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NREL modifies organism to produce ethylene via photosynthesis: alternative to fossil-fuel based ethylene for chemicals and transportation fuels
26 September 2012
Scientists at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a new photo-biological process for the sustained production of ethylene from CO2. The NREL team introduced a modified gene sequence encoding an ethylene-forming enzyme from Pseudomonas syringae pv. into a cyanobacterium—Synechocystis sp. PCC 6803—and demonstrated that the organism remained stable through at least four generations, producing ethylene gas that could be easily captured. Research results were published in the RSC journal Energy & Environmental Science.
Ethylene—a valuable commodity two-carbon chemical that can be oligomerized into transportation fuels—is the most widely produced petrochemical feedstock globally. The organism produced ethylene at a high rate and is still being improved. The laboratory demonstrated rate of 171 milligrams of ethylene per liter per day is greater than the rates reported for the photosynthetic production by microorganisms of ethanol, butanol or other algae biofuels.
The same ethylene production rate was sustained across four successive sub-cultures without apparent loss of ethylene-forming ability. Up to 5.5% of the fixed carbon was directed to ethylene synthesis, surpassing the published carbon-partition rate into the TCA cycle. Nitrogen- and phosphorus-enriched seawater can support both growth and ethylene production. Factors limiting ethylene production, including efe expression levels, light intensity and nutrient status, were identified and alleviated, resulting in a peak production rate of 5,650 μL L−1 h−1 (7,125 μg L−1 h−1, 252 μmol L−1 h−1, or 171 mg L−1 day−1), which is higher than that reported for other algae biofuels and chemicals. This study suggests that Synechocystis, expressing the modified efe gene, has potential to be an efficient biological catalyst for the uptake and conversion of CO2 to ethylene.—Ungerer et al.
The process does not release carbon dioxide into the atmosphere. Conversely, the process recycles carbon dioxide, a greenhouse gas, since the organism utilizes the gas as part of its metabolic cycle.
Ethylene is currently produced exclusively from fossil fuels, and its production is the largest CO2-emitting process in the chemical industry. Steam cracking of long-chain hydrocarbons from petroleum produces 1.5 to 3 tons of carbon dioxide for every ton of ethylene produced.
The NREL process, by contrast, produces ethylene by using carbon dioxide, which is food for the bacteria. That could mean a savings of six tons of carbon dioxide emissions for every ton of ethylene produced—the three tons that would be emitted by tapping fossil fuels and another three tons absorbed by the bacteria.
Ten years ago, a group of Japanese scientists led by Takahira Ogawa at Sojo University was the first to try to produce ethylene via photosynthetic conversion in the cyanobacterium Synechococcus 7942. But by the fourth generation, the bacteria were defunct, producing no ethylene at all, said NREL principal investigator, Jianping Yu.
NREL turned to a different cyanobacterium, Synechocystis 6803, which scientists had been researching for a long time, knowing how to change its DNA sequences. They manipulated the sequence to design an ethylene-producing gene to be more stable and more active than the original version.
This process resulted in an organism that uses carbon dioxide and water to produce ethylene, but doesn’t lose its ability to produce ethylene over time. The product ethylene is non-toxic to the producing microorganisms and is not a food source for other organisms that could potentially contaminate an industrial process.
Our peak productivity is higher than a number of other technologies, including ethanol, butanol, and isoprene. We overcame problems encountered by past researchers. Our process doesn’t produce toxins such as cyanide and it is more stable than past efforts. And it isn’t going to be a food buffet for other organisms.—Jianping Yu
After the culture reaches maximum growth, it’s possible that it could keep producing for months at a time, said Rich Bolin, who is a member of NREL’s partnerships group. The ethylene gas it produces naturally leaves the organism, spurring the organism to keep producing more.
The ethylene would be produced in an enclosed photobioreactor containing seawater enriched with nitrogen and phosphorous. The ethylene gas would rise and be captured from the reactor’s head space. It could then undergo further processing, including a catalytic polymer process to produce fuels and chemicals. The continuous production system improves the energy conversion efficiency and reduces the operational cost.
NREL is initiating discussions with potential industry partners to help move the process to commercial scale. Interested companies include those in the business of producing ethylene or transportation fuels, as well as firms that build photobioreactors.
Separations in biotechnology are complicated and costly. The nice thing about this system is that it is a gas that just separates from the culture media and rises to the head space. That’s a huge advantage over having to destroy the valuable culture that is taking carbon dioxide and light and water to make your product. It’s much easier than a liquid-liquid separation like in ethanol.—Jim Brainard, director of NREL’s Biosciences Center
Justin Ungerer, Ling Tao, Mark Davis, Maria Ghirardi, Pin-Ching Maness and Jianping Yu (2012) Sustained photosynthetic conversion of CO2 to ethylene in recombinant cyanobacterium Synechocystis 6803. Energy Environ. Sci., 5, 8998-9006doi: 10.1039/C2EE22555G
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