A seemingly unremarkable ocean microbe turns out to be a multitasker — it can not only photosynthesize, but can also produce large amounts of hydrogen, opening up a potential way to make the gas cheaply for fuel.
The single-celled cyanobacterium Cyanothece 51142 can make hydrogen in air, Himadri Pakrasi of Washington University in St Louis, Missouri, and his colleagues report in Nature Communications1. Until now, the only organisms known to make hydrogen could only produce it in an oxygen-free environment — making it a potentially expensive process to scale-up.
Cyanothece 51142 was discovered in 1993, off the coast of Texas, by Louis Sherman of Purdue University in West Lafayette, Indiana, a co-author on the study. Pakrasi later discovered that the bacterium has a two-stage daily cycle. During the day it undergoes photosynthesis, using sunlight and carbon dioxide to make oxygen and branching chains of glucose molecules called glycogen. When the Sun goes down, the microbe's nitrogenase enzyme kicks into action, using the energy stored in the glycogen to fix nitrogen from the air into ammonia. Hydrogen is formed as a by-product.
The two mechanisms are different in that photosynthesis is an aerobic process — one that requires oxygen — whereas nitrogen fixation, and, consequently, hydrogen production, can take place only anaerobically, because contact with oxygen destroys the nitrogenase enzyme. But Cyanothece 51142 manages to fix nitrogen even in the presence of atmospheric oxygen by burning cellular oxygen to produce energy. Because no photosynthesis is taking place, the bacterium uses up its cellular oxygen so that the nitrogenase enzyme is effectively in a largely oxygen-free environment.