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Engineered Bacteria Create Fertilizer Using Natural Sources Of Nitrogen
August 7, 2018

According to information provided by Washington University in St. Louis, researchers there, led by Himadri Pakrasi, the Glassberg-Greensfelder Distinguished University Professor in the Department of Biology in Arts & Sciences and director of the International Center for Energy, Environment and Sustainability (InCEES), are predicting that in the future, plants will be able to create their own fertilizer.

Himadri Pakrasi (left), led a team of researchers that has created a bacteria that uses photosynthesis to create oxygen during the day, and at night, uses nitrogen to create chlorophyll for photosynthesis. The team included Michelle Liberton (second from left), Deng Liu and Maitrayee Bhattacharyya-Pakrasi. Credit: Photo: Joe Angeles/Washington University.

Published recently in the journal mBio, the research, “Engineering Nitrogen Fixation Activity in an Oxygenic Phototroph,” indicates that it might soon be possible to engineer plants to develop their own fertilizer. This discovery could have a revolutionary effect on agriculture and the health of the planet.

Creating fertilizer is energy intensive, produces greenhouse gases, and as a nitrogen delivery system, is inefficient as less than 40 percent of the nitrogen in commercial fertilizer makes it to the plant. Additionally, applying fertilizer creates another environmental problem, runoff, where rain washes the fertilizer into waterways.

However, the Earth’s atmosphere, composed of about 78 percent nitrogen, offers an abundant alternative, and the Prof. Pakrasi’s lab has engineered a bacterium that can make use of that atmospheric gas — a process known as “fixing” nitrogen — in a significant step toward engineering plants that can do the same.

With funding provided through a $3.7 million grant from the National Science Foundation (NSF), the research was rooted in the fact that, although there are no plants that can fix nitrogen from the air, there is a subset of cyanobacteria (bacteria that photosynthesize like plants) that is able to do so. Cyanobacteria can do this even though oxygen, a byproduct of photosynthesis, interferes with the process of nitrogen fixation.

The bacteria used in this research, Cyanothece, is able to fix nitrogen because of something it has in common with people.

“Cyanobacteria are the only bacteria that have a circadian rhythm,” Prof. Pakrasi noted. Interestingly, Cyanothece photosynthesize during the day, converting sunlight to the chemical energy they use as fuel, and fix nitrogen at night, after removing most of the oxygen created during photosynthesis through respiration.

The research team wanted to take the genes from Cyanothece, responsible for this day-night mechanism, and put them into another type of cyanobacteria, Synechocystis, to coax this bug into fixing nitrogen from the air, too.

To find the right sequence of genes, the team looked for the telltale circadian rhythm. “We saw a contiguous set of 35 genes that were doing things only at night,” Prof. Pakrasi said, “and they were basically silent during the day.”

The team, which also included Maitrayee Bhattacharyya-Pakrasi, research associate Michelle Liberton, former research associate Jingjie Yu, and postdoctoral researcher Deng Liu manually removed the oxygen from Synechocystis and added the genes from Cyanothece. The researchers discovered that Synechocystis was able to fix nitrogen at 2 percent of Cyanothece. However, when Liu began to remove some of those genes; with just 24 of the Cyanothece genes, Synechocystis was able to fix nitrogen at a rate of more than 30 percent of Cyanothece.

Nitrogen fixation rates dropped markedly with the addition of a little oxygen (up to 1 percent), but rose again with the addition of a different group of genes from Cyanothece, although it did not reach rates as high as without the presence of oxygen.

“This means that the engineering plan is feasible,” Prof. Pakrasi said. “I must say, this achievement was beyond my expectation.”

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