Scientists are analyzing the molecular pathways that plants use for photosynthesis.
For decades, researchers have been trying to improve upon Mother Nature’s favorite solar-power trick — photosynthesis — but now they finally think they see the sunlight at the end of the tunnel.
“We now understand photosynthesis much better than we did 20 years ago,” said Richard Cogdell, a botanist at the University of Glasgow who has been doing research on bacterial photosynthesis for more than 30 years. He and three colleagues discussed their efforts to tweak the process that powers the world’s plant life today in Vancouver, Canada, during the annual meetingof the American Association for the Advancement of Science.
The researchers are taking different approaches to the challenge, but what they have in common is their search for ways to get something extra out of the biochemical process that uses sunlight to turn carbon dioxide and water into sugar and oxygen. “You can really view photosynthesis as an assembly line with about 168 steps,” said Steve Long, head of the University of Illinois’ Photosynthesis and Atmospheric Change Laboratory.
Revving up Rubisco
Howard Griffiths, a plant physiologist at the University of Cambridge, just wants to make improvements in one section of that assembly line. His research focuses on ways to get more power out of the part of the process driven by an enzyme called Rubisco. He said he’s trying to do what many auto mechanics have done to make their engines run more efficiently: “You turbocharge it.”
Some plants, such as sugar cane and corn, already have a turbocharged Rubisco engine, thanks to a molecular pathway known as C4. Geneticists believe the C4 pathway started playing a significant role in plant physiology in just the past 10 million years or so. Now Griffiths is looking into strategies to add the C4 turbocharger to rice, which ranks among the world’s most widely planted staple crops.
The new cellular machinery might be packaged in a micro-compartment that operates within the plant cell. That’s the way biochemical turbochargers work in algae and cyanobacteria. Griffiths and his colleagues are looking at ways to create similar micro-compartments for higher plants. The payoff would come in the form of more efficient carbon dioxide conversion, with higher crop productivity as a result. “For a given amount of carbon gain, the plant uses less water,” Griffiths said.
Making the grid more efficient
Anne K. Jones, a biochemist at Arizona State University, wants to make use of the power that goes to waste during photosynthesis. On a sunny day, a plant’s molecular machinery generates more electrons than the Rubisco carbohydrate-producing engine can handle. “A lot of those electrons get thrown away,” she said.
In this sense, photosynthesis is like “a badly connected electrical grid,” Jones said. She’s studying ways to use biological nanowires to transfer the extra energy from the light-harvesting cell into another cell that’s genetically engineered to produce fuel or food. The nanowires would be analogous to electrical transmission lines, distributing power from one part of the grid to another.
Jones said filaments found on the surface of many bacterial species, known as pili, could be adapted for this purpose. Other researchers have already been looking into using those filaments as the basis for bioelectronic circuits.
“Components in future systems need not even be biological, so long as they interface with the wires developed in this project, paving the way for hybrid biological/inorganic photosynthetic systems,” Jones explained in an abstract for her presentation.
Creating an artificial leaf
Jones’ research meshes with Cogdell’s efforts to adapt the chemistry of photosynthesis ujsing synthetic biology. Cogdell’s project, backed by Britain’s Biotechnology and Biological Sciences Research Council, is aimed at developing an artificial leaf that produces a dense, portable fuel you could put in your car.
“We would aim to produce hydrocarbon fuel from carbon dioxide,” he said. His favorite candidate is terpene, the main ingredient in the plant resins that are today distilled into turpentine. Under the right conditions, terpene behaves “rather like octane,” Cogdell said.
He envisions a process in which carbon dioxide and water are chemically processed to produce a scummy sheen of terpene, which could be skimmed off and turned into fuel. Even though the end product is a hydrocarbon, the process would be carbon-neutral because of the CO2 capture, Cogdell said.
“We can’t do it yet, but we have a dream,” he told me.
Whether the future belongs to artificial leaves, or nanowired bacteria, or turbocharged rice, all these researchers believe that coming up with a better way to turn sunlight into energy is a crucial challenge for the next generation. They estimated that there was only a 30- to 50-year window for completing the transition from the fossil-fuel era to the age of total renewable energy.
Griffiths said the next generation will need more food as well as more fuel. He referred to the “green revolution” that has transformed global agriculture over the past half-century, and added that “what we now need is a new green revolution for the next 50 years.”
Cogdell echoed that view: “This is one of the grand challenges that mankind faces,” he said.