A OTUs enriched in rock biofilm OTUs enriched in water OTUs enriched in microplastic biofilm OTUs enriched in leaf biofilm Not significantly differen log2(fold change) log2(fold change) log2(fold change) B 14 123 > 151 135 16 32 Enriched OTUs in biofilm Depleted OTUs in biofilm Bacteroidetes Bacteroidetes Cyanobacteria Firmicutes Chlorobi Acidobacteria charibacteria Parcubac Saccharibacteria Actinobacteria Proteobacteria Actinobacteria Gemmatimonadetes Gemmatimonadetes Enriched OTUs Depleted OTUs paredherthee neach type o 产之 tially enric eeorncfoaidntigtotokemodlamunrexiesr and OTUs！ has amodula rrelation of 0.8 OTUs(red for a m stru an.200 61 microbial taxa, network analysis of significant taxon co-occurrence patterns was used to decipher the structure of biofilm commu￾nities. By calculating the pairwise Spearman's correlation co￾efficients (r), the topology of the resulting network was calculated and OTUs with a correlation of 0.8 or greater to other OTUs were shown (Fig. 4). Nodes represent OTUs and edges represent the correlation between OTUs (red for positive correlation, grey for negative correlation). The size of the node is proportional to the number of edges linking to it. The modularity index is used to determine whether the network has a modular structure. A modularity index >0.4 suggests that the network has a modular structure (Barberan et al., 2012; Newman, 2006). The modularity index of 0.847 (rock biofilm), 0.708 (microplastic biofilm), and 0.719 (leaf biofilm) indicated that the formed networks had a modular structure (Newman, 2006). Evidently, the correlation pattern of microplastic biofilm was Fig. 3. Different types of biofilm enriched and depleted for certain OTUs from river water. The enriched/depleted OTUs were OTUs with significantly different abundance compared to OTUs in river water. Each point represents an individual OTU, and the position along the y axis represents the abundance fold change compared with river water. For each type of community (rock biofilm, microplastic biofilm, leaf biofilm and planktonic communities in river water), there were 5 replicates. (A) Enrichment and depletion of the OTUs included in each type of biofilm; (B) Numbers of differentially enriched and depleted OTUs between each type of biofilm compared with river water. (C) Phylum annotation of the differentially enriched or depleted OTUs. 6 X. Wu et al. / Water Research 165 (2019) 114979