Integrated bioprocess for conversion of gaseous substrates to liquids

Abstract

In the quest for inexpensive feedstocks for the cost-effective production of liquid fuels, we have examined gaseous substrates that could be made available at low cost and sufficiently large scale for industrial fuel production. Here we introduce a new bioconversion scheme that effectively converts syngas, generated from gasification of coal, natural gas, or biomass, into lipids that can be used for biodiesel production. We present an integrated conversion method comprising a two-stage system. In the first stage, an anaerobic bioreactor converts mixtures of gases of CO 2 and CO or H 2 to acetic acid, using the anaerobic acetogen Moorella thermoacetica. The acetic acid product is fed as a substrate to a second bioreactor, where it is converted aerobically into lipids by an engineered oleaginous yeast, Yarrowia lipolytica. We first describe the process carried out in each reactor and then present an integrated system that produces microbial oil, using synthesis gas as input. The integrated continuous bench-scale reactor system produced 18 g/L of C16-C18 triacylglycerides directly from synthesis gas, with an overall productivity of 0.19 g·L −1 ·h −1 and a lipid content of 36%. Although suboptimal relative to the performance of the individual reactor components, the presented integrated system demonstrates the feasibility of substantial net fixation of carbon dioxide and conversion of gaseous feedstocks to lipids for biodiesel production. The system can be further optimized to approach the performance of its individual units so that it can be used for the economical conversion of waste gases from steel mills to valuable liquid fuels for transportation.two-stage bioprocess | lipid production | microbial fermentation | gas-to-liquid fuel | CO 2 fixation C oncerns over diminishing oil reserves and climate-changing greenhouse gas emissions have led to calls for clean and renewable liquid fuels (1). One promising direction has been the production of microbial oil from carbohydrate feedstocks. This oil can be readily converted to biodiesel and recently there has been significant progress in the engineering of oleaginous microbes for the production of lipids from sugars (2-5). A major problem with this approach has been the relatively high sugar feedstock cost. Alternatively, less costly industrial gases containing CO 2 with reducing agents, such as CO or H 2 , have been investigated. In one application, anaerobic Clostridia have been used to convert synthesis gas to ethanol (6), albeit at low concentration requiring high separation cost. Here we present an alternative gas-to-lipids approach that overcomes the drawbacks of previous schemes.We have shown previously that acetate in excess of 30 g/L can be produced from mixtures of CO 2 and CO/H 2 , using an evolved strain of the acetogen Moorella thermoacetica, with a substantial productivity of 0.55 g·L −1 ·h −1 and yield of 92% (7). We also have demonstrated that the engineering of the oleaginous yeast Yarrowia lipolytica can yield biocatalysts that can produce lipids from...

Application of metabolic controls for the maximization of lipid production in semicontinuous fermentation

“…S4). These results outcompeted the previously published data (46 g/L lipids in 170 h fermentation), which were the best reported to date (11). Furthermore, our fermentation used only dilute acetate as feed and, more importantly, the carbon-restrained operation eliminated by-product generation (Fig.…”

“…S2). This result is a significant improvement over previous carbon-rich cultures where by-production of citrate was observed in the late stage, accounting for more than 10% of the overall lipid titer (11).…”

“…generation of by-products due to excessive carbon flux through undesired pathways (11). Consequently, with these considerations in mind, we believe that the engineering of a bioprocess that can produce lipids at high yield, titer, and productivity from dilute acetate streams only is paramount in overcoming this bottleneck in the bio-GTL conversion scheme.…”

“…In particular, oleaginous microorganisms can produce medium-and longchain triacylglycerols for biodiesel preparation. Currently the best reported results for this step were produced using an engineered Yarrowia lipolytica strain with the cofeeding of concentrated and dilute acetate under nitrogen starvation conditions, resulting in a yield, titer, and productivity of 0.16 g/g, 46 g/L, and 0.27 g·L −1 ·h −1 , respectively (10,11). Overall, the bio-GTL platform ( Fig.…”

Syngas Fermentation: A Microbial Conversion Process of Gaseous Substrates to Various Products

“…Butanol is sought as a "drop-in" biofuel for use in existing petroleum infrastructure, as a solvent and as precursor for subsequent synthesis. Propionic acid, propanol, hexanoic acid, hexanol, acetone, isobutanol, butanediol, amino and fatty acids are other potential products proposed from syngas fermentation [34,35,[105][106][107]. A biological water-gas shift is proposed to produce H 2 [108], and syngas can be biologically converted to methane [109] so that syngas energy and subsequent products might be obtained from biological conversion of natural gas.…”