RESUMO
In recirculating aquaculture systems (RAS), maintaining water quality in aquaculture tanks is a paramount factor for effective fish production. A down-flow hanging sponge (DHS) reactor, a trickling filter system used for water treatment of RAS that employs sponges to retain biomass, has high nitrification activity. However, nitrification in seawater RAS requires a long start-up time owing to the high salinity stress. Therefore, this study aimed to evaluate the nitrification characteristics and changes in the microbial community during the conversion of freshwater to seawater in a DHSreactor fed with ammonia-based artificial seawater. The total ammonia nitrogen concentration reached 1.0â¯mg-N·L-1 (initial concentration 10â¯mg-N·L-1) within 11 days of operation, and nitrate production was observed. The 16â¯S rRNA gene sequence of the DHS-retained sludge indicated that the detection rate of the ammonia-oxidizing archaeon Candidatus Nitrosocosmicus decreased from 23.9â¯% to 14.0â¯% and 25.8-17.6â¯% in the upper and lower parts of the DHS reactor, respectively, after the introduction of seawater. In contrast, the nitrite-oxidizing bacteria Nitrospira spp. increased from 0.1â¯% to 9.5â¯% and from 0.5â¯% to 10.5â¯%, respectively. The ammonia oxidation rates of 0.12 ± 0.064 and 0.051 ± 0.0043â¯mg-N·g-MLVSS-1·h-1 on the 37th day in the upper and bottom layers, respectively. Thus, nitrification in the DHS reactor performed well, even under high-salinity conditions with short operational days. This finding makes the transition from freshwater to saltwater fish in the RAS system simple and economical, and has the potential for early start-up of the RAS.