sediment samples physicochemical properties
Determination of physical and chemical properties of sediment samples, including pH, oxidation-reduction potential (ORP), moisture content, ammonium content, nitrite content, nitrate content, total inorganic nitrogen and total nitrogen content (TN) are presented in Table 1. All of the sediments had acidic pH values (i.e., 5.45-6.56). The pH value of the upper layer sediment samples (S0-10, S10-20) were relatively lower than the middle (S20-30, S30-40) and deep layer (S40-50, S50-60), which probably resulted from the higher nitrification rates in the upper layer sediments. The ORP of the upper layer sediments were much higher than the rest of the samples. For the moisture, the middle layer was the highest. The ammonium content of the six sediment samples ranged from 1.32 to 6.21 mg/kg and the peak was found in the sample of S30-40. All the sediment samples had low nitrite content as expected. The nitrate content ranged from 4.99 to 16.89 mg/kg, and the nitrate content in the deep layer were higher than that of the upper layer. As for the TN, the content ranged from 857.77 to 1604.05 mg/kg, and the TN content increased as the depth went deeper.
Table 1
The physiochemical properties of the sediment samples
Sample names
|
pH
|
ORP (mV)
|
Moisture (%)
|
NH4+-N (mg/kg)
|
NO2−-N (mg/kg)
|
NO3−-N (mg/kg)
|
TIN (mg/kg)
|
TN (mg/kg)
|
S0 − 10
|
5.45
|
149.07
|
25.12
|
1.32
|
0.18
|
10.75
|
12.25
|
857.77
|
S10 − 20
|
5.91
|
105.88
|
35.23
|
1.30
|
0.18
|
4.99
|
6.47
|
1243.61
|
S20 − 30
|
6.01
|
76.86
|
41.44
|
1.55
|
0.48
|
12.73
|
14.76
|
1279.37
|
S30 − 40
|
6.56
|
75.10
|
44.02
|
6.21
|
0.79
|
16.89
|
23.90
|
1521.07
|
S40 − 50
|
6.23
|
83.17
|
39.50
|
3.77
|
0.67
|
16.21
|
20.65
|
1269.46
|
S50 − 60
|
6.29
|
89.39
|
39.56
|
4.98
|
0.51
|
11.61
|
17.11
|
1604.05
|
Abundance of AOA and AOB
In this study, the functional genes (amoA gene) of AOA and AOB in six sediment samples were quantitatively analyzed . The number of AOA amoA genes of these sediment samples ranged from1.26×107 to 2.82×108 and the number of AOB amoA genes of the sediment samples ranged from 3.33×105 to 3.22×108 (Table S1). The number of AOA and AOB amoA genes observed in this work was similar to previous study in the intertidal zones in Zhoushan, Zhejiang province (Hu et al., 2019). Hu found that the AOA and AOB amoA gene copy numbers in the intertidal zone were both ranging from 107 to 108. The AOB amoA gene copy numbers in this study was relatively lower since samples S40-50 and S50-60 were collected in the deep layers of the sediments. In the intertidal zones in Zhoushan, only surface sediments were collected for analysis. The AOA amoA gene copy numbers in the upper layers (S0-10, S10-20) were much lower than that in the middle (S20-30, S30-40) and deep (S40-50, S50-60) layer (Fig. 1). On the contrary to AOA amoA genes, the number of AOB amoA genes in the upper layers (S0-10, S10-20) were much higher than that in the middle (S20-30, S30-40) and deep (S40-50, S50-60) layer (Fig. 1). The ratio of AOA/AOB amoA gene copy numbers ranged from 0.04 to 150.75 in the 6 sediment samples and the ratio increased dramatically from upper layers to deep layers. The ratios of AOA/AOB amoA genes in the 6 sediment samples were 0.04 (S0-10), 0.11 (S10-20), 8.20 (S20-30), 7.07(S30-40), 86.77 (S40-50) and 150.75 (S50-60), respectively (Table S1). The ratios of AOA/AOB amoA genes increased as the sediments depth went deeper, which indicated that AOA may dominated the ammonia oxidation in the deep layer. This founding was in accordance with the previous work focusing on the ammonia-oxidizing microbes in soils (Leininger et al., 2006). Leininger reported that the ratios of AOA/AOB amoA genes increased from 55 to 842 with soil depth went deeper. This phenomenoncould be explained by the higher affinities of AOA for oxygen compared to AOB (Martens-Habbena et al. 2009; Junget al. 2011; Kim et al. 2012). In the upper layers (S0-10, S10-20) where the oxygen was sufficient, AOB won the competition with AOA and may be the main driver of ammonia oxidation. However, in the middle (S20-30, S30-40) and deep layer (S40-50, S50-60) where the environment became anoxic, the higher affinities of AOA for oxygen could help them to win the competition with AOB. AOA outnumbered AOB in oxygen-limited environment was also previously mentionedin Zhoushan intertidal zones. In the middle tidal zone where the oxygen was relatively sufficient, the number of AOB amoA genes was higher than that of AOA. In the subtidal zones where oxygen was limited, AOA outnumbered AOB (Hu et al., 2019). From the point of abundance, oxygen concentration indeed led to the niche specialization of AOA and AOB.
Diversity of AOA and AOB
High-quality AOA and AOB sequences after quality control procedures were applied to diversity analysis. Aa for AOA, a total of 17144 high-quality sequences were obtained for the 6 sediment samples. Using the 15% cut-off, which was recommended previously (Pester et al., 2012), a total of 18 OTUs were obtained for the amoA gene of archaeal. The coverages of the 6 AOA amoA gene clone libraries were all higher than 99.0%, ranging from 99.80% to 99.96% (Table S2). The AOA OTU numbers of the 6 sediment samples ranged from 10 to 15, with S30-40 (10 OTUs) and S50-60 (15 OTUs) displayed the lowest and highest diversity, respectively (Table S2). The OTU numbers in the deep layers were higher than that in the upper and middle layer (Fig. 2). Overall, the change rule of the AOA OTU numbers was similar to the AOA abundance, both of them increased with sediment depth. AOA showed higher diversity as oxygen concentration became lower, which was in accordance with previous work in water-level-fluctuating zones in Three Gorges Reservoir (Liu et al., 2015). At higher oxygen concentration of non-flooded areas, AOA OTU were lower value; In the relatively low oxygen concentration in flooded areas, AOA OTU values higher. This indicated that AOA was more adaptive to the oxygen-limited environments. A total of18865 high-quality AOA sequences were obtained form 6 sediment samples. Using 15% as cutoff value (Purkhold et al., 2000), 18865 sequences were assigned to 11 OTUs. The coverages of the 6 AOB amoA gene clone libraries were ranging from 99.96% to 100.00% (Table S3). The sample S0-10 showed the highest diversity, holding 11 OTUs in total, which was higher than the rest 5 sediment samples (Fig. 2). The OTU numbers in the rest of the 5 samples were all the same, holding 8 OTUs (Table S3). Similar to the change rules of abundance, the AOB OTU numbers decreased as sediment depth went deeper, which indicated that AOB was not tolerant or adaptable to the hypoxic environments. The research in the area of water-level-fluctuating zones in Three Gorges Reservoir also showed the similar pattern. The AOB OTU numbers were always higher in the non-flooded zones where the oxygen concentration was sufficient than that in the flooded zones where the environment was anoxic. The diversity of AOA was always higher than AOB in all the sediment samples (Fig. 2) and the ratio of AOA/AOB OTU numbers increased from 1.09 to 1.88 as the sediment depth went deeper, which was similar to the change pattern of the abundance.
Phylogenetic analysis and community structure of AOA and AOB
After all the quality control procedures, for AOA, totally 17144 high-quality sequences were obtained for the six sediment samples. According to the amoA genes classification of Archaea (Pester et al., 2012), the representative sequences of the 18 OTUs were grouped into four different clusters as shown in Fig. 3. Neither Nitrosotalea nor Thermal-related AOA was found in all the sediment samples. Nitrosophaera cluster contained 3 OTUs (OTU 12, OTU15, OTU 18), totally 9 sequences. OTU 12(containing 3 seqs) belonging to Nitrosophaera cluster was retrieved from sample S20-30 and S40-50, OTU 15 (containing 2 seqs)was detected in sample S0-10 and S50-60, and OTU 18 (containing 4 seqs) was only retrieved from deep layer samples (S40-50, S50-60). The rest of the 15 OTUs were all affiliated with Nitrosopumilus cluster. Among the 17144 sequences, 17135 sequences were affiliated with Nitrosopumilus cluster, accounted for 99.94% of all the sequences obtained. Allthe six sediment samples were composed dominantly by sequences belonging to Nitrosopumilus cluster. The dominance of the Nitrosopumilus cluster over the Nitrosophaera clusterin this study was similar to previous studies in Chongming eastern intertidal sediments and in the intertidal zones in Zhoushan Island (Zheng et al., 2013; Hu et al., 2019). Phylogeny and meta-data analyses of archaeal amoA sequences also showed that the most ofAOA belong to the Nitrosopumilus cluster (73%), which was nearly twice as much as the Nitrososphaera cluster (37%) (Alves et al., 2018). The failure of the Nitrososphaera cluster was more likely to form due to their inability to adapt to salty conditions or the fluctuation of intertidal environmental factors. AOB obtained 18865 sequences from 6 sediment samples by the same quality control procedure as AOA Using the 15% cut-off that was recommended in a previous study (Purkhold et al., 2000), 18865 sequences were assigned to 11 OTUs. The representative sequences of the 11 OTUs were grouped into two clusters, with 2 OTUs (OTU 9, OTU 10) clustered into the Nitrosomonas cluster and the remaining 9 OTUs clustered into the Nitrosospira cluster (Fig. 4). Only 35 sequences were affiliated with the Nitrosomonas cluster. All the six AOB communities were primarily composed of Nitrosospira-related sequences. The dominance of the Nitrosospira cluster over the Nitrosomonas cluster in this study was similar to previous studies in the coastal Pearl River estuary (Cao et al., 2011). The lower concentration of ammonia nitrogen (0.03 ~ 0.11 mm NH4 +) in the intertidal zone of Qingdao may help to explain the absolute dominance of nitrosoma and nitrosoma. The Nitrosomonas cluster has a lower affinity for the substrate and adapts to higher substrate concentrations. In contrast, the Nitrosospira cluster have relatively higher affinity with the substrate and are more likely to survive environments with low substrate concentrations (Zheng et al., 2014; Yu et al., 2016).
Environmental factors that influence the diversity and communities of AOA and AOB
Pearson correlation coefficient was used to analyze the linear relationship between different environmental factors and amoA gene abundance, OTU number, AOA, AOB diversity index, as shown in Table 2.. ORP was significantly positively correlated with AOB abundance and AOB OTU numbers (P < 0.01), which mean that AOB preferred the niches with higher oxygen concentrations. On the contrary, ORP was negatively correlated with AOA abundance, which mean that AOA preferred the niches with lower oxygen concentrations. All this could be explained by the higher affinity of AOA to oxygen compared to AOB. MC was significantly negatively correlated with AOB abundance (P < 0.01) and AOB OTU numbers (P < 0.05). TN was also found to be significantly negatively correlated with AOB abundance (P < 0.01) and AOB OTU numbers (P < 0.05). The pH value was negatively correlated with the abundance of AOB (P < 0.05).
Table 2
Correlation analysis of environmental factors and AOA, AOB abundance, OTU numbers and diversity index
Environmental factors
|
Pearson correlation coefficient
|
Relative abundance
|
Ratio of abundance
|
Number of OTUs
|
AOA
|
AOB
|
AOA:AOB
|
AOA
|
AOB
|
pH
|
0.300
|
-0.020*
|
0.423
|
-0.919
|
-0.055
|
ORP
|
-0.184
|
0.002**
|
-0.576
|
0.976
|
0.001**
|
MC
|
0.250
|
-0.004**
|
0.605
|
-0.871
|
-0.014*
|
NH4+-N
|
0.503
|
-0.178
|
0.320
|
-0.887
|
-0.392
|
NO2−-N
|
0.090
|
-0.051
|
0.519
|
-0.727
|
-0.242
|
NO3−-N
|
0.083
|
-0.273
|
0.676
|
-0.600
|
-0.755
|
TIN
|
0.142
|
-0.187
|
0.516
|
-0.670
|
-0.583
|
TN
|
0.674
|
-0.029*
|
0.252
|
0.760
|
-0.045*
|
Significance levels are indicated as follows: **, 0.01; *, 0.05. |