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NREL Cyanobacteria Ramps Up Photosynthesis—and New Challenges

June 3, 2015

Photosynthesis is nature’s solar energy conversion process—it makes O2 from water while turning CO2 into organic compounds. Cyanobacteria were the first organisms to develop this capability in the course of evolution, and they passed it to other photosynthetic microbes and green plants. Photosynthesis powers biomass growth in plants and algae, which are potential feedstocks for bioenergy production. In addition, photosynthetic organisms can be engineered to convert CO2 directly to target chemicals, bypassing the middle-man biomass, as in the case of a cyanobacterium engineered by NREL to convert CO2 directly to ethylene gas.

In a recent project, an NREL research team, led by Jianping Yu, improved ethylene productivity so that up to 10% of photosynthetically fixed carbons become ethylene—with one unexpected, yet welcome, surprise. While cellular metabolism was expected to change, possibly inhibiting growth due to significant and continuous loss of carbon, growth seems to be unaffected. So the team faced the puzzle: how can cells adapt to loss of carbon and maintain growth rate?

NREL Researcher Wei Xiong and team accepted this challenge. The study started by developing advanced analytical methods to measure flow of carbons among numerous metabolic pathways in a cyanobacterium, similar to a real-time traffic report on TV. Modeling carbon metabolism requires heavy-duty computing, so the team used NREL’s super computer to speed up the process. Experimental data and the derived metabolic models showed dramatic changes of carbon flow associated with ethylene production, as detailed in a new Nature Plants paper. The team was thrilled to find that cells were able to fully compensate for the carbon loss by ramping up photosynthesis and increasing carbon fixation rate. This is music to the ears of bioengineers, as the cyanobacterium is not only producing a useful chemical, it is fixing more CO2 as well!

The discovery is exciting—but it leads to even more questions, as we now know that photosynthesis under normal conditions does not run as fast as it could. The team now faces another challenge: find the trigger to unleash photosynthesis, and apply it to increase photosynthetic productivity of fuels and chemicals. Stay tuned.

—Kathy Cisar