Micro Algal Hydrogen Production

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Microalgal hydrogen production Current Opinion in Biotechnology April, 2010 SACHIN KUMAR VERMA

Why look for alternative sources of energy? : 

Why look for alternative sources of energy? What are we going to do after 50-75 years?? Conventional sources would be depleted. Conventional sources are non – renewable, even for the next few years fuel cost is going to shoot up – unaffordable. Create a lot of pollution because all are carbon-based. Copenhagen Climate Conference – global temperature rise must be restricted to 2*C to avoid dangerous climate change. Developing countries want it to be less than 1.5*C, which requires reduction of concentration of CO2 to 350ppm from the present 450ppm

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Conventional sources Alternative Sources

Why choose algae as a source? : 

Why choose algae as a source? Cheap input, less capital used compared to other fuel sources like plants Do not require arable land, can be grown using sea water and waste water Doubling time is 4-6 hours in case of microalgae Easy gene manipulation possible to increase yields Conditions can be controlled to maximize yields Chlamydomonas reinhardtii, Nostoc, Synechocystis PCC6803

Other advantages : 

Other advantages Improved water use efficiencies Ability to produce the feedstocks for a wide range of fuels (e.g. biodiesel, methane, ethanol, and H2) Closed reactor for H2 production Open system for biodiesel production

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Various modes of algal cultivation and Bio-fuel production

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Synthesis of hydrogen in microalgae Important to know this because For gene manipulation to increase yields For optimizing conditions to maximize production 2 sources of H+ and e- for HydA – photosynthesis & Carbon metabolism

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Figure 2. Hydrogenase-related electron transport pathways in green algae. Electrons may originate either at PSII upon photooxidation of water, or at the plastoquinone pool upon oxidation of cellular endogenous substrate (e.g. via glycolysis and the tricarboxylic acid cycle). Electrons in the electron transport chain are transported via PSI to ferredoxin, which serves as the physiological electron donor to the Fe hydrogenase. P680, Reaction center of PSII; P700, reaction center of PSI; Q, primary electron acceptor of PS II; A, primary electron acceptor of PSI; PQ, plastoquinone; Cyt, cytochrome; PC, plastocyanin; Fd, ferredoxin; Red, NAD(P)H oxido-reductase; H2ase, hydrogenase; FNR, ferredoxin-NADP reductase. *Source - Hydrogen Production. Green Algae as a Source of Energy, Anastasios Melis and Thomas Happe, Plant Physiology Nov 2001

The hydrogenase enzyme : 

The hydrogenase enzyme Oxygen sensitive Therefore oxygen production and hydrogen production should be compartmentalized Achieved by sulphur depletion and repletion D1 reaction center protein – target Damaged on illumination Repair requires S-containing amino acids Sulphur depletion prevents this and oxygen synthesis is reduced Oxygen insensitive HydA analogs used But dangerous to have both O2 and H2 production together – explosive Therefore O2 absorption systems employed

Plastaquinone pool : 

Plastaquinone pool From photosynthesis (direct) Delivers substrate to HydA High light levels deplete PQ pool Mutants with large PQ favorable for H2 production From starch metabolism (indirect) Sulfur deprived conditions H+ and e- fed into PQ and then taken to PS I Starch lacking mutants produce high amounts of H2

Continuous hydrogen production : 

Continuous hydrogen production The normal mode involves the use of 3 photons For transfer of electron through PS II Through PS I to starch From starch to HydA Continuous mode involves only two photons For transfer through PS II From PS I to HydA 33% higher H2 production This happens in starch mutants Happens under micro-oxic conditions Key to success is to balance O2 uptake with O2 evolution by engineering the organism

Economics : 

Economics This process is viable only if the input costs are less than the output To make it an economically viable process, combine Oil Biomass Hydrogen production High value products e.g. glycerol Animal feed Sequestered carbon as bio-char Light to fuel conversion maximization Only for Hydrogen - 5%

Light capture and cell cultivation : 

Light capture and cell cultivation Maximum light intensity – higher production But photoinhibition in PS II (above 100mE; what is available in temperate and tropical regions is 1000mE – 1500mE) To minimize – reduce cross section of accessory pigments – genetic mutants 80-90% radiation usually wasted by photoprotective mechanisms This is reduced by genetic manipulation Also, light dilution – high incident solar radiation is spread over a large surface area

Other practical considerations : 

Other practical considerations 140mE ; 2.4x108 cells/L (24 mg Chl ‘a’ per L) with good culture mixing were found to be most effective for H 2 production in Chlamydomonas reinhardtii Low cost bio-reactors – aerobic and anaerobic designed – for biomass and Hydrogen production respectively

High efficiency H2 production systems : 

High efficiency H2 production systems Improved strains used – stm6 – blocked cyclic ETC stm6Glc4, stm6Glc4T7 – having hexose sugar symporter mechanism Antenna size reduced Direct and Indirect mechanisms can occur simultaneously 20-30% higher growth rates; 50% higher H2 production Yields – 400-600 mL / L /3-4days; 8mL / L /hr!!!!!!!!!!!!! Mixotrophic conditions; immobilized cells with pH based sulfur controlled conditions; genetic manipulation based on genomic, proteomic, metabolomic studies

Successful biofuel industry : 

Successful biofuel industry Biorefinery systems; 1.5%-5% increase in production CO2 recycling Input for biofuel (oil), biogas (methane), hydrogen, animal feed, bio-char etc combined Profit maximized Food vs fuel considerations when Minimized when non arable lands used Minimum money spent Yield is higher

References : 

References Microalgal Hydrogen production Olaf Kruse and Ben Hankamer, Current Opinion in Biotechnology, April, 2010 Functional integration of the HUP1 hexose symporter gene into the genome of C. reinhardtii: impacts on biological H(2) production - J Biotechnol 2007 Prolongation of H-2 photoproduction by immobilized, sulfur-limited Chlamydomonas reinhardtii cultures Laurinavichene TV, Kosourov SN, Ghirardi ML, Seibert M, Tsygankov AA: J Biotechnol 2008

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