Chain elongation under low-temperature conditions: Performance and bacterial-fungal interactions.

Chain elongation (CE) technology has attracted widespread attention for its ability to achieve high-value recovery of carbon resources from organic waste. However, there is still a lack of knowledge regarding the stable operation and microbial synergies of CE for its potential application at low temperature environment. This study demonstrates that a continuous CE reactor can operate stably over 180 days at 20°C using lactate as substrate. by adjusting the hydraulic retention time (10 d-5 d-10 d). The results indicate that the system can achieve stable and efficient caproate production at 20°C (maximum concentration of 149.16 mmol-C L-1 and maximum selectivity of 56.5%). Kinetic modeling suggested half-saturation constant for the butyrate synthesis is tens of times higher than lactate oxidation or caproate synthesis. Microbial community analysis revealed that Caproiciproducens consistently remained the dominant genus, with a relative abundance peaking at 72.9%. Additionally, the enrichment of Cutaneotrichosporon moniliiforme showed a positive correlation with caproate production, suggesting that fungi may have a potential complementary role in supporting low-temperature chain elongation. Metagenomic analysis further indicated that key functional genes of the reverse β-oxidation pathway maintained high abundance under low-temperature conditions. Furthermore, a large number of cobW-related genes were detected, suggesting that vitamin B12 metabolism and cobalt ion utilization may support reverse β-oxidation-associated functions. Overall, bacterial-fungal synergy and cofactor-related functional potentials provide possible explanations for efficient caproate production at low temperature, although it requires further experimental validation.