We constructed ammonia monooxygenase alpha subunit (amoA) gene clone libraries of ammonia-oxidizing
archaea (AOA) and bacteria (AOB) from three biofiltration tanks used for closed marine fish culture systems.
The number of operational taxonomic units (OTUs) found in any one place was 76%–80% of the total OTUs in
each tank for AOA and 100% for AOB when OUTs were defined on the basis of a 5% nucleotide difference. In a
phylogenetic tree, all of the AOA amoA sequences fell into a cluster, which contained Candidatus
Nitrosopumilus maritimus. All of the AOB amoA sequences were related to the Nitrosospira lineage. These
results indicated that different ammonia oxidizers were present in different tanks, but that the dominant
phylogenetic types were stable. In a biofiltration tank to which a high concentration of ammonium chloride
was added periodically to condition the biofilter materials, most of the AOA amoA sequences were different
from the dominant one observed in the fish culture tanks. The AOB amoA sequences were also different, and
were similar to those of Nitrosomonas aestuarii. These findings suggest that high concentration ammonia
loads have a considerable affect on ammonia-oxidizer community composition