Context: Wastewater treatment plants (WWTPs) process large quantities of waste via complex microbial communities that degrade, transform and/or take-up diverse molecules. Secreted proteins (exoenzymes) are key for such functions and usually catalyze the rate-limiting step because many are first to interact with and/or initiate degradation of various external macromolecules.
Gap: Little is known about what types of secreted proteins/enzymes are produced by different microorganisms in WWTPs.
Aims: To gain insights into complements of predicted secreted proteins encoded by microorganisms from Danish WWTPs, with focus placed on secreted hydrolases that target different macromolecules.
Methods: Encoded proteins from 1083 high-quality metagenome assembled genomes (MAGs) were subjected to sub-cellular location predictions. Proteins from predicted extracellular, outer-membrane, cell-wall and periplasmic locations were then specifically analysed bioinformatically.
Findings: Numbers and types of predicted secreted proteins differed markedly among phylogenetic groups, with the phyla Bacteroidota and Myxococcota having especially high numbers of extracellular proteins, suggesting high capacities to transform external molecules and/or interact with extracellular environment. Abundant Gammaproteobacteria encode few predicted secreted hydrolases or proteins, but numerous predicted periplasmic proteins, indicating they generally use simple molecules and/or few macromolecules. Bacteroidota, Gemmatimonadota and Acidobacteriota are well equipped to digest diverse polysaccharides. Secreted nucleases are encoded by diverse taxa (40% of MAGs), possibly related to high amounts of extracellular DNA in flocs. Predicted secreted lipases are the most restricted macromolecule-targeting hydrolases, encoded mainly by Gammaproteobacteria, Actinobacteriota and Myxococcota. Secreted peptidases are encoded by a highly diverse array of taxa, highlighting proteins as sought-after nutrients. Additional results reveal potential for bacterial interspecies competition, and insights into redox flexibility among different taxa.
Utilization: This study provides new functional perspectives on microbial communities in WWTPs, revealing distinct arrays of secreted proteins are encoded by different taxa, with evidence for specialised use of specific macromolecules by certain groups.
