Introduction
The maintenance of good water quality is crucial for production success in recirculating aquaculture systems (RAS). Ozone (O3) has seen an increase in interest among aquafarmers worldwide as a water treatment tool with a wide array of beneficial effects. Besides its germicidal properties, O 3 can enhance solid removal, oxidise toxic nitrogen compounds and degrade a broad spectrum of biogenic and man-made molecules. While its application in freshwater is mostly unproblematic, ozonation of seawater creates by-products termed ‘ozone-produced oxidants’ (OPO). These can accumulate in a system when process water is recirculated and are detrimental to animal health if species-specific thresholds are exceeded.
Material & Methods
In this experiment, we tested the seaweed Ulva sp. for its OPO- bioremediation capacity in an outdoor IMTA (Integrated Multi-Trophic Aquaculture)-RAS-setup with Gilthead seabream (Sparus aurata ). Effluent sea water was ozonated and subsequently led through an Ulva cultivation unit. OPO-concentrations in the sea water were measured before and after the seaweed unit and the decline was compared to controls without Ulva . Additionally, the impacts of OPO on growth, metabolic composition and photosynthetic efficiency of Ulva were investigated by comparing OPO-exposed seaweed to controls cultivated without ozonation. 16S rRNA gene sequence analyses were performed to monitor the Ulva-associated microbiomes in response to ozonation.
Results
While seaweed-containing systems exhibited a significantly higher reduction in OPO, ozonation also diminished Ulva growth (32 %) and caused a darker, fringier, and more rigid morphology. Moreover, ozonation elevated chlorophyll -, total phenolic – and flavonoid contents, and shifted the amino acid composition in Ulva towards more glutamine, glutamic acid and serine.
Conclusion
With this study we showed that Ulva can reduce OPO-concentrations in marine RAS, but with impacts on growth , chemical composition, and morphological properties of algal biomass.
Acknowledgements
The authors would like to thank Dr. Antonio Vieira and Prof. Wolfram Weckwerth for the provision of space, resources, and consultancy.