Abstract
BackgroundH2–ethanol-coproducing bacteria, as primary fermenters, play important roles in the microbiome of bioreactors for bioenergy production from organic wastewater or solid wastes. Ethanoligenens harbinense YUAN-3 is an anaerobic ethanol–H2-fermenting bacterium. Ethanol is one of the main end-products of strain YUAN-3 that influence its fermentative process. Until recently, the molecular mechanism of metabolic regulation in strain YUAN-3 during ethanol accumulation has still been unclear. This study aims to elucidate the metabolic regulation mechanisms in strain YUAN-3, which contributes to effectively shape the microbiome for biofuel and bioenergy production from waste stream.ResultsThis study reports that ethanol stress altered the distribution of end-product yields in the H2–ethanol-coproducing Ethanoligenens harbinense strain YUAN-3. Decreasing trends of hydrogen yield from 1888.6 ± 45.8 to 837 ± 64.7 mL L−1 and acetic acid yield from 1767.7 ± 45 to 160.6 ± 44.7 mg L−1 were observed in strain YUAN-3 with increasing exogenous ethanol (0 mM–200 mM). However, the ethanol yield of strain YUAN-3 increased by 15.1%, 30.1%, and 27.4% in 50 mM, 100 mM, and 200 mM ethanol stress, respectively. The endogenous ethanol accounted for 96.1% (w/w) in liquid end-products when exogenous ethanol of 200 mM was added. The molar ratio of ethanol to acetic acid increased 14 times (exogenous ethanol of 200 mM) compared to the control. iTRAQ-based quantitative proteomic analysis indicated that 263 proteins of strain YUAN-3 were differentially expressed in 50 mM, 100 mM, and 200 mM of exogenous ethanol. These proteins are mainly involved in amino acid transport and metabolism, central carbon metabolism, and oxidative stress response.ConclusionThese differentially expressed proteins play important roles in metabolic changes necessary for growth and survival of strain YUAN-3 during ethanol stress. The up-regulation of bifunctional acetaldehyde-CoA/alcohol dehydrogenase (ADHE) was the main reason why ethanol production was enhanced, while hydrogen gas and acetic acid yields declined in strain YUAN-3 during ethanol stress. This study also provides a new approach for the enhancement of ethanologenesis by H2–ethanol-coproducing bacteria through exogenous ethanol addition.
Highlights
H2–ethanol-coproducing bacteria, as primary fermenters, play important roles in the microbiome of bioreactors for bioenergy production from organic wastewater or solid wastes
There was a clear trend of decreasing hydrogen yield from 1888.6 ± 45.8 to 837 ± 64.7 mL L−1 with increasing exogenous ethanol from 0 to 200 mM; the final cell dry weight of strain YUAN-3 increased in the presence of exogenous ethanol and the maximum cell dry weight reached 544 ± 9.7 mg L−1 (Fig. 1b)
Exogenous ethanol altered the yields of end-products in E. harbinense strain YUAN-3
Summary
H2–ethanol-coproducing bacteria, as primary fermenters, play important roles in the microbiome of bioreactors for bioenergy production from organic wastewater or solid wastes. This study aims to elucidate the metabolic regulation mechanisms in strain YUAN-3, which contributes to effectively shape the microbiome for biofuel and bioenergy production from waste stream. Ethanoligenens harbinense is an anaerobic ethanol–H2-fermenting bacterium, and the type strain YUAN-3 forms autoaggregating granules which is benefit to the formation of anaerobic granular sludge and maintenance of high cell density in continuous-flow bioreactors [8,9,10]. The end-products are composed of ethanol, acetic acid, H2, and C O2 [11], which can be subsequently used by other microbial species through cross-feeding interactions in anaerobic digestion and bioelectrochemical systems (BES) to achieve higher energy recovery from organic wastes [12,13,14]. MCFAs are easier to extract from water and are more versatile than ethanol and acetic acid [15, 16]
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