Nitrification characteristics and microbial community changes during conversion of freshwater to seawater in down-flow hanging sponge reactor.

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 1.0 mg-N·L ^-1 (initial concentration 10 mg-N·L ^-1 ) within 11 days of operation, and nitrate production was observed. The 16 S rRNA gene sequence of the DHS-retained sludge indicated that the detection rate of the ammonia-oxidizing archaeon Candidatus Nitrosocosmicus decreased from 23.9 % to 14.0 % and 25.8-17.6 % 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 0.1 % to 9.5 % and from 0.5 % to 10.5 %, respectively. The ammonia oxidation rates of 0.12 ± 0.064 and 0.051 ± 0.0043 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.
Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Takahiro Watari reports financial support was provided by Japan Society for the Promotion of Science. Takahiro Watari reports financial support was provided by Japan Science and Technology Agency. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)