Fig. 4

Networks of co-occurring prokaryotic OTUs in all sludge samples based on correlation analysis. Nodes were colored by a modularity class with labeled genera names, and b occurrence in networks of Cluster I (C1) and Cluster II (C2). A connection stands for a strong (Spearman’s ρ > 0.6) and significant (p < 0.01) correlation. For each panel, the size of each node is proportional to the number of connections (degree); the thickness of each connection between two nodes (edge) is proportional to the value of Spearman’s correlation coefficients, ranging from 0.60 to 0.93. Other: OTUs did not occur in networks of Cluster I or II