Building artificial chloroplasts drop-by-drop
In nature, photosynthesis takes place in specialized compartments, in plant chloroplasts. Using synthetic biology and microfluidic technology, a team of scientists from the Max Planck Institute and the University of Bordeaux have developed cell-sized artificial photosynthetic systems at the interface of biological and synthetic worlds. Their results were published in the May 8th issue of the journal, "Science".
Over billions of years, micro-organisms and plants have evolved the remarkable process known as photosynthesis. It converts solar energy into chemical energy, thus providing all life on earth with food and oxygen. The cellular compartments housing the molecular machines, the chloroplasts, are probably the most important natural engines on earth. Many scientists have considered artificially rebuilding and controlling the photosynthetic process, the “Apollo project of our time”. It would mean the ability to produce clean energy - clean fuel, clean carbon compounds such as antibiotics, and other products simply from light and carbon dioxide.
However, how would it be possible to build a living, photosynthetic cell from scratch? The key to mimicking the processes of a living cell is to get its components to work together at the right time and in the right place. Tobias Erb, director of the Max Plank Institute for terrestrial microbiology in Marburg, Germany and Jean-Christophe Baret, professor at the University of Bordeaux and the Centre de recherche Paul Pascal (CRPP - CNRS - UBx) have pursued this ambitious goal in an interdisciplinary multi-lab initiative, the MaxSynBio network. Along with their teams, they created a platform for the automated construction of cell-sized photosynthetically active compartments, “artificial chloroplasts”,that are able to capture and convert the greenhouse gas carbon dioxide with light.
Microfluidics meets synthetic biology
The researchers used two recent technological developments: firstly, synthetic biology for the design and construction of novel biological systems, such as reaction networks for the capture and conversion of carbon dioxide, and secondly, microfluidics, for the assembly of soft materials, such as cell-sized droplets.
The Max Planck scientists isolated the photosynthesis apparatus from the spinach plant and demonstrated that it was able to provide chemical energy that could be used to power single reactions and more complex reaction networks with light. But this was only the first step towards an artificial chloroplast.
“A long standing goal in our lab is to use synthetic biology to create sustainable solutions for the great challenges of our time, such as global warming. A few years ago, we developed the CETCH cycle, a metabolic module for the conversion of CO2 that we constructed from 18 individual enzymes and that is more efficient than the carbon metabolism naturally evolved by plants” explains Tobias Erb. “With the energy module at our disposal, we could now combine the two essential processes of photosynthesis into one system”. After several rounds of optimization of the two modules, the authors finally demonstrated the light-driven capture of the greenhouse gas CO2 with their integrated system.
More efficient than nature’s photosynthesis
While the team successfully prototyped their artificial photosynthesis in the reaction tube, they still lacked a technology to miniaturize and assemble their system on a microscale within a defined compartment. This was the role of Jean-Christophe Baret and his team. Automation is key to mass-production. The novel microfluidics platform served as a small factory to create thousands of standardized cell-like droplets, which can be individually filled with content. “We can build thousands of similar droplets, or equip these droplets with specific properties so that their functions differ from each other. We can synchronize them in their activity or program distinct, dynamic behavior,” says Tarryn Miller. The PhD candidate, whose supervisor is Tobias Erb, came to the CRPP to do these experiments with Thomas Beneyton, research engineer at the laboratory. “This allows us to create autonomous micro-reactors from the bottom-up that we can control in time and space using light.”
The artificial chloroplast features several new-to-nature enzymes and reactions, which allows it to capture CO2 over 100 times faster than other synthetic-biological approaches. It is also more energy-efficient than natural photosynthesis. “For now, the products of this artificial photosynthesis remain encapsulated in the drops, the next challenge is to use the spatial organization of the drops and to couple these systems to extract the molecules of interest”, explains Jean-Christophe Baret. In the long term, life like systems could be applied to practically all technological areas, including material science, biotechnology and medicine.
Tarryn E. Miller, Thomas Beneyton, Thomas Schwander, Christoph Diehl, Mathias Girault, Richard McLean, Tanguy Chotel, Peter Claus, Niña Socorro Cortina, Jean-Christophe Baret, Tobias J. Erb - Light-powered CO2fixation in a chloroplast mimic with natural and synthetic parts.
Professor at the University of Bordeaux (CRPP)