How Does Hot Pretreatment Alter Microbes in Manure Digestion?

At an industrial plant that turns dairy manure into biogas, researchers tested a new trick: heating the material to 70‑72 °C with the bacterium Caldicellulosiruptor, a true extreme‑heat lover. This step, called Extremophilic Biological Process (EBP), makes the substrate easier to digest, but what happens to the microbes that later produce methane?

The plant is split into three reactor types. First are the EBP tanks, where the heating occurs. Then come the mesophilic CSTRs, which treat manure without pretreatment, and finally the IBRs, which receive the combined flow from the CSTR and EBP tanks. All have a two‑day hydraulic retention time and were monitored for 60 days.

Results show that heating raises acetate levels, a key molecule for methane production. However, acetate‑using methanogens like Methanosarcina drop in the IBRs. In their place, syntrophic methanogens from the Methanobacteriaceae family increase, tightly linked with hydrogen‑producing bacteria.

Network analysis confirms these collaborations are far more organized in pretreated digestate, while they’re absent in the CSTRs. In short, the hot pretreatment pushes the system toward syntrophic methanogenesis, where multiple microbial partners cooperate. To fully benefit, plants must keep acetoclastic methanogens active, preventing acetate waste and maximizing methane yield.

EBP exploits the enzymatic arsenal of Caldicellulosiruptor, a thermophilic genus that produces cellulases, xylanases, and other glycosylases capable of breaking down complex polysaccharides in manure. Heating at 70‑72 °C not only disrupts plant fibers but also solubilizes volatile fatty acids (VFA), especially acetate, the main precursor for acetoclastic methanogens. However, the acetate boost does not automatically translate into Methanosarcina growth, as these organisms require stable pH (6.8‑7.2) and limited H₂ concentrations to stay active. When H₂ levels rise, syntrophic methanogens from the Methanobacteriaceae family (e.g., Methanobacterium, Methanobrevibacter) become more competitive, consuming H₂ with CO₂ to form CH₄, lowering H₂ partial pressure and facilitating the degradation of more complex fatty acids by fermentative bacteria. Co‑occurrence analyses using Spearman correlations and SparCC network models identified central microbial hubs in pretreated digestate, with Methanobacteriaceae nodes linked to genera such as Syntrophomonas and Desulfovibrio. These syntrophic interactions improve the thermodynamic efficiency of the process, as H₂ removal drives fermentation toward higher acetate production. To maximize methane yield, selective retention strategies—like membrane filters or sedimentation zones—should be employed to maintain a stable acetoclastic methanogen population, preventing acetate loss as unconverted substrate.