Examinando por Autor "Takumi Akamine"

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    Nitrification characteristics and microbial community changes during conversion of freshwater to seawater in down-flow hanging sponge reactor
    (2024) Takumi Akamine; Mami Nagai; Takahiro Watari; Hirotoshi Netsu; Nur Adlin; Penpicha Satanwat; Carlos Riquelme; Masashi Hatamoto; Takashi Yamaguchi
    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.