Context:Granular sludge-based upflow anaerobic sludge blanket (UASB) reactors are an established technology for treatment of high strength wastewaters due to excellent biomass retention, allowing for compact reactor design. The globally growing population and closure of water cycles results in increasing salinity of wastewater. Elevated salinity is often reported inhibitory to UASB treatment process, resulting in granules disintegration and washout. Recently, it was shown that Methanothrix-rich granules are able to withstand elevated salinities (20 g/L Na+) probably due to the ability of Methanothrix to synthesize osmolytes and adapt the composition of extracellular polymeric substances (EPS).
Gap:Understand the mechanisms behind the salinity resistance of the Methanotrix-rich granules.
Aim:Investigate the full metabolic potential of the Methanothrix ssp. identified within the salinity resistant granules via reconstruction and analysis of metagenome-assembled genomes (MAGs).
Methods:Genome-centric metagenomics analysis was conducted on nine DNA samples taken along 217 days operation of saline (5 and 20 g/L Na+) UASB reactors, after Illumina sequencing. Recovered Methanothrix MAGs were probed for salinity stress-related functions using pathway analysis and homology to pathways from halotolerant and halophilic archaea using HMM models. Confocal laser scanning microscopy (CLSM) was applied to visualize and validate EPS production functions.
Findings:Among the three dominant subspecies of Methanothrix identified in our analysis, Methanothrix harundinacea sp., and Methanothrix_A MAGs encode for several osmoadaptation pathways. In particular, we identified the potential to produce and excrete different EPS glycoconjugates in response to salinity via the N-glycosylation process, and to synthetize compatible solutes such as Nε-acetyl-β-lysine and ectoine. Moreover, the two Methanothrix MAGs also have the potential to reinforce membrane stability by the production of isoprenoids and carotenoid precursors.
Utilization:This is the first study reporting salinity stress-related genomic features in a Methanothrix ssp., a milestone further supporting its key role in driving anaerobic granulation.
