Concentration-dependent variation in carbon allocation and microbial assembly in chain elongation microbiota:...

This study evaluated the effects of acetate, ethanol, and lactate on anaerobic chain elongation (CE) using pit mud microbiota as a model inoculum. Substrate-normalized product carbon allocation analysis showed high acetate primarily yielded butyrate (31%). Ethanol concentrations of 15.8-31.6 g/L optimized caproate selectivity. High lactate (30 g/L) coincided with a shift in product distribution from caproate to odd-chain propionate (31%) and valerate (19%). Microbial succession and influencing factors were elucidated via linear mixed-effects models and Mantel tests. Acetate appeared to function as a basal substrate with relatively weak selective pressure. At 15.8-31.6 g/L ethanol concentrations, CE-related functional genera (e.g., Clostridium, Rummeliibacillus, and Caproicibacterium) were enriched, promoting efficient caproate synthesis. Conversely, higher ethanol levels (63.1 g/L) promoted Lactobacillus while suppressing CE functional genera via acidification and toxicity. High lactate concentrations (30 g/L) enriched Haloimpatiens, which was associated with odd-chain fatty acid pathways that produced propionate and valerate. Network topology analysis revealed that carbon sources differentially influenced microbial interaction networks. High acetate simplified network connectivity. Ethanol induced nonlinear effects: at 15.8 and 47.3 g/L, it fostered robust and highly connected networks that balanced competition and cooperation, while the 63.1 g/L concentration led to network sparsification and stability decay. In contrast, lactate concentration positively correlated with network complexity and stability. These findings provide a theoretical basis for managing carbon flux and community stability in anaerobic fermentation.