OXYGEN TOLERANCE AND PERFORMANCE OF MARINE ANAMMOX CULTURE (CANDIDATUS SCALINDUA) IN RECIRCULATING AQUACULTURE SYSTEMS

Sammanfattning

Recirculating aquaculture systems (RAS) are increasingly used as an alternative to open cage aquaculture because of their reduced environmental impact and improved water-use efficiency. However, maintaining water quality remains challenging due to the accumulation of nitrogen compounds, including ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻). Anaerobic ammonium oxidation (anammox) has emerged as a sustainable nitrogen removal process, although the oxygen (O₂) sensitivity of marine anammox bacteria, particularly Candidatus Scalindua (Ca. Scalindua), limits its application in highly oxygenated systems such as RAS. This study investigated the effect of dissolved oxygen (DO) on the nitrogen removal performance and O₂ tolerance of Ca. Scalindua under conditions relevant to cold-water marine RAS. Two laboratory-scale marine reactors with different initial anammox biomass (~8% and ~25% in reactors 1 and 2, respectively) were operated in a two-phase experiment. In phase 1 (stabilization), DO concentrations were maintained below 0.1 mg L⁻¹, while in phase 2 (O₂ testing) DO levels were progressively increased. The reactors were continuously fed synthetic wastewater containing NH₄⁺, NO₂⁻, and trace elements. Both reactors maintained high nitrogen removal performance despite increasing DO concentrations. Average NH₄⁺ and NO₂⁻ removal efficiencies were 94.27 ± 2.80% and 99.77 ± 0.35% in reactor 1, and 97.4 ± 2.1% and 99.83 ± 0.20% in reactor 2, respectively. At maximum tested DO concentrations of 4.9 and 4.4 mg L⁻¹, NH₄⁺ and NO₂⁻ removal remained above 88% and 99%, respectively. The results demonstrate that Ca. Scalindua can sustain efficient nitrogen removal under moderate aerobic conditions, considerably higher than previously reported inhibitory levels. No clear O2 tolerance threshold was identified within the tested DO range. Nevertheless, the study suggest that the O₂ tolerance threshold is at least 4.4–4.9 mg L⁻¹, corresponding to the maximum DO concentration achieved, and the effect on microbial community is yet to be assessed. These findings highlight the potential for integrating marine anammox processes into RAS, where maintaining strictly anoxic conditions is technically challenging.