January 30, 2021. By Kolemann Lutz
Researchers from the University of Colorado Boulder developed light-powered nano-hybrid microbial factories that eat and convert CO2 by using light-activated quantum dots that fire specific microbial enzymes to generate biofuels, chemicals, and biodegradable plastics.
Intensive research has been conducted in combining multiple combinations of light, voltage, magnetic field stimulation to inorganic nanomaterials with metabolic networks in living cells. These attempts have been made to use direct light-activation in cell free extracts or purified enzymes for biocatalysis or bioelectrocatalysis. However, these strategies have limitations due to enzyme deactivation in air or during chemical conversion, without the ability to regenerate enzymes.
In 2013, the UC Boulder team began researching the applications of quantum dots (QDs): tiny nanoscopic (1X10-9 meter) semiconductors that are critical to nanotechnology systems. These QDs are injected into cells and later adapted, binded, and self-assembled to the electrochemical potential of the enzymes to wirelessly activate enzyme proteins using light, sound, or magnetic fields. The researchers investigated whether emitting artificial light to genetically engineered microbial cells with quantum dots could activate enzymes to convert airborne carbon dioxide and nitrogen even though the microbes cannot do so naturally because they are not photosynthetic.
The researchers designed and formed living nano-organisms using various strains of gram-negative soil bacteria, Azotobacter vinelandii (which fix nitrogen when grown aerobically) and Cupriavidus necator, hydrogen-oxidizing “knallgas” bacterium.
These non-photosynthetic microbes are typically dormant in water and release their gaseous byproducts to the surface, which are then harvested off the surface for manufacturing. These microbes and synthetic bacteria can generate electrons, reduce renewable chemical feedstocks like CO2, H2O, nitrogen, and air to carry out industrially important reactions.
By suspending normally nonphotosynthetic bacteria in buffered water without any sugar, the team engineered nanorganisms (nanorgs) to convert renewable feedstocks of air and CO2 directly into biofuels such as Isopropyl alcohol (IPA), hydrogen (H2), Butanediol (BDO), chemicals including formic acid, ammonia (NH3), and ethylene (C2H4), and the degradable bioplastic polyhydroxybutyrate (PHB).
By diffusing tailored quantum dots into the cells, the common microbes could accelerate their appetite for CO2 without sugar and with minimal sunlight to carry out the energy-intensive biochemical conversions. With an artificial light source radiating a light flux of 1.6mW/cm^2 for each of the ~10,000 biohybrid enzymes per nanorg, researchers estimated a 6-fold incident photons per enzyme turnover and achieved a high 13% quantum efficiency of enzyme activation using light.
The researchers also discovered that different combinations of quantum dots and absorption of light sources ranging from white, ultraviolet, blue, green, and near-infrared LEDs could yield different efficiencies for producing biofuels, chemicals, and bioplastics. Green wavelengths cause the microbes to consume more nitrogen, producing ammonia. On the other hand, red infrared wavelengths enable the bacteria to create more bioplastics after eating CO2.
By coupling the QD-hybrid nanorgs with a variety of bacterial strains (A. vinelandii and genetically modified C. necator strains), researchers created tailored nanorgs for each biofuel with a high turnover number (TON) of up to 10^7 moles of produce per mol of cells.
Throughout the experiment, the microbial factories rarely showed signs of exhaustion or depletion when they were activated consistently for multiple hours, leading to the belief that the cells can regenerate to form self sustaining colonies.
“Each cell is making millions of these chemicals and we showed they could exceed their natural yield by close to 200 percent,” said Prashant Nagpal, lead author of the research and an assistant professor in CU Boulder's Department of Chemical and Biological Engineering.
More research is required to genetically engineer nanorgs to accommodate the half solar irradiance present on the Martian surface in anaerobic, low pressure, and low-nitrogen environments with greater CO2 substrate concentrations. Additionally, innovative sensor architecture and integration with photobiomodulation technology that radiates infrared wavelengths hold the potential to yield greater CO2 conversion efficiencies of these bacteria on celestial bodies and throughout space.
On Earth, CO2 emissions from vehicles and houses could be pumped to a nearby holding pond with microbes to convert waste into bioplastics and sequester local carbon output. Improving CO2 capture of organisms can help replace carbon-intensive manufacturing for plastics and fuels to combat climate change on Earth while advancing the mission to become multiplanetary.
Research Paper. Yuchen Ding, et al. Nanorg Microbial Factories: Light-Driven Renewable Biochemical Synthesis Using Quantum Dot-Bacteria Nanobiohybrids. Journal of the American Chemical Society. June, 2019