Bioremediation of ozone-produced oxidants in marine RAS using Ulva ohnoi: benefits and impacts on seaweed physiology and associated microbiomes

Maintaining good water quality is essential to the success of recirculating aquaculture systems (RAS). Among water treatment tools, ozone (O₃) has garnered interest from aquafarmers worldwide due to its various beneficial effects. Beyond its germicidal properties, ozone improves solid removal, oxidises toxic nitrogen compounds, and degrades a broad spectrum of biogenic and artificial molecules. However, the ozonation of seawater produces by-products (‘ozone-produced oxidants’ (OPO)), which can pose significant risks to animal health. In this experiment, we evaluated the capacity of the seaweed Ulva ohnoi to bioremediate OPO in an Integrated Multi-Trophic Aquaculture (IMTA)–RAS setup cultivating gilthead seabream (Sparus aurata). Effluent water was ozonated and then passed through a cultivation unit containing U. ohnoi. OPO concentrations in the water were measured before and after the seaweed unit, and the reduction in OPO was compared to control systems without U. ohnoi. Additionally, we assessed the effects of OPO on growth, metabolic composition, photosynthetic efficiency, and associated microbiomes of U. ohnoi by comparing seaweed exposed to ozonated water with controls grown without ozonation. The results showed that Ulva-containing systems achieved an 11% higher reduction in OPO than controls. However, OPO induced oxidative stress in U. ohnoi, leading to reduced growth, altered morphology, and elevated levels of chlorophyll, phenolic compounds, soluble sugars, and selected amino acids. The microbiome associated with U. ohnoi shifted, with a reduction in complex carbohydrate-metabolising bacteria. This study demonstrates that U. ohnoi can reduce OPO concentrations in marine RAS, but with impacts on the chemical composition, morphology, and microbiome of the seaweed.