Temporal and spatial variations in anaerobic microbiomes from different inoculum sources for carbon monoxide conversion in thermophilic trickle bed reactor

Trickle Bed Reactors (TBRs) are effective systems that use microorganisms in biofilm as biocatalysts to valorize biodegradable gases such as syngas and CO. While microbial CO conversion has been studied in batch tests, the suitability of inoculum source on CO conversion in continuous TBR has not been tested. This study compared microbial conversion of CO from three inoculum sources in identical TBRs: anaerobic sludge (AS) from a wastewater treatment plant (WWTP), anaerobic digestate (AD) from a biogas plant, and cow manure (CM). TBR performance was monitored daily. The microbial composition of the inoculum and biofilm samples were analyzed using 16S/18S rRNA gene sequencing. AS showed the fastest adaptation, stabilizing within 10 days, while AD and CM took longer. Overall, AS and AD reactors achieved similar CO conversion and methane production rates, reaching 6 Nm³m⁻³d⁻¹ and 1 Nm³m⁻³d⁻¹, respectively, at a CO loading rate of 7.75 Nm³m⁻³d⁻¹. All the reactors showed reduced microbial diversity as compared to inocula. The reactor with AS showed relatively lower spatial microbial dissimilarity than CM. Despite initial diversity differences, Euryarchaeota and Firmicutes became the dominant phyla in all TBR biofilms, with Methanothermobacter as the main genus in the CO conversion process. These findings suggest that although essential microbes for CO utilization were present in all reactors, inoculum composition significantly impacted the time required to reach stable CO conversion and reactor performance.