title: Enhanced photo-fermentative H2 production using Rhodobacter sphaeroides by ethanol addition and analysis of soluble microbial products.
abstract: BACKGROUND
  Biological fermentation routes can provide an environmentally friendly way of producing H2 since they use renewable biomass as feedstock and proceed under ambient temperature and pressure. In particular, photo-fermentation has superior properties in terms of achieving high H2 yield through complete degradation of substrates. However, long-term H2 production data with stable performance is limited, and this data is essential for practical applications. In the present work, continuous photo-fermentative H2 production from lactate was attempted using the purple non-sulfur bacterium, Rhodobacter sphaeroides KD131. As a gradual drop in H2 production was observed, we attempted to add ethanol (0.2% v/v) to the medium.
  
  
  RESULTS
  As continuous operation went on, H2 production was not sustained and showed a negligible H2 yield (< 0.5 mol H2/mol lactateadded) within two weeks. Electron balance analysis showed that the reason for the gradual drop in H2 production was ascribed to the increase in production of soluble microbial products (SMPs). To see the possible effect of ethanol addition, a batch test was first conducted. The presence of ethanol significantly increased the H2 yield from 1.15 to 2.20 mol H2/mol lactateadded, by suppressing the production of SMPs. The analysis of SMPs by size exclusion chromatography showed that, in the later period of fermentation, more than half of the low molecular weight SMPs (< 1 kDa) were consumed and used for H2 production when ethanol had been added, while the concentration of SMPs continuously increased in the absence of ethanol. It was found that the addition of ethanol facilitated the utilization of reducing power, resulting in an increase in the cellular levels of NAD(+) and NADP(+). In continuous operation, ethanol addition was effective, such that stable H2 production was attained with an H2 yield of 2.5 mol H2/mol lactateadded. Less than 15% of substrate electrons were used for SMP production, whereas 35% were used in the control.
  
  
  CONCLUSIONS
  We have found that SMPs are the key factor in photo-fermentative H2 production, and their production can be suppressed by ethanol addition. However, since external addition of ethanol to the medium represents an extra economic burden, ethanol should be prepared in a cost-effective way.
authors.affiliation: Biomass and Waste Energy Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Daejeon, Yuseong-gu 305-343, Republic of Korea.
authors.forename: Dong-Hoon
authors.initials: DH
authors.lastname: Kim
authors.affiliation: Biomass and Waste Energy Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Daejeon, Yuseong-gu 305-343, Republic of Korea.
authors.forename: Ji-Hye
authors.initials: JH
authors.lastname: Lee
authors.affiliation: Department of Civil Engineering, Kyung Hee University, 1732 Deokyoungdaero, Yongin, Giheung, Gyeonggi-do 446-701, Republic of Korea.
authors.forename: Seoktae
authors.initials: S
authors.lastname: Kang
authors.affiliation: Departement of Microbiology and Immunology, University of Montreal, CP 6128, Succursale Centre-Ville, Montreal, QC H3C 3J7, Canada.
authors.forename: Patrick C
authors.initials: PC
authors.lastname: Hallenbeck
authors.affiliation: Department of Life Science and Basic Science Institute for Cell Damage Control, Sogang University, Mapo, Shinsu 1, Seoul 121-742, Republic of Korea.
authors.forename: Eui-Jin
authors.initials: EJ
authors.lastname: Kim
authors.affiliation: Department of Life Science and Basic Science Institute for Cell Damage Control, Sogang University, Mapo, Shinsu 1, Seoul 121-742, Republic of Korea.
authors.forename: Jeong K
authors.initials: JK
authors.lastname: Lee
authors.affiliation: Biomass and Waste Energy Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Daejeon, Yuseong-gu 305-343, Republic of Korea ; Division of Renewable Energy Engineering, University of Science and Technology, 217 Gajeong-ro, Daejeon, Yuseong-gu 305-350, Republic of Korea.
authors.forename: Mi-Sun
authors.initials: MS
authors.lastname: Kim
keywords: Electron balance; Ethanol; Lactate; Photo-fermentative hydrogen production; Size exclusion chromatography; Soluble microbial products