Non-photosynthetic Biological CO 2 Fixation

Transcription

1 Non-photosynthetic Biological CO 2 Fixation Developing a Research Agenda for Utilization of Gaseous Carbon Waste Streams National Academies Tuesday, March 6 th, 2018 Dr. Benjamin M. Woolston Postdoctoral Associate in Prof. Greg Stephanopoulos Lab MIT Department of Chemical Engineering

2 Sources of electrons for biological CO 2 fixation Photosynthetic CO 2 Fixation Technological Maturity CO 2 Hydrolysis Fermentation Solar Biomass Sugars Products Solar Photosynthesis <1% efficiency E. coli S. cerevisiae ++++ Products Photosynthetic fermentation ++ CO 2 Algae, cyanobacteria Non-photosynthetic CO 2 Fixation Technological Maturity H 2 O CO 2 Gas Fermentation Solar Electricity H 2 Products + Photovoltaics Electrolysis Acetogens ~20% efficiency Knallgas Bacteria Waste Industrial Gases CO, CO 2, H 2 Gas Fermentation Products ++ Solar Solar Photovoltaics ~20% efficiency CO 2 Electricity Electrocatalysis CO HCOOH CH 3 OH CO 2 Acetogens Electricity Products Microbial Electrosynthesis Fermentation Products + + 2

3 Advantages of biological gas fermentation Biology provides specificity, avoiding the need to separate and sell a wide range of products Typical FT Product Distribution Dirty gas streams poison traditional catalysts, but not syngas-fermenting microbes Feedstock flexibility Comparatively low CapEx, allows monetization of smaller point sources Kibby C et. al Catal. Today 215: Perego et.al. Catalysis Today (2009):

4 Major Players in Non-photosynthetic CO 2 Fixation Acetogens Knallgas Bacteria 4H 2 + 2CO 2 CH 3 COOH + 2H 2 O (ΔG 0 = -95 kj/mol) Anaerobic microbes High electron efficiency of CO 2 fixation: ~92% Relatively lower rates: 6.2 g Acetate L -1 hr -1, μ = 0.05 hr -1 Low titer: ~30 g L -1 Rate & Titer Efficiency 2H 2 + O 2 2H 2 O (ΔG 0 = -477 kj/mol) Aerobic microbes High rates: 1.6 g PHB L -1 hr -1, μ = 0.42 hr -1 Lower efficiency of CO 2 fixation pathway: ~27% High titer: 62 g L -1 (intracellular) Kantzow, et al. J. Biotechnol. 212 (2015) Ishizaki, et al. Appl. Microbiol. Biotechnol. 57 (2001),

5 Acetogenic CO 2 fixation with the Wood-Ljungdahl Pathway CO 2, H 2, CO, CH 2 O 2, CH 3 OH Ac-CoA Acetate (+ATP) Ethanol Products Wood-Ljungdahl Pathway Feedstock flexibility High electron efficiency Commercial interest Drivers: Preliminary genetic tools, native ethanol production 5

6 Technical Challenge: H 2 /CO Mass Transfer Efficient feedstock utilization requires high mass transfer rates to deliver substrate to microbe CO and H 2 poorly soluble Reactor design for high mass transfer rate benefits from wellestablished literature and industrial practice Source: LanzaTech Presentation 6

7 Technical Challenge: Genetic Tools and Metabolic Understanding Genetic Tools Plasmids and transformation procedures (Kopke, 2010) Gene knockouts (Leang, 2013) CRISPR-cas9 knockouts (Huang, 2016) Temperature-sensitive replicons (Molitor, 2016) CRISPRi (Woolston, 2018) Metabolic Understanding Discovery of Flavin-based electron bifurcation (Thauer, 2008) First GSM of acetogen (Nagarajan, 2013) Transcriptomics ( ) Proteomics (Richter, 2016) Technologies for metabolic engineering in acetogens are new, but developing rapidly 7

8 Technical Challenge: Low ATP Yield Constrains Portfolio of Target Molecules ~0.5 ATP CO 2 CO 2 CH 3 -THF WL Acetyl-CoA pta ack Acetate CO ATP Product Hydrogen Cost ($/kg) $5.00 $4.00 $3.00 $2.00 $1.00 $0.00 Maximum H2 Price ($/kg) Ethanol 3HB (tesb) 3HB (ptb/buk) Butyrate Butanol Butanol (AOR) Product Molar Selectivity 2,3, BDO Lactate 4HB ATP Produced ½ ½ 0 0 1,4 BDO 100% 80% 60% 40% 20% 0% Product Selectivity 4H 2 + 2CO 2 CH 3 COOH + 2H 2 O (ΔG 0 = -95 kj/mol) 8

9 Overcoming ATP limitation in gas fermentation Separate metabolic capabilities of multiple strains Bench-Scale Metrics 18 g L -1 Lipids (C16-C18) 0.19 g L -1 hr -1 Energetic efficiency: 10% Two-stage bioreactor system optimizes division of labor, and intracellular product overcomes challenge with dilute product stream Hu P, Chakraborty S, Kumar A, Woolston BM, Liu H, Emerson D and Stephanopoulos G. PNAS (113)

10 Overcoming ATP limitation in gas fermentation Bolster ATP production through additional supplements No supplement, μ = 0.04 hr -1, Y = 0.11 Supplemented with 15 mm nitrate, μ = 0.08 hr -1, Y = 0.48 Emerson D, Woolston BM, et al., and Stephanopoulos G. (2018) Submitted Nitrate respiration improves growth and yield 10

11 Overcoming ATP limitation in gas fermentation Bolster ATP production through additional supplements Arginine eliminates autotrophic acetate production in C. autoethanogenum Mixotrophy Co-feeding of fructose enhances CO 2 fixation Valgepea, et al. and Marcellin Met. Eng 41: Jones et al. and Papoutsakis Nat Comm. DOI: /ncomms12800 Eventual solution will be decided by process economic considerations 11

12 Major Players in Non-photosynthetic CO 2 Fixation Acetogens Knallgas Bacteria 4H 2 + 2CO 2 CH 3 COOH + 2H 2 O (ΔG 0 = -95 kj/mol) Anaerobic microbes High electron efficiency of CO 2 fixation: ~92% Relatively lower rates: 6.2 g Acetate L -1 hr -1, μ = 0.05 hr -1 Low titer: ~30 g L -1 Rate & Titer Efficiency 2H 2 + O 2 2H 2 O (ΔG 0 = -477 kj/mol) Aerobic microbes High rates: 1.6 g PHB L -1 hr -1, μ = 0.42 hr -1 Lower efficiency of CO 2 fixation pathway: ~27% High titer: 62 g L -1 (intracellular) Kantzow, et al. J. Biotechnol. 212 (2015) Ishizaki, et al. Appl. Microbiol. Biotechnol. 57 (2001),

13 Knallgas Bacteria Technical Challenges Explosive Gas Mixtures: Maintaining O 2 below explosion limit leads to drop in productivity Use Calvin Cycle for CO 2 fixation: Lower efficiency 13

14 New pathways for improved CO 2 fixation performance Synthetic biology is allowing us to design new (non-natural) CO 2 fixation pathways CETCH De Novo designed pathway for CO 2 fixation in vitro rate similar to those measured in CBB pathway Schwander, et al. and Erb. J. Science (2016) 354,

15 Microbial Electrosynthesis for CO 2 Fixation Anaerobic microbes (acetogens) grown on cathode; accept electrons directly Bypasses need for intermediate electron carrier Technical Challenges Lack of mechanistic understanding of microbecathode interface Scale-up. Best rates ~0.13 g Acetate L -1 hr -1 Curr. Opinion. in Biotech (42) Compare to 6.2 g Acetate L -1 hr -1 Aryal et. al. Green Chem., 2017, 19,

16 Summary Non-photosynthetic CO 2 fixation using H 2, CO is an attractive technology for converting CO 2 to products with high specificity, bypassing some of the challenges of photosynthesis Major remaining technical challenges are overcoming energy limitations to improve rate and access higher-value chemicals, and dilute product streams Questions? 16