Abstract
An ammonia-oxidizing bacterium was isolated from the sludge of a closed recirculating seawater system for fish culture. The isolated cells were gram negative large coccus of 2.0 to 2.5μm in diameter, and they occurred as single cells, diploids, and tetrads. The cells had central lamellar cytomembranes composed of a stack of 5 to 8 parallel flattened vesicles. The cells grew well in natural seawater and distilled water containing NaCl, MgCl2, and CaCl2. However, they could not grow in distilled water containing only NaCl. The optimum temperature and salinity for growth were 30°C and 40, respectively. The highest ammonia-oxidation rate was obtained at 30°C and a salinity of 20. The isolated bacterium was classified as a member of the genus Nitrosococcus, and was considered to be closely related to N. oceanus.
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