Scientists creating plants that make their own fertilizer
A proof of principle
As a proof of principle Pakrasi and his colleagues plan to develop the synthetic biology tools needed to excise the nitrogen fixation system in one species of cyanobacterium (a phylum of green bacteria formerly considered to be algae) and paste it into a second cyanobacterium that does not fix nitrogen.
The team includes: Tae Seok Moon, PhD, and Fuzhong Zhang, PhD, both assistant professors of energy, environmental and chemical engineering in the School of Engineering at Washington University, and Costas D. Maranas, Donald B. Broughton Professor in the Chemical Engineering Department at Pennsylvania State University.
“Ultimately what we want to do is take this entire nitrogen-fixation apparatus—which evolved once and only once—and put it in plants,” Pakrasi says. “Because of the energy requirements of nitrogen fixation we want to put it in chloroplasts, because that’s where the energy-storing ATP molecules are produced.” In effect, the goal is to convert all crop plants, not just the legumes, into nitrogen fixers.
Amazing cycling chemistry
All cyanobacteria photosynthesize, storing the energy of sunlight temporarily in ATP molecules and eventually in carbon-based molecules, but only some of them fix nitrogen. Studies of the evolutionary history of 49 strains of cyanobacteria suggest that their common ancestor was capable of fixing nitrogen and that this ability was then repeatedly lost over the course of evolution.
The big hurdle to redesigning nitrogen fixation, however, is that photosynthesis and nitrogen fixation are incompatible processes. Photosynthesis produces oxygen as a byproduct and oxygen is toxic to nitrogenase, the enzyme needed to fix nitrogen. This is why most organisms that fix nitrogen work in an anaerobic (oxygenless) environment.
Cyanobacteria that both photosynthesize and fix nitrogen separate the two activities either in space or in time. Cyanothece 51142, a cyanobacterium Pakrasi’s lab has studied for more than 10 years, does it through timing.
Cyanothece 51142 has a biological clock that allows it to photosynthesize during the day and fix nitrogen at night. During the day the cells photosynthesize as fast as they can, storing the carbon molecules they create in granules. Then, during the night, they burn the carbon molecules as fast as they can. This uses up all the oxygen in the cell, creating the anaerobic conditions needed for nitrogen fixation.
Thus the environment within the cell oscillates daily between the aerobic conditions needed for capturing the energy in sunlight and the anaerobic conditions needed for fixing nitrogen.