Sustainable and inexpensive technologies for treating mariculture effluents are needed in both integrated multi-trophic aquaculture (IMTA), and recirculating aquaculture systems (RAS). Marine periphyton can be used to remove excess nutrients from fishponds effluent while converting waste into edible biomass but the technology has not been developed commercially so far. Toward this goal, we developed and examined a marine periphyton-based biofilter in both an IMTA system and intensive RAS. Growth performances, nutrient removal, and biochemical content were studied along with the cultivation of periphyton in the biofilter. In addition, we examined the assembly processes of the microbial community in the periphyton to expand our understanding of the forces that govern community composition, functionality, and networking.
In the IMTA, the periphyton-based biofilter removed up to 76% of the total N and P in the effluents of the fishponds (Mugil cephalus). In RAS, low levels of toxic ammonia, nitrite, and nitrate were maintained even at a high stocking density of fish (Sparus aurata) of 50 kg m-3 and minimal addition of fresh water (10% day-1) to compensate for evaporation.
The periphyton prokaryotic and eukaryotic communities revealed significant differences in their succession patterns. The bacterial community consisted of 23 phyla, but Proteobacteria, Bacteroidetes, Planctomycetes, and Cyanobacteria accounted for >95% of the total ASVs. The bacterial successional process became more stochastic over time while the eukaryotes were more deterministic. The bacterial community assembly was governed by both nutrient regimes and temporal dynamics. However, the force of time produced a stronger effect on the differentiation in prokaryotic community composition compared to nutrient input. Interestingly, the lowest diversity was detected in the mid-time of the periphyton development course, implying a competition process before the community reaches a stable stage. This event coupled with a drastic reduction of Vibrio prevalence and an increase in bacterial genes related to resilience and stress resistance. Further research about pathogenic occurrence in this system will complete a full picture of community dynamics, which in turn will support a more holistic mariculture practice.