Abstract
Microbial fuel cells (MFCs) are eco-friendly bio-electrochemical reactors that use exoelectrogens as biocatalyst for electricity harvest from organic biomass, which could also be used as biosensors for long-term environmental monitoring. Glucose and xylose, as the primary ingredients from cellulose hydrolyzates, is an appealing substrate for MFC. Nevertheless, neither xylose nor glucose can be utilized as carbon source by well-studied exoelectrogens such as Shewanella oneidensis. In this study, to harvest the electricity by rapidly harnessing xylose and glucose from corn stalk hydrolysate, we herein firstly designed glucose and xylose co-fed engineered Klebsiella pneumoniae-S. oneidensis microbial consortium, in which K. pneumoniae as the fermenter converted glucose and xylose into lactate to feed the exoelectrogens (S. oneidensis). To produce more lactate in K. pneumoniae, we eliminated the ethanol and acetate pathway via deleting pta (phosphotransacetylase gene) and adhE (alcohol dehydrogenase gene) and further constructed a synthesis and delivery system through expressing ldhD (lactate dehydrogenase gene) and lldP (lactate transporter gene). To facilitate extracellular electron transfer (EET) of S. oneidensis, a biosynthetic flavins pathway from Bacillus subtilis was expressed in a highly hydrophobic S. oneidensis CP-S1, which not only improved direct-contacted EET via enhancing S. oneidensis adhesion to the carbon electrode but also accelerated the flavins-mediated EET via increasing flavins synthesis. Furthermore, we optimized the ratio of glucose and xylose concentration to provide a stable carbon source supply in MFCs for higher power density. The glucose and xylose co-fed MFC inoculated with the recombinant consortium generated a maximum power density of 104.7 ± 10.0 mW/m2, which was 7.2-folds higher than that of the wild-type consortium (12.7 ± 8.0 mW/m2). Lastly, we used this synthetic microbial consortium in the corn straw hydrolyzates-fed MFC, obtaining a power density 23.5 ± 6.0 mW/m2.
Highlights
Shewanella oneidensis, one of the most well-studied exoelectrogens (Hau and Gralnick, 2007; Fredrickson et al, 2008; Li et al, 2018a,c), is capable of conducting extracellular electrons transfer (EET) to anodes via direct electron transfer pathways mediated by c-type cytochromes, and electron transfer pathways mediated by diffusible electron shuttles (Shi et al, 2016; Kumar et al, 2017)
A lactate transporter encoded by lldP gene from E. coli was introduced in K. pneumonia to increase the lactate transportation across the hydrophobic cell membrane
To promote power generation of the microbial consortium in cellulosic hydrolyzates-fed microbial fuel cells (MFCs), we rationally engineered the co-culturing strains in the consortium by redirecting carbon flux distribution toward lactate biosynthesis in K. pneumoniae and enhancing flavins-mediated EET efficiency of S. oneidensis, respectively. In this synthetic microbial consortium, the byproducts were eliminated, and a lactate biosynthesis pathway was assembled into K. pneumonia for overproducing lactate
Summary
Microbial electrochemical technologies are green and sustainable that enabled many practical applications in environments and energy fields (Wang and Ren, 2013; Wang et al, 2015), including microbial fuel cells (MFCs) to harvest electricity production from organic wastes treatment (Bond et al, 2002; Lovley, 2006; Logan, 2009; Logan and Rabaey, 2012), microbial electrolysis cells to product hydrogen (Cheng and Logan, 2007; Kadier et al, 2016), microbial electrosynthesis to produce value-added chemicals and biofuels from CO2 bio-reduction (Rabaey and Rozendal, 2010; Li et al, 2012; Liu et al, 2016), and MFC-based biosensors for remote environmental monitoring in long-term (Golitsch et al, 2013), and so on. The glucose metabolic pathways from Zymomonas mobilis were heterogeneously expressed into S. oneidensis, allowing it to use glucose as the sole carbon and energy source (Choi et al, 2014). These efforts only used either glucose or xylose for harvesting energy, not achieving co-utilization of pentoses and hexoses from cellulose hydrolyzates for electricity production. Many virulence factors that contribute to its pathogenicity including lipopolysaccharide (LPS), capsular antigens, fimbrial adhesins, siderophores, and O antigens, have been inactivated by genetically engineering approaches (Kumar and Park, 2017)
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