Nitrate accumulation in biofloc aquaculture systems

Minimizing ammonium is one of the most important concerns in the practice of aquaculture. Chemoautotrophic bacterial nitrification (CBN) and heterotrophic bacterial assimilation (HBA) are the two main ammonium pathways in aquaculture water under light-limited conditions. CBN oxidizes ammonia–nitrogen (NH4+-N) to nitrite–nitrogen (NO2−-N) and finally to nitrate–nitrogen (NO3−-N), and this is the primary strategy employed in recirculating aquaculture systems using fixed-cell bioreactors. Zero-exchange management systems based on HBA and biofloc technology have been developed referred as biofloc aquaculture systems. HBA is encouraged in biofloc aquaculture systems by increasing the C/N ratio. However, NO3−-N build-up was found in many studies examining these systems, with CBN proposed as the responsible mechanism and documenting the factors leading to the occurrence of the CBN process. This paper examines the relative importance of CBN and HBA in biofloc aquaculture systems. A mechanism is proposed and future study aimed at mediating NH4+-N pathways in biofloc aquaculture systems is recommended.