Abstract
BackgroundBiotechnological processes for efficient resource recovery from residual materials rely on complex conversions carried out by reactor microbiomes. Chain elongation microbiomes produce valuable medium-chain carboxylates (MCC) that can be used as biobased starting materials in the chemical, agriculture and food industry. In this study, sunflower oil is used as an application-compatible solvent to accumulate microbially produced MCC during extractive lactate-based chain elongation. The MCC-enriched solvent is harvested as a potential novel product for direct application without further MCC purification, e.g., direct use for animal nutrition. Sunflower oil biocompatibility, in situ extraction performance and effects on chain elongation were evaluated in batch and continuous experiments. Microbial community composition and dynamics of continuous experiments were analyzed based on 16S rRNA gene sequencing data. Potential applications of MCC-enriched solvents along with future research directions are discussed.ResultsSunflower oil showed high MCC extraction specificity and similar biocompatibility to oleyl alcohol in batch extractive fermentation of lactate and food waste. Continuous chain elongation microbiomes produced the MCC n-caproate (nC6) and n-caprylate (nC8) from l-lactate and acetate at pH 5.0 standing high undissociated n-caproic acid concentrations (3 g L−1). Extractive chain elongation with sunflower oil relieved apparent toxicity of MCC and production rates and selectivities reached maximum values of 5.16 ± 0.41 g nC6 L−1 d−1 (MCC: 11.5 g COD L−1 d−1) and 84 ± 5% (e− eq MCC per e− eq products), respectively. MCC were selectively enriched in sunflower oil to concentrations up to 72 g nC6 L−1 and 3 g nC8 L−1, equivalent to 8.3 wt% in MCC-enriched sunflower oil. Fermentation at pH 7.0 produced propionate and n-butyrate instead of MCC. Sunflower oil showed stable linoleic and oleic acids composition during extractive chain elongation regardless of pH conditions. Reactor microbiomes showed reduced diversity at pH 5.0 with MCC production linked to Caproiciproducens co-occurring with Clostridiumtyrobutyricum, Clostridiumluticellarii and Lactobacillus species. Abundant taxa at pH 7.0 were Anaerotignum, Lachnospiraceae and Sporoanaerobacter.ConclusionsSunflower oil is a suitable biobased solvent to selectively concentrate MCC. Extractive reactor microbiomes produced MCC with improved selectivity and production rate, while downstream processing complexity was reduced. Potential applications of MCC-enriched solvents may include feed, food and biofuels purposes.
Highlights
Biotechnological processes for efficient resource recovery from residual materials rely on complex conversions carried out by reactor microbiomes
Sunflower oil and oleyl alcohol biocompatibility and carboxylates extraction efficiency in batch extractive chain elongation First, batch experiments were carried out to compare the biocompatibility and extraction performance of sunflower oil and oleyl alcohol in batch extractive fermentation using an inoculum from food waste fermentation [4]
After 20 days of incubation, experiments with sunflower oil and oleyl alcohol resulted in similar amounts of carboxylates produced compared to the blank experiments without solvent (Fig. 1A)
Summary
Biotechnological processes for efficient resource recovery from residual materials rely on complex conversions carried out by reactor microbiomes. Novel resource recovery bioprocesses using chain elongation microbiomes produce medium-chain carboxylates (MCC), saturated monocarboxylic acids with 6 to 12 carbon atoms that find applications in lubricants, biodegradable plastics, antimicrobials, feed additives and biofuels production [2]. Chain-elongating bacteria utilize energyrich substrates (e.g., ethanol, lactate, sugars) as electron donors to elongate short-chain carboxylates (SCC, 1–5 carbon units) to MCC such as n-caproate (nC6) or n-caprylate (nC8) through a series of biochemical reactions in the reverse β-oxidation pathway [2] These electron donors can be obtained from low-cost organic residual materials to achieve MCC production directly from the residues [4, 5] or with externally added electron donors [6, 7]. The terms extractive fermentation and extractive chain elongation are used interchangeably
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