Analysis of nitrification potential and nitrifier communities in commercial aquaponic systems

Aquaponics is a type of recirculatory aquaculture system which integrates the hydroponic with a high potential to recover nitrogenous wastes. The nitrogen recovery in aquaponic systems mostly relies on the chemolithoautotrophicnitrifiers. In the present study we have analysed the nitrification potential and the nitrifier communities in the growbed of three commercial aquaponic systems.In all the systems, the concentration of ammonia, nitrite, and nitrate in the water decreased drastically (35-50%) when filtered through the growbed. Based on functional gene analysis comprising amoA,norA, andhzogene, we have analysed various nitrifying communities present in thegrowbed. We have detected the presence of ammonia oxidizing bacteria(AOB), ammonia oxidizing archaea(AOA), nitrite oxidizing bacteria(NOB), complete ammonia oxidizing bacteria (comammox). However all the systems were devoid of anaerobic ammonia oxidizing (anammox)bacteria. The abundance of AOB, AOA, and NOB were carriedout by the real-time PCR.The qPCR results shows a high abundance of AOA (8.7× 10² & 7.2 × 10²) in two auaponic systems compared to the AOB (7.3 × 10² & 2.2 × 10²µg of DNA). However, in the aquaponic system connected with a bioreactor, exhibit a high abundance of AOB (4.6 × 10² & 2.65 × 10²µg of DNA) than the AOA (1.45 × 10² & 5 × 10¹µg of DNA). But in general, all the systems the presence of NOB (0.62 – 1.135 µg of DNA) were very low than the ammonia oxidizers. To our knowledge this is the first report on the presence of AOA, and comammox in aquaponic systems. To get an idea about the entire bacterial communities in the growbed, we have amplified the V3-V4 region, and given for metagenomic analysis. This study demonstrated that the growbed of an aquaponic system is the major place for the nitrification and it contains diverse group of nitrifiers including AOB, AOA, COMAMMOX, and NOB. With this high diversity of nitrifiers, growbed of an aquaponic system can be an ideal source for the enrichment of chemolithoautrophicnitrifiers.