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Highlights: What are the main findings? For the first time, this study demonstrates a shift in DIET-related genes (pilA and hgtR) alongside the established mcrA/16SrRNA ratio as a key indicator of OLR stress. Methane production halted after OLR shock, correlating with significant reduction in the mcrA/16SrRNA ratio, and pilA (1.43-log) and hgtR (1.34-log) gene abundances. mcrA and pilA are robust indicators of reactor performance under both stable and disturbed states. Partial recovery of pilA and hgtR gene abundance to the near-steady-state level was observed in the control reactor. Propionate, a critical intermediate in anaerobic digestion, and its syntrophic removal, is sensitive to stress. To our knowledge, this study investigates for the first time the response of a metabolic gene panel to organic loading rate (OLR) stress in propionate-degrading methanogenic consortia in lab-scale upflow anaerobic sludge blanket (UASB) reactors. The experimental phases included stabilisation (1.4–2.8 g COD/L/day), electroactive enrichment, OLR shock (6 g COD/L/day), and early recovery. Quantitative PCR was used to assess the abundance of key functional genes (16SrRNA, mcrA, pilA, and hgtR). During stabilisation, ~200 mLCH₄/h was produced, the mcrA/16SrRNA ratio was 0.78–2.64, and pilA and hgtR abundances were 1.29–2.27 × 105 and 2.12–4.37 × 104 copies/gVS. Following the OLR shock, methane production ceased entirely, accompanied by a sharp decline in the mcrA/16S ratio (0.08–0.24) and significant reductions in pilA (1.43-log) and hgtR (1.34-log) abundance. Partial recovery of pilA and hgtR abundance (1.19 × 105 and 8.57 × 104) was observed in the control reactor after the early recovery phase. The results highlight the utility of mcrA, 16SrRNA, pilA, and associated ratios, as reliable indicators of OLR stress in lab-scale UASB reactors. This study advances the understanding of molecular stress responses in propionate-degrading methanogenic consortia, focusing on direct interspecies electron transfer in process stability and recovery. [ABSTRACT FROM AUTHOR]

The accumulation of volatile fatty acids (VFAs) through the anaerobic fermentation of organic waste, such as strawberry extrudate, is proposed for this work. A hydrothermal treatment was carried out, and it was proposed to break the complex matrix of the strawberry extrudate to favour the hydrolysis stage of the anaerobic digestion process. The production of volatile fatty acids from treated and untreated strawberry was evaluated by adjusting the pH to 5 and 9. After the hydrothermal treatment of the strawberry extrudate, an increase in the solubilisation of organic matter, such as sugars and phenols, was observed. In the production of VFAs by means of anaerobic digestion of the pretreated strawberry extrudate, a significant increase in the accumulation of volatile fatty acids was demonstrated at a pH of 9 with respect to the untreated strawberry extrudate. In addition, the operational pH also had a strong effect on the individual VFA profile. A stream enriched in acetic acid was obtained at a pH of 9 (around 65% of the VFAs), whereas the operation at a pH of 5 resulted in a more complex composition with a high percentage of propionic acid (29% of the VFAs). [ABSTRACT FROM AUTHOR]