There and back again: genomic insights into microbial life in a recirculating petroleum refinery wastewater biotreatment system

Petroleum refinery wastewater biotreatment relies on microbes to remediate carbon, nitrogen, and sulfur compounds, yet their life strategies and ecological roles remain unclear. This study characterises the ecological functions of 20 metagenome-assembled genomes (MAGs) from a full-scale petroleum refinery wastewater treatment plant in southern China. The taxonomic identity, nutrient metabolism genes (including C/N/S cycling), carbohydrate-active enzymes, and CRISPR-Cas systems of these MAGs were analysed. The recovered MAGs represented bacteria primarily from the Pseudomonadota and Bacteroidota phyla. The major carbon sources for the represented organisms are likely aromatic and aliphatic compounds, as well as carbohydrates including peptidoglycan, chitin, and starch. Almost all MAGs contained genes for nitrate or nitrite reduction, while metabolic pathways for sulfur metabolism were generally less prevalent. Meiothermus sp. bin.89 was the most metabolically versatile MAG. This organism possessed genes that allowed it to recycle biomass, break down aliphatic and monoaromatic compounds, and perform anaerobic respiration using nitrate. However, it was likely the most susceptible to viral predation, as indicated by the high abundance of CRISPR spacers. Overall, the results revealed that stress-tolerant ecological traits were common among organisms in this microbiome, showcasing the ability of the microbes to obtain carbon from aromatic and aliphatic compounds. This study provides a substantial contribution towards future efforts in optimising microbiome stability for pollutant removal in petroleum refinery wastewater biotreatment systems.