Looking for lipases and lipolytic organisms in low-temperature anaerobic reactors treating domestic wastewater

Poor lipid degradation limits low-temperature anaerobic treatment of domestic wastewater even when psychrophiles are used. We combined metagenomics and metaproteomics to find lipolytic bacteria and their potential, and actual, cold-adapted extracellular lipases in anaerobic membrane bioreactors treating domestic wastewater at 4°C and 15°C. Of the 40 recovered putative lipolytic metagenome-assembled genomes (MAGs), only three ( Chlorobium, Desulfobacter , and Mycolicibacterium ) were common and abundant (relative abundance ≥ 1%) in all reactors. Notably, some MAGs that represented aerobic autotrophs contained lipases. Therefore, we hypothesised that the lipases we found are not always associated with exogenous lipid degradation and can have other roles such as polyhydroxyalkanoates (PHA) accumulation/degradation and interference with the outer membranes of other bacteria. Metaproteomics did not provide sufficient proteome coverage for relatively lower abundant proteins such as lipases though the expression of fadL genes, long-chain fatty acid transporters, was confirmed for four genera ( Dechloromonas, Azoarcus, Aeromonas and Sulfurimonas ), none of which were recovered as putative lipolytic MAGs. Metaproteomics also confirmed the presence of 15 relatively abundant (≥1%) genera in all reactors, of which at least 6 can potentially accumulate lipid/polyhydroxyalkanoates. For most putative lipolytic MAGs, there was no statistically significant correlation between the read abundance and reactor conditions such as temperature, phase (biofilm and bulk liquid), and feed type (treated by ultraviolet light or not). Results obtained by metagenomics and metaproteomics did not confirm each other and further work is required to identify the true lipid degraders in these systems. ### Competing Interest Statement The authors have declared no competing interest. * AnMBRs : Anaerobic membrane bioreactors Bp : Base pair CCR : Carbon catabolite repression COD : Chemical oxygen demand EPS : Extracellular polymeric substances fadL : Long-chain fatty acid transporter FDR : False discovery rates MAGs : Metagenome-assembled genomes PHAs : Polyhydroxyalkanoates VSS : Volatile suspended solid