New insights into the r/K selection theory achieved in methanogenic systems through continuous-flow and sequencing batch operational modes

Anaerobic digestion is achieved through cooperation among various types of microorganisms, and the regulation of microbial communities is key to achieving stable system operation. In this study, the r/K selection theory was adopted to examine the system performance and microbial characteristics in anaerobic reactors with different operating modes (continuous-flow reactors, CFRs; sequencing batch reactors, SBRs) and sludge retention times (25 and 10 days). Four lab-scale reactors (CFR_25d, CFR_10d, SBR_25d, and SBR_10d) were operated. In the cycle reaction, CFR_25d achieved the highest methane yield (678.0 mL/L) and methane production rate (140.8 mL/(L·h)); while those in CFR_10d were the lowest, which could have been due to an accumulation of volatile fatty acids. CFR could wash out r-strategists efficiently, such as Methanosarcina. CFR_25d and CFR_10d significantly enriched the K-strategist Geobacter, with the relative abundances of 34.0% and 72.6%, respectively. In addition, the hydrogenotrophic methanogens of Methanolinea and Methanospirillum (K-strategists) dominated in CFR_25d and CFR_10d. Methanobacterium adapted to the diverse operational conditions, but the slow grower Methanosaeta only accounted for 0.9% in CFR_10d. Failure to enrich propionate oxidizers resulted in a functional absence of propionate degradation in the CFRs.