Cell Density-dependent Anammox Activity of Candidatus Brocadia sinica Regulated by N-acyl Homoserine Lactone-mediated Quorum Sensing

The activity of anaerobic ammonia-oxidizing (anammox) bacteria is considered to depend on cell density; however, this has not yet been confirmed due to the fastidious nature of anammox bacteria (e.g., slow growth, oxygen sensitivity, and rigid aggregate formation). In the present study, the cell density-dependent occurrence of anammox activity (14-15N2 gas production rate) was investigated using planktonic enrichment cultures of Candidatus Brocadia sinica. This activity was detectable when the density of cells was higher than 107‍ ‍cells‍ ‍mL–1 and became stronger with increases in cell density. At the cell densities, the transcription of the BROSI_A1042 and BROSI_A3652 genes, which are potentially involved in the biosynthesis and reception of N-acyl homoserine lactone (AHL), was detectable in Brocadia sinica cells. The presence of AHL molecules in the MBR culture of B. sinica was confirmed by an AHL reporter assay and gas chromatography mass spectrometry analysis. The exogenous addition of the MBR culture extract and AHL molecules (a cocktail of C6, C8, C10, and C12-homoserine lactones) increased the specific 14-15N2 production rate of B. sinica. These results suggest that the specific anammox activity of B. sinica is regulated by AHL-mediated quorum sensing.