Managing photosynthesis' traffic jams

 

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In the drive to create sustainable energy sources, there is a push to get  cyanobacteria and algae to produce biofuels. But a major barrier to market adoption has been simple math: production costs are too prohibitive, as the efficiency of converting solar energy into useful compounds is not high enough.

The Ducat lab, however, has coaxed a lab-grown cyanobacteria to increase photosynthetic efficiency while simultaneously producing bioproducts, a remarkable technique that might remove that long-standing economic barrier.

The results have been published in the Plant and Cell Physiology journal.

 

A potential new source of energy

Sugar is at the heart of modern biotechnology. Take bioethanol: it is a biofuel made by taking sugar molecules in a plant, such as corn or sugarcane, and fermenting them into alcohol.

But there is a limit to how much we can squeeze out from plants.

Dr. Danny Ducat, an Assistant Professor at the Department of Biochemistry and Molecular Biology, says, “Think about it, we use plants, such as corn, to feed people and animals and also to produce biofuels like ethanol. These are competing markets that can make certain resources scarce, like fertile land and fresh water.”

“Cyanobacteria are highly productive and could potentially become an alternative source of sugars. This would be ideal for biotechnology, with cyanobacteria taking some of the load away from crops and plants while not competing with them for soil or water.”

 

Cyanobacteria can grow in environments that would be hostile to land plants: salt water, hot springs, even icebergs.
By NASA, CC BY-SA 2.0

 

A mysteriously productive synthetic organism

With that in mind, the Ducat lab created a sucrose-exporting cyanobacteria years ago, with the idea it would eventually produce useful bioproducts.

“The lab strains were very productive but did things we did not understand. For example, we noticed an unexpected increase in productivity, which seemed non-intuitive,” Danny says.

Danny expected the lab cells would get sickly, because forcing the cyanobacteria to produce and export sucrose was not part of their usual function nor useful for their survival. Instead they increased in productive efficiency.

Bradley Abramson, a PhD student in Danny’s lab, was interested in exploring why, and the answer was in the process of photosynthesis.

 

Photosynthesis traffic

 Photosynthesis is the source of energy for most life on our planet. And photosynthetic organisms have to manage the process very much like highway traffic, where the cars are energy and carbon. And much like on our roads, backups do happen.

For example, an apple tree captures energy with photosynthesis in the leaves, and stores some of that energy - in the form of sugars – in apples. A longstanding observation is that, if you chop off all of the apples, the sugars have fewer places to go, and the ensuing sugar “traffic jam” can lead to a reduction of production in the leaves.

Conversely, if any leaves are removed - say, blown away in a storm - the remaining leaves may pick up the pace in the short-term to make up for their lower numbers.

 

A cyanobacterial cell, 25 times smaller than a human hair. By the Ducat lab

 

Off-ramps

What Bradley found was that sucrose export led to an increase in photosynthetic efficiency to compensate for the amount of sucrose exported. In other words, more of the energy from sunlight was used towards producing the sugars.

"By introducing sugar export to a cell’s activity, we have essentially built a new off-ramp, using the traffic analogy,” according to Bradley. “The off-ramp relieves traffic jams by letting more cars (energy) exit the road. This lets traffic (photosynthesis) flow more regularly and efficiently because the road is less crowded.”

Call it a win-win: the cyanobacteria produce lots of a useful product for society while being more effective at photosynthesis.

 

Where to build off-ramps

Although other scientists have observed similar results in their efforts to produce biofuels, Bradley’s study is the first to understand the mechanism behind the productivity bump.

Next, the Ducat lab wants to examine the links between energy production and export, how these links are sensed by the cyanobacteria, and how this all affects cyanobacteria’s productivity and health.

And the potential industrial applications, which is Bradley’s passion, are exciting.

“Now we need to understand where and how to build new off-ramps,” Bradley adds. “This is not just about sucrose. It is becoming evident this happens in many cyanobacteria strains engineered to produce something. By understanding what is happening in these strains, we can begin to engineer greater efficiency, which means we can expect greater production at a lower cost."