, associated biodiversity) (Kamali et al., 2022) . This equilibrium could become more complex to achieve with the development of the polyculture in RAS (Thomas et al., 2020).
The fish microbiota (tract, skin, and gills) is highly diverse, including bacteria, fungi, viruses (Merrifield and Rodiles , 2015; Dulski et al., 2020; Dai et al., 2021) and is affected by intrinsic factors including trophic level, fish species (mainly linked with gastrointestinal tract morphology and diet) and environmental factor as season and captive-state (Egerton et al., 2018) . Fish have an intimate interaction with their surrounding environment resulting in a shaping effect on water microbial communities (Fourrier et al., 2022). This study suggest also that the RAS history (linked with biofilm memory) seems to influence water microbiome. A reliable protocol is required to stabilize and equilibrate initial microbial communities before studying the effects of fish communities on microbial communities.
An experiment was performed to select a standardized procedure to obtain tanks with a microbial diversity as close as possible regardless of the history of the tanks. The objective here is not to have a perfect disinfection of the tanks , but rather to have a starting situation (t0) in the tanks that is as similar as possible either by homogenizing the filtration substrates or either by replacing them, limiting variability due to fish rearing .
Materials and methods
We compared 3 disinfection/homogenization modalities (M) using 4 replicates for each. That’s why 12 tanks were selected based on their rearing history of different fish species over the past 3 years .
The experiment lasted 30 days and water was sampled (days 0, 16, 23 and 30) to access physico-chemical parameters of water and microbial communities dynamic. The physicochemical parameters of water: temperature, dissolved oxygen, pH, NH4+, NO2- and NO3- will be measured during the experiment. The microbial diversity of tank water ( sampled on days 0, 16, 23 and 30) was assessed by metabarcoding approaches. Sequencing (Miseq 2x300 bp) was performed at the GIGA Institute (University of Liège) and bioinformatic analysis was performed using FROGS and R software.
Results
We observed a significant effect of the treatment on the dynamic of the microbial communities. The most homogenous starting point in all replicates was obtained using the second modality M2. The results allowed us to select the most reliable tested protocol that can be used to study microbial communities regardless of the tank rearing history.
References
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