Metagenomic Reconstruction of Key Anaerobic Digestion Pathways in Municipal Sludge and Industrial Wastewater Biogas-Producing Systems

Abstract

Anaerobic digestion (AD) is a microbial process widely used to treat organic wastes. While the microbes involved in digestion of municipal sludge are increasingly well characterized, the taxonomic and functional compositions of AD digesters treating industrial wastewater have been understudied. This study examined metagenomes from a biogas-producing digester treating municipal sludge in Shek Wu Hui (SWH), Hong Kong and an industrial wastewater digester in Guangzhou (GZ), China, and compared their taxonomic composition and reconstructed biochemical pathways. Genes encoding carbohydrate metabolism and protein metabolism functions were overrepresented in GZ, while genes encoding functions related to fatty acids, lipids and isoprenoids were overrepresented in SWH, reflecting the plants' feedstocks. Mapping of genera to functions in each community indicated that both digesters had a high level of functional redundancy, and a more even distribution of genera in GZ suggested that it was more functionally stable. While fermentation in both samples was dominated by Clostridia, SWH had an overrepresentation of Proteobacteria, including syntrophic acetogens, reflecting its more complex substrate. Considering the growing importance of biogas as an alternative fuel source, a detailed mechanistic understanding of AD is important and this report will be a basis for further study of industrial wastewater AD.

Keywords: anaerobic digestion; biogas; industrial wastewater; metagenomes; municipal sludge.

FIGURE 1

Significant differences in microbial communities at the (A) phylum and (B) class level between GZ (blue) and SWH (yellow). Significance was determined by Fisher’s exact test with Story’s FDR correction for multiple comparisons, with corrected p < 0.05 considered significant. Only taxa with difference between proportions >0.2 (i.e., considered large effect) are shown.

FIGURE 2

SEED subsystems (level 1) with significantly different abundances in the GZ (blue) and SWH (yellow) metagenomes. Significance was determined by Fisher’s exact test with Storey’s FDR correction for multiple comparisons, with corrected p < 0.05 considered significant.

FIGURE 3

Genera in the GZ (genus name in blue) and SWH (genus name in red) metagenomes mapped to each major AD step (acidogenesis, acetogenesis, and methanogenesis). The five most abundant genera mapped to the last step in the formation of the major intermediates are shown (labeled with a letter). The log₁₀-normalized relative abundance of each genus is indicated by the colored tile beside its name.

FIGURE 4

Clustering of the GZ and SWH metagenomes with other AD metagenomes at (A) the genus level, representing taxonomic similarity and (B) level 3 SEED subsystems, representing functional similarity. Metagenomes were clustered based on the Bray–Curtis similarity with square-root transformed abundances using PRIMER software. Dashed lines are inserted for visualizing the similarity comparisons.