Microbial community response to temperature reduction during anaerobic treatment of long chain fatty acids-containing wastewater

[Display omitted] •Mesophilic biomass acclimated to 15°C with efficient COD removal (>80%).•Robust microbiome underpinned the system stability during acclimation.•Fermentative bacteria played significant role in transient stages of acclimation.•Smithella was the predominant genus responsible for LCFA degradation at 15°C.•Membrane modifications and osmotic balance supported cold acclimation. Acclimating mesophilic biomass to low temperatures have been used to start-up psychrophilic anaerobic reactors, but limited microbial information is available during the acclimation. To investigate microbial responses to temperature reductions, duplicate lab-scale anaerobic digestion (AD) reactors were operated for 166 days, with the temperature being reduced from 37°C to 15°C, using synthetic long chain fatty acid (LCFA)-containing wastewater as the feedstock. The acclimated biomass at 15°C exhibited efficient removal of organic matter (total COD>75%, soluble COD>88%, and LCFA>99%). Temperature reductions lead to significant reductions in microbiome diversity. Fermentative bacteria were highly dynamic and functional redundant during temperature reductions. Smithella was the dominant syntrophic bacteria involved in LCFA degradation coupled with Methanothrix and Methanocorpusculum at 15°C. Membrane modifications and compatible cellular solutes production were triggered by temperature reductions as microbial response to cold stress. This study provided molecular insights in microbial acclimation to low temperatures for psychrophilic AD.