Overview and future perspectives of nitrifying bacteria on biofilters for recirculating aquaculture systems

Intensive fish culture is a practice in aquaculture systems; however, it can produce negative environmental impacts due to the accumulation of organic nitrogen from feeding procedures, but also from fish wasting products. Recirculating aquaculture systems have emerged as one strategy to reduce these impacts, maintaining a healthy environment for fish. These reduce toxicity level through nitrifying biofilters, which use a microbial consortium to convert ammonia into nitrate. The main component of bacterial consortium is comprised by aerobic ammonia-oxidizing bacteria and aerobic nitrite-oxidizing bacteria, which are part of the whole nitrification process. These nitrifying bacteria grow in suspension or in fixed form, the latter characterized by a biofilm formation where bacteria adhere to a physical substrate in a self-produced polymer matrix. The biofilm formation requires tight communication among bacteria to regulate the transcriptional circuits underlying the production of chemical signal molecules (e.g. N-acetyl-homoserine lactones) that control biofilm formation. This coordination is known as quorum sensing and can be considered as a mechanism that contributes to the coupling and maintenance of nitrification rate among bacteria by regulating expression levels of relevant genes associated with nitrification process. Therefore, the control of this process is crucial in recirculating aquaculture systems and its incorrect manipulation can produce a detriment of water quality. We here present an overview of the nitrification process in recirculating aquaculture systems. Subsequently, we describe nitrifying biofilters and nitrifying bacteria. Finally, we discuss how quorum sensing controls the efficiency of nitrifying bacteria and potential applications of nitrifying biofilters in intensive aquaculture systems.