Abstract
To distinguish between the distribution and activity of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in the Pearl River estuary (PRE), we investigated the DNA- and cDNA-based β-proteobacterial and archaeal amoA genes on three size-fractionated particles of >3.0 μm, 0.45–3.0 μm, and 0.22–0.45 μm. Results showed that AOB were more abundant in the freshwater with high concentrations of ammonium (NH4+) and low dissolved oxygen, whereas AOA were dominant in the NH4+-depleted seawater and sensitive to temperature. Obvious shifts in ammonia-oxidizing communities were found along the salinity gradient in the PRE. AOB clearly presented a particle-associated nature, as evidenced by higher relative abundance of amoA genes attached to the large particles (>3.0 μm) and their transcripts exclusively detected on this fraction. Moreover, higher transcriptional activity (indicated by the cDNA/DNA ratio) of AOB on the large particles, suggesting AOB were actively involved in ammonia oxidation despite their lower abundance in the mid- and lower estuarine regions. In contrast, AOA exhibited higher transcriptional activity on the 0.45–3.0 μm and 0.22–0.45 μm particles, implying the free-living strategy of these microbes. Together, these findings from field observations provide useful information on the ecological strategies of ammonia-oxidizing communities in response to different environmental conditions.
Highlights
Nitrification is an important process in the marine nitrogen cycle, which determines the distribution of oxidized and reduced nitrogen components in the ocean (Ward, 2008)
The bivariate correlations between environmental parameters and total abundances of the amoA genes on the three size-fractionated particles in the Pearl River estuary (PRE) were described by Pearson correlation coefficients (R value, see Table 1)
The DNA-based β-Ammonia-oxidizing bacteria (AOB) amoA genes on the three size fractions were significantly correlated (P < 0.05) with temperature (R = 0.776), NO2− (R = 0.700), and salinity (R = −0.720), while the cDNA-based β-AOB amoA genes that were only detected on the large particles were positively correlated (P < 0.05) with NH4+ (R = 0.844) and NO2− (R = 0.786)
Summary
Nitrification is an important process in the marine nitrogen cycle, which determines the distribution of oxidized and reduced nitrogen components in the ocean (Ward, 2008) It is an oxygen- and alkalinity-consuming process, contributing to hypoxia development and the enhancement of ocean acidification especially in coastal waters (Hu and Cai, 2011). A year-round pattern of dramatic decrease in NH4+ and increase in NO3− is found in the upper estuary This feature suggests an elevated level of nitrification (Dai et al, 2006, 2008; He et al, 2014), which corresponds to a high concentration of N2O in the water column (Lin et al, 2016; Ma et al, 2019). Filters of three pore sizes (3.0 μm, 0.45 μm, and 0.22 μm) were applied to fractionate particles in the PRE
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