Due to concerns about the unsustainability and predictable shortage of fossil feedstocks, research efforts are currently being made to develop new processes for production of commodities using alternative feedstocks. 3-Hydroxypropionic acid (CAS 503-66-2) was recognised by the US Department of Energy as one of the most promising value-added chemicals that can be obtained from biomass. This article aims at reviewing the various strategies implemented thus far for 3-hydroxypropionic acid bioproduction. Special attention is given here to process engineering issues. The variety of possible metabolic pathways is also described in order to highlight how process design can be guided by their understanding. The most recent advances are described here in order to draw up a panorama of microbial 3-hydroxypropionic acid production: best performances to date, remaining hurdles and foreseeable developments. Important milestones have been achieved, and process metrics are getting closer to commercial relevance. New strategies are continuously being developed that involve new microbial strains, new technologies, or new carbon sources in order to overcome the various hurdles inherent to the different microbial routes.

Interspecies electron transfer is a common way to couple metabolic energy balances between different species in mixed culture consortia. Direct interspecies electron transfer (DIET) mechanism has been recently characterised with Geobacter species which couple the electron balance with other species through physical contacts. Using this mechanism could be an efficient and cost-effective way to directly control redox balances in co-culture fermentation. The present study deals with a co-culture of Geobacter sulfurreducens and Clostridium pasteurianum during glycerol fermentation. As a result, it was shown that Geobacter sulfurreducens was able to grow using Clostridium pasteurianum as sole electron acceptor. C. pasteurianum metabolic pattern was significantly altered towards improved 1,3-propanediol and butyrate production (+37% and +38% resp.) at the expense of butanol and ethanol production (−16% and −20% resp.). This metabolic shift was clearly induced by a small electron uptake that represented less than 0.6% of the electrons consumed by C. pasteurianum. A non-linear relationship was found between G. sulfurreducens growth (i.e the electrons transferred between the two species) and the changes in C. pasteurianum metabolite distribution. This study opens up new possibilities for controlling and increasing specificity in mixed culture fermentation.

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