title: Conversion of L-lactate into n-caproate by a continuously fed reactor microbiome.
abstract: Conversion of lactate to n-caproate had been described for the type strain Megasphaera elsdenii in batch systems. Recently, investigators have also described production of n-caproate from endogenous or exogenous lactate with batch-fed reactor microbiome systems. However, no reports exist of lactate to n-caproate conversion within a continuously fed bioreactor. Since continuously fed systems are advantageous for biotechnology production platforms, our objective was to develop such a system. Here, we demonstrated continuous lactate to n-caproate conversion for more than 165 days. The volumetric n-caproate production rate (productivity) was improved when we decreased the operating pH from 5.5 to 5.0, and was again improved when we utilized in-line product recovery via pertraction (membrane-based liquid-liquid extraction). We observed a maximum n-caproate productivity of 6.9 g COD/L-d for a period of 17 days at an L-lactate loading rate of 9.1 g COD/L-d, representing the highest sustained lactate to n-caproate conversion rate ever reported. We had to manage two competing lactate conversion pathways: 1) the reverse β-oxidation pathway to n-caproate; and 2) the acrylate pathway to propionate. We found that maintaining a low residual lactate concentration in the bioreactor broth was necessary to direct lactate conversion towards n-caproate instead of propionate. These findings provide a foundation for the development of new resource recovery processes to produce higher-value liquid products (e.g., n-caproate) from carbon-rich wastewaters containing lactate or lactate precursors (e.g., carbohydrates).
authors.affiliation: Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: lak244@cornell.edu.
authors.forename: Leo A
authors.initials: LA
authors.lastname: Kucek
authors.affiliation: Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: mtn29@cornell.edu.
authors.forename: Mytien
authors.initials: M
authors.lastname: Nguyen
authors.affiliation: Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: la249@cornell.edu.
authors.forename: Largus T
authors.initials: LT
authors.lastname: Angenent
mesh_terms: D018533:Biomass; D019149:Bioreactors / Q000382:microbiology*; D002208:Caproates / Q000378:metabolism*; D003013:Clostridium / Q000254:growth & development / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000378:metabolism*; D000070101:Megasphaera elsdenii / Q000254:growth & development / Q000378:metabolism; D053858:Metabolic Networks and Pathways; D064307:Microbiota; D010084:Oxidation-Reduction; D045857:Ruminococcus / Q000254:growth & development / Q000378:metabolism; D013997:Time Factors
chemical_list: D002208:Caproates; D019344:Lactic Acid
keywords: Acrylate pathway; Carboxylate platform; Lactate; Reactor microbiome; Reverse β-oxidation pathway; n-Caproate