Competition Between Butyrate Fermenters and Chain-Elongating Bacteria Limits the Efficiency of Medium-Chain Carboxylate Production

Abstract

Medium-chain carboxylates such as n-caproate and n-caprylate are valuable chemicals, which can be produced from renewable feedstock by anaerobic fermentation and lactate-based microbial chain elongation. Acidogenic microbiota involved in lactatebased chain elongation and their interplay with lactic acid bacteria have not been characterized in detail yet. Here, the metabolic and community dynamics were studied in a continuous bioreactor with xylan and lactate as sole carbon sources. Four succession stages were observed during 148 days of operation. After an adaptation period of 36 days, a relatively stable period of 28 days (stage I) was reached with n-butyrate, ncaproate and n-caprylate productivities of 7.2, 8.2 and 1.8 gCOD L −1 d −1 , respectively. After a transition period, the process changed to another period (stage II), during which 46% more n-butyrate, 51% less n-caproate and 67% less n-caprylate were produced. Co-occurrence networks of species based on 16S rRNA amplicon sequences and correlations with process parameters were analyzed to infer ecological interactions and potential metabolic functions. Diverse functions including hydrolysis of xylan, primary fermentation of xylose to acids (e.g., to acetate by Syntrophococcus, to n-butyrate by Lachnospiraceae, and to lactate by Lactobacillus) and chain-elongation with lactate (by Ruminiclostridium 5 and Pseudoramibacter) were inferred from the metabolic network. In stage I, the sub-network characterized by strongest positive correlations was mainly related to the production of n-caproate and n-caprylate. Lactic acid bacteria of the genus Olsenella co-occurred with potentially chain-elongating bacteria of the genus Pseudoramibacter, and their abundance was positively correlated with n-caproate and n-caprylate concentrations. A new sub-network appeared in stage II, which was mainly related to n-butyrate production and revealed a network of different lactic acid bacteria (Bifidobacterium) and potential n-butyrate producers (Clostridium sensu stricto 12). The synergy effects between lactate-producing and lactate-consuming bacteria constitute a division of labor cooperation of mutual benefit. Besides cooperation, competition between different taxa determined the bacterial community assembly over

Machine Learning-assisted Identification of Bioindicators Predicts Medium-chain Carboxylate Production Performance of an Anaerobic Mixed Culture

“…xylooligosaccharides, from corncob; Roth, Karlsruhe, Germany) and lactic acid (85%, FCC grade; Sigma Aldrich, St. Louis, USA) as defined carbon sources and produced C4, C6 and C8 over 150 days [10]. For the present study, two 1-L bioreactors (A and B; BIOSTAT® A plus, Sartorius AG, Göttingen, Germany) were filled up with 0.5 L of the enriched culture.…”

“…Samples for determining cell mass concentrations were collected from the reactor effluent. Concentrations of xylan, carboxylates and alcohols were measured in the effluent supernatants [10]. In total, samples were collected on time points for each bioreactor.…”

“…CE is a microbial process that produces medium-chain carboxylates (6 to 8 carbon atoms) through reverse ꞵ-oxidation [9]. Recently we enriched a mixed culture that produces n-butyrate (C4), ncaproate (C6) and n-caprylate (C8) from xylan and lactate in a daily-fed reactor system [10], to simulate the feedstock conditions of anaerobic fermentation of ensiled plant biomass [11]. For this bioprocess to be viable, it needs to include diverse functions such as xylan hydrolysis, xylose fermentation and CE with lactate as electron donor.…”

“…Species involved in these interactions can drive shifts in community structure and function [1]. During the long-term stable reactor operation, the community developed towards predominating C4 and biomass production at the cost of C6/C8 production [10]. We wanted to explore how process parameter changes shape the existing microbiota to optimise the process towards the target products C6 and C8.…”

Lactate-based microbial chain elongation for n-caproate and iso-butyrate production: genomic and metabolic features of three novel Clostridia isolates

“…Anaerobic fermentation broth from a caproate-producing reactor (38 °C, pH 5.5 and hydraulic retention time of 4 d) fed with corn silage was initially taken as the inoculum. Serum bottles (120 mL) with 45 mL mineral medium [18] containing 5 g/L lactic acid (initial pH 5.5) were inoculated with 5 mL of the sieved reactor broth (mesh size 2 mm). After replacing the headspace by N 2 /CO 2 (80:20 in a volume ratio, 100 kPa), the bottles were statically incubated at 37 °C in the dark.…”