Comparison and interpretation of freshwater bacterial structure and interactions with organic to nutrient imbalances in restored wetlands

Abstract

Chemical oxygen demand to nitrogen (COD/N) and nitrogen to phosphorus (N/P) ratios have distinct effects on bacterial community structure and interactions. However, how organic to nutrient imbalances affect the structure of freshwater bacterial assemblages in restored wetlands remains poorly understood. Here, the composition and dominant taxa of bacterial assemblages in four wetlands [low COD/N and high N/P (LH), low COD/N and low N/P (LL), high COD/N and high N/P (HH), and high COD/N and low N/P (HL)] were investigated. A total of 7,709 operational taxonomic units were identified by high throughput sequencing, and Actinobacteria, Proteobacteria, and Cyanobacteria were the most abundant phyla in the restored wetlands. High COD/N significantly increased bacterial diversity and was negatively correlated with N/P (R ² = 0.128; p = 0.039), and the observed richness (Sobs) indices ranged from 860.77 to 1314.66. The corresponding Chao1 and phylogenetic diversity (PD) values ranged from 1533.42 to 2524.56 and 127.95 to 184.63. Bacterial beta diversity was negatively related to COD/N (R ² = 0.258; p < 0.001). The distribution of bacterial assemblages was mostly driven by variations in ammonia nitrogen (NH₄ ⁺-N, p < 0.01) and electrical conductivity (EC, p < 0.01), which collectively explained more than 80% of the variation in bacterial assemblages. However, the dominant taxa Proteobacteria, Firmicutes, Cyanobacteria, Bacteroidetes, Verrucomicrobia, Planctomycetes, Chloroflexi, and Deinococcus-Thermus were obviously affected by variation in COD/N and N/P (p < 0.05). The highest node and edge numbers and average degree were observed in the LH group. The co-occurrence networkindicated that LH promoted bacterial network compactness and bacterial interaction consolidation. The relationships between organic to nutrient imbalances and bacterial assemblages may provide a theoretical basis for the empirical management of wetland ecosystems.

Keywords: bacterial assemblages; co-occurrence network; nutrient imbalance; organic matter; restored wetlands.

FIGURE 1

(A) Sampling locations in the four wetlands. LH: Shanghu Wetland; LL: Shajiabang Wetland; HH: Taihu Wetland; and HL: Nanhu Wetland. (B,C) One-way analysis of variance (ANOVA) of Chemical oxygen demand to nitrogen (COD/N) and nitrogen to phosphorus (N/P) in the four wetlands. Different superscripted lowercase letters indicate p < 0.05.

FIGURE 2

Relationship between bacterial alpha diversity (Chao1) and (A) Chemical oxygen demand to nitrogen (COD/N) and (B) nitrogen to phosphorus (N/P). R² represents the correlation coefficient, p < 0.05 indicates significance and p > 0.05 indicates non-significance.

FIGURE 3

(A) Principal coordinate analysis (PCoA) of bacteria based on Bray–Curtis dissimilarity in freshwater of the four wetlands; redundancy analysis (RDA) diagram illustrating the relationships between the compositions of freshwater bacteria at the (B) phylum, and (C) class levels from different sampling sites with variable environments. Blue arrows show bacterial community composition; red arrows show physicochemical properties in the freshwater.

FIGURE 4

Relationships between bacterial beta diversity and (A) Chemical oxygen demand to nitrogen (COD/N), (B) nitrogen to phosphorus (N/P), (C) Chemical oxygen demand (COD), (D) ammonia nitrogen (NH₄⁺-N), (E) total P (TP), and (F) electrical conductivity (EC). R² represents the correlation coefficient; p < 0.05 indicates significance and p > 0.05 indicates non-significance.

FIGURE 5

The relative abundances of the 10 most abundant bacteria at the (A) phylum and (B) class levels in freshwater of the four wetlands. Different superscripted lowercase letters indicate p < 0.05.

FIGURE 6

Heatmap depicting correlations between physicochemical properties of the freshwater and the 10 most abundant bacteria at the (A) phylum and (B) class levels. Red indicates negative correlations, while cyan indicates positive correlations. ** represents p < 0.01, and * represents p < 0.05.

FIGURE 7

Co-occurrence network of freshwater bacterial assemblages in different wetlands. (A) high N/P (LH), (B) low N/P (LL), (C) high N/P (HH), and (D) low N/P (HL). Each node represents a bacterial genus, the size of the node represents the degree, and nodes of the same color represent a network module. The connections between nodes represent significant relationships between genera (r > 0.6 or r < –0.6, p < 0.05), and the thickness of the lines connecting nodes represent the size of the correlation coefficient.