Possible Organisms

• Caldicellulosiruptor saccharolyticus
  • inputs: C. saccharolyticus can metabolize various carbon sources ranging from monomers, such as xylose, arabinose, glucose, fructose and galactose to α- and β-linked di- and polysaccharides, such as maltose, lactose, sucrose, starch, pullulan, threhalose, xylan and cellulose [32]. C. saccharolyticus can also grow and produce H2 from complex lignocellulosic materials, both pre-treated, such as Miscanthus hydrolysate [2], sugar beet juice [34] and paper sludge [23], and untreated, such as wheat straw [35], pine wood [22] and bagasse
    • When C. saccharolyticus was cultivated on a mixture of monosaccharides, they were consumed simultaneously but at different rates, i.e., fructose > arabinose > xylose > mannose > glucose > galactose.
    • To maintain high growth rate conditions, cells of C. saccharolyticus should obtain optimal energy gain from the substrate to fuel both anabolism and sugar transport, and thus lactate dehydrogenase (LDH) and alcohol dehydrogenase (ADH) should be kept inactive 
    • during exponential growth, H2, CO2 and acetate are the only fermentation products in C. saccharolyticus.
    • It revealed that LDH activity in C. saccharolyticus is strongly regulated by the levels of the energy carriers PPi and ATP, in addition to the NADH/NAD ratio (Figure (Figure2)2) [47]. When the cells are growing at the maximum specific growth rate, PPi levels are high and ATP levels are low, keeping LDH inactive and its affinity for NADH low. It further assures that the catabolic flux is directed to acetate and H2 (Figure (Figure2).2). However, as soon as the anabolic activity declines, the PPi/ATP ratio drops by an order of magnitude [71], which results in an increase in LDH activity as well as its affinity for NADH and hence lactate starts being formed [47].
    • It thus might be questioned whether deleting the ldh gene would improve H2 yields during sugar fermentation
  • outputs: The fermentation of these raw materials by C. saccharolyticus has yielded H2, CO2 and acetate as the main metabolic end products 
• E. Coli
• Geobacter
• rhodobacter
  • This paper describes hydrogen gas production by Rhodobacter sphaeroides O.U.001 using a column photobioreactor in batch and continuous operation (http://link.springer.com/chapter/10.1007%2F978-0-585-35132-2_18)
  • inputs: light, L-malic acid, and sodium glutamate
  • outputs: 
• enterobacter
• Clostridium thermocellum
  • identified as a specific bacteria with consolidated bioprocessing capabilities, ferments and produces ethanol which may be helpful as a supplimentary bacteria: http://www.biotechnologyforbiofuels.com/content/7/1/75
• The tables in this article list different bacteria with the consolidated bioprocessing approach and the levels of product they produce: http://www.sciencedirect.com/science/article/pii/S0960852412015490

More synbio approaches

• Getting another organism to take away the byproducts that hinder hydrogen production
  • combining dark fermentation and photo fermentation - Combined fermentation
  • getting an organism to consume the acetate
• containment - making the bacteria dependent on each other to survive
  • syntrophic exchange (http://www.pnas.org/content/111/20/E2149.short?rss=1)
  • down-regulate the pathways except for the hydrogen production pathway
• protein scaffolding to increase metabolic flux
• making recombinant proteins
• one bacteria degrading proteins/complex sugars that the other can use for production (http://www.sciencedirect.com/science/article/pii/S0960852414017301)
• metabolic wiring: bacteria in a closed system produce molecules that regulate the growth of the other bacteria http://2014.igem.org/Team:Edinburgh/project/population
• co-culture of anaerobic thermophiles producing Hydrogen: http://www.sciencedirect.com/science/article/pii/S0360319908003844

• Engineering a Synthetic Dual-Organism System for Hydrogen Production - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2663214/

  • conversion of biomass into formate, which can subsequently be processed into hydrogen by Escherichia coli.

Important hydrogen production review article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3757257/

Another review article: http://www.sciencedirect.com/science/article/pii/S0167779909000572