Steering ecological interactions in microbial chain elongation: functions and impacts of carbon- and iron-based...

The increasing accumulation of organic waste and the demand for renewable chemicals highlight the production of medium-chain fatty acids (MCFAs) as an effective strategy for waste valorization. Microbial chain elongation (CE) converts short-chain fatty acids (SCFAs) into MCFAs. However, its efficiency in mixed cultures is frequently limited by unstable carbon flow, restricted electron transfer, and shifts in functional microbial communities. While exogenous materials are increasingly applied to regulate CE, their mechanisms of action have not been as systematically reviewed as traditional process optimization strategies. This review examines how material-mediated CE enhances MCFA production by facilitating metabolic pathways, directing microbial enrichment, and promoting electron transfer. Specifically, the discussion focuses on carbon-based materials, iron-based materials, and engineered composites, detailing their role in electron shuttling, interfacial redox buffering, microbial attachment, and functional microbial community assembly. Furthermore, we summarize material design principles, their enhancement effects, and the key factors governing operational stability in practical mixed-culture systems. Rather than focusing solely on the enrichment of single functional microbes, this review emphasizes the application of exogenous materials to direct microbial community assembly and enhance interfacial microbe-material interactions. These mechanisms provide a basis for understanding and regulating caproate selectivity during organic waste conversion. Finally, by incorporating economic and environmental considerations, we propose strategies for scaling up material-assisted CE from the laboratory to practical applications. Ultimately, this review builds a framework connecting material properties, microbial ecology, and process efficiency, facilitating the selective conversion of organic waste into MCFAs.