Abstract
The maintenance of good water quality is essential for the success of recirculating aquaculture systems (RAS). Among water treatment tools, ozone (O₃) has garnered increasing interest from aquafarmers worldwide due to its wide array of beneficial effects. Beyond its germicidal properties, ozone improves solid removal, oxidises toxic nitrogen compounds, and degrades a broad spectrum of biogenic and artificial molecules. While its application in freshwater systems is rather unproblematic, the ozonation of seawater produces by-products known as ‘ozone-produced oxidants’ (OPO). These by-products can accumulate in recirculating systems and pose significant risks to animal health. In this experiment, we evaluated the bioremediation capacity of the seaweed Ulva sp. for OPO removal in an outdoor Integrated Multi-Trophic Aquaculture (IMTA) RAS setup that cultivated gilthead seabream (Sparus aurata). Effluent water was ozonated and then passed through a cultivation unit containing Ulva. OPO concentrations in the water were measured both before and after the seaweed unit, and the reduction in OPO was compared to control systems without Ulva. Additionally, we assessed the effects of OPO on the growth, metabolic composition, and photosynthetic efficiency of Ulva by comparing seaweed exposed to ozonated water with controls grown without ozonation. The results showed that systems containing Ulva achieved a significantly higher reduction in OPO levels, with an 11.1% greater decline compared to controls. However, ozonation also negatively affected Ulva biomass productivity, altered its morphology, and increased its chlorophyll-, phenolic compound-, and soluble sugar contents. Furthermore, the amino acid composition of Ulva shifted under ozonation, with increases in proline, glutamic acid, glutamine, leucine, serine, valine, and phenylalanine. The microbiome associated with Ulva also experienced notable changes, with a marked reduction in complex carbohydrate-metabolising bacteria, likely due to the biofilm-degrading properties of OPO. This study demonstrates that Ulva can effectively reduce OPO-concentrations in marine RAS. However, this bioremediation process impacts the chemical composition, morphological properties, and microbiome of the seaweed, highlighting potential trade-offs in its use.