Dynamics of ammonia-oxidizing microorganisms in gradient temperature-regulated reactors under low ammonia loading condition

Ammonia-oxidizing microorganisms (AOMs) play a crucial role in nitrogen removal in engineered systems. However, temperature fluctuations can lead to ammonia oxidation failure. This study investigated the effects of a broad temperature range (10.5, 14, 17.5, 21, 26.4, 35, 38.5, 42, and 45.5 °C) on the distribution of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and comammox together with Nitrospira and Nitrobacter in gradient temperature-regulated reactors operated under low ammonia loading condition. Quantitative PCR and high throughput sequencing revealed that the AOA amoA gene numbers (∼10⁴ copies/ng DNA) were relatively high at 38.5 °C to 42 °C, suggesting AOA favoring higher temperature than lower temperature. The amoA genes of AOA (∼10³ - 10⁴ copies/ng DNA) outcompeted those of AOB (∼10³ copies/ng DNA) and comammox (<LOD - 10⁴ copies/ng DNA) at the higher temperature ranges from 35 °C toward 45.5 °C. Comammox was sensitive to increasing temperatures comparing to AOA and AOB. Sharp drop of the comammox amoA gene numbers arose when the temperature increased at 35 °C. The comammox amoA genes (∼10³ - 10⁴ copies/ng DNA) dominated at the lower temperature ranges of 14–35 °C. Surprisingly, AOB showed good adaptability to temperature change since stable numbers of the AOB amoA genes (∼10³ - 10⁴ copies/ng DNA) were observed at all temperature levels, which may be caused by the competitive capability among temperature ranges across AOB lineages. Nitrospira was the major NOB at all temperatures studied. However, Nitrobacter exhibited an increasing trend from 14 °C to 10.5 °C. The results of this study lead to a better understanding of the potential response of AOMs to different temperature levels; therefore, the selection of temperature range for the operation of nitrogen removal processes could be better projected.