Chain elongators, friends, and foes

[Display omitted] •A meta-analysis of chain elongation microbiomes shows community divergence.•Key microbiome members are often not classified or characterized.•Isolated chain elongators do not represent suspected catalysts in many microbiomes.•Interactions between chain elongators and other community members are analysed.•Research gaps, research needs, and the potential path forward are discussed. Bioproduction of medium chain carboxylic acids has recently emerged as an alternative strategy to valorize low-value organic waste and side-streams. Key to this route is chain elongation, an anaerobic microbial process driven by ethanol, lactic acid, or carbohydrates. Because these technologies use wastes as feedstocks, mixed microbial communities are often considered as biocatalysts. Understanding and steering these microbiomes is key to optimize bioprocess performance. From a meta-analysis of publicly available sequencing data, we (i) explore how the current collection of isolated chain elongators compares to microbiome members, (ii) discuss the main beneficial and antagonistic interactions with community partners, and (iii) identify the key research gaps and needs to help understand chain elongation microbiomes, and design/steer these novel bioproduction processes.