Influence of ammonia and NaCl on nitrifying community and activity: Implications for formulating nitrifying culture augmentation.

Bioaugmentation of nitrifying cultures can accelerate nitrification during startup and transition periods of recirculating aquaculture system (RAS) operations. To formulate nitrifying cultures for RASs, impacts of ammonia and salinity (NaCl) on culturing nitrifying microorganisms were comprehensively investigated by including currently known groups of nitrifying microorganisms (ammonia oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), comammox, Nitrospira, and Nitrobacter). By varying ammonia loading rate (ALRs of 1.6, 8, 20, 40, 60 and 150 mgN/L/d) of continuous-flow bioreactors fed with inorganic medium experimented for culture preparation, cultures containing distinct patterns of nitrifying communities were produced. Operating the reactors at the ALRs of ≤40 mgN/L/d, resulting in the effluent total ammonia nitrogen (TAN) and nitrite concentrations of ≤2.64 and ≤0.53 mgN/L, respectively, delivered the consortia consisting of a broad spectrum of substrate affinity nitrifying microorganisms. At the lower ranges of these ALRs (≤8 mgN/L/d), the most desirable consortia comprising comparable numbers of AOB, AOA, and comammox could be produced (the effluent TAN concentrations of ≤0.20 mgN/L), which would be resilient for applying in various RAS types. Enriching the cultures at the ALRs of ≥60 mgN/L/d allowed only the nitrifying microorganisms with low substrate affinity to dominate, incongruent with the consortia found in actual RASs. AOB were adaptable at all salinity studied (2, 15, and 30 g/L), while AOA and comammox were sensitive to elevated salinity (15 and 30 g/L, respectively). The ammonia removal rate of a culture prepared at 2 g/L salinity decreased largely when applied at 15 and 30 g/L. In contrast, those prepared at 15 and 30 g/L were more robust to different salinity. Separately preparing the cultures at different salinity for uses in freshwater-low salinity and brackish-marine RASs is recommended. The findings of this work enhance our understanding on how to formulate nitrifying culture augmentation for used in different RAS types.