Abstract
Biohydrogen production (BHP) can be achieved by direct or indirect biophotolysis, photo-fermentation and dark fermentation, whereof only the latter does not require the input of light energy. Our motivation to compile this review was to quantify and comprehensively report strains and process performance of dark fermentative BHP. This review summarizes the work done on pure and defined co-culture dark fermentative BHP since the year 1901. Qualitative growth characteristics and quantitative normalized results of H2 production for more than 2000 conditions are presented in a normalized and therefore comparable format to the scientific community.Statistically based evidence shows that thermophilic strains comprise high substrate conversion efficiency, but mesophilic strains achieve high volumetric productivity. Moreover, microbes of Thermoanaerobacterales (Family III) have to be preferred when aiming to achieve high substrate conversion efficiency in comparison to the families Clostridiaceae and Enterobacteriaceae.The limited number of results available on dark fermentative BHP from fed-batch cultivations indicates the yet underestimated potential of this bioprocessing application. A Design of Experiments strategy should be preferred for efficient bioprocess development and optimization of BHP aiming at improving medium, cultivation conditions and revealing inhibitory effects. This will enable comparing and optimizing strains and processes independent of initial conditions and scale.
Highlights
A possibility to circumvent the production of noncarbon neutral greenhouse gasses, such as carbon dioxide (CO2), is the development and continuous investigation of alternative biofuels
Chemostat culture and fed-batch Molecular hydrogen (H2) production We provide an overview of dark fermentative Biohydrogen production (BHP) in bioreactors and similar set-ups, such as modified Erlenmeyerflasks and refer to these examination techniques as open systems, because gas sparging, offgas composition determination, pH titration and medium supplementation can be performed
Comparison of H2 production performance of strains related to Clostridiaceae, Enterobacteriaceae and Thermoanaerobacterales (Family III) As summarized in Additional files 2, 3, 4, 5 quantitative examination of dark fermentative BHP is largely performed on strains phylogenetically related to either the family Clostridiaceae or Enterobacteriaceae, and strains belonging to the family Thermoanaerobacterales (Family III) receive increasing scientific attention, because they comprise certain beneficial metabolic features [46,48-51]
Summary
A possibility to circumvent the production of noncarbon neutral greenhouse gasses, such as carbon dioxide (CO2), is the development and continuous investigation of alternative biofuels. Closed batch H2 production Dark fermentative BHP is found to be most often performed by strain cultivation in closed vessels (Additional file 2). Chemostat culture and fed-batch H2 production We provide an overview of dark fermentative BHP in bioreactors and similar set-ups, such as modified Erlenmeyerflasks and refer to these examination techniques as open systems, because gas sparging, offgas composition determination, pH titration and medium supplementation can be performed. The limited number of results available for dark fermentative BHP from fed-batch fermentation is due to the fact that usually the application of this technique leads to massive accumulation of organic acids and other reduced end products (i.e. alcohols) in the culture broth, strongly inhibiting the growth and H2 production kinetics. The quantitative potential of fed-batch cultivation has to be exploited in more detail for dark fermentative BHP by using biochemical engineering principles
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