## 4 Discussion
This suggests that these two bacterial phyla were more tolerant to heavy metal toxicity, as compared to other bacterial phyla, which was consistent with the results of previous studies [10; 25]. Nitrospirae as important nitrite-oxidizing bacteria plays a special role in nitrogen cycling [26]. The studies also revealed that heavy metal could decrease bacterial diversity and abundance. The Venn diagram of bacterial OTUs (Figure 7c) showed that there were 10,725 core OTUs in the investigated area. The higher-polluted soils (Location 1-4) had 3103 and 4257 unique OTUs for July 2018 and January 2019, respectively. This was slightly lower than those in the lower-polluted soils (Location 5-8), which had 3231 and 4357 unique OTUs for July 2018 and January 2019, respectively. The bacterial diversity was generally greater in the lower-polluted soils (Location 5-8) than in the higher-polluted soils (Location 1-4; Figure 7d). [PERSON]'s correlation analysis (Table 3) showed  
Figure 7: Relative abundance of Acidobacteria (**a**) and Nitrospirae (**b**) at the phylum level for four groups. The Venn diagram (**c**) and Alpha diversity analysis (**d**) for four groups. Note: A and B represented the high-polluted group (Location 1–4) and the low-polluted level group (Location 5–8) soil collected in July 2018; C and D represented the high-polluted level group (Location 1–4) and low-polluted level group (Location 5–8) soil collected in January 2019; Error bars indicate standard error (\(n=4\)). Small letters indicate significant differences among the four groups by one-way ANOVA tests (\(p<0.05\)). that there was a stronger (\(p<0.05\)) negative relationship between Cd and diversity index (ACE and Shannon). This confirms that increased heavy metal concentration could reduce bacterial diversity and abundance in the study area, which was in good agreement with the work by [PERSON] et al. (2013). The results from this study suggested that temporal variation in microbial structure was not remarkable. Proteobacteria was found to be the most predominant phyla in the soils collected during both sampling campaigns, which is consistent with previous research in other mangrove soils ([PERSON] _et al._, 2018). However, most of the diversity indices were significantly higher in the summer (July 2018) than in the winter (January 2019; Table 2 and Figure 7d), indicating a less diverse bacterial community in the winter. It was likely that the low temperature during winter time inhibited the activities of the bacteria ([PERSON] _et al._, 2018). There was work showing that there was no significant seasonal variation in bacterial abundance ([PERSON] _et al._, 2018; [PERSON] _et al._, 2018). However, both OTUs number and Venn figure in this study indicated that the bacterial abundance was higher during the winter (January 2019) than during the summer (July 2018; Figure 7c and Table 2). Mai Po Mangrove Wetland in Hong Kong, China is adjacent to the study area. In comparison, the Shannon index was higher in the Mai Po than in the current investigated area (Table 4). This may be attributable to the less contaminated status in the former (Jul _et al._, 2018). Both sites shared 4 out of the 5 most abundant bacterial phyla with a slightly different order. The bacterial community structure in the current study area was also comparable to other mangrove sites along the South China coast with some levels of difference in terms of diversity index and community structure (Table 4), reflecting the difference in local environmental conditions that influence microbial activities. By comparison with some of tropical mangrove wetlands around the world, it is interesting to note that the community structure in the mangrove soils along the subtropical South China coast was, to a remarkable extent, different from those shown in the tropical reference sites though the most abundant phylum-level bacteria are the same (Table 4).