|Schematic diagram showing the process of biohydrogen production by Cyanothece 51142 cells using solar energy and atmospheric CO2 and/or glycerol. Bandyopadhyay et al. Click to enlarge.|
Researchers from Washington University and Purdue University report on the ability of the single-celled cyanobacterium Cyanothece 51142 to produce biohydrogen under aerobic conditions in a paper published 14 December in the journal Nature Communications. Until now, the only organisms known to produce biohydrogen could only produce it in an anaerobic environment—making such a pathway for biohydrogen production potentially expensive 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. Himadri Pakrasi at Washington University 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. At night, 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.
...we describe Cyanothece sp. ATCC 51142, a unicellular, diazotrophic cyanobacterium with the capacity to generate high levels of hydrogen under aerobic conditions. Wild-type Cyanothece 51142 can produce hydrogen at rates as high as 465 µmol per mg of chlorophyll per hour in the presence of glycerol. Hydrogen production in this strain is mediated by an efficient nitrogenase system, which can be manipulated to convert solar energy into hydrogen at rates that are several fold higher, compared with any previously described wild-type hydrogen-producing photosynthetic microbe.
The two mechanisms are different in that photosynthesis is an aerobic process, 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.
Cyanothece 51142 also has a natural circadian rhythm that can be optimized to produce even more hydrogen. After a single 12-hour-day and 12-hour-night cycle, Pakrasi and his team kept the lights on for a further 48 hours straight. During this time, the microbes continued with their night-time nitrogen fixation and hydrogen production in the period that would normally have been dark, but made more fuel for the process by photosynthesizing. The researchers found that under these conditions the microbes adjusted their photosynthetic capacity to maximize nitrogen fixation.
The amount of hydrogen produced in this way—150 micro moles per milligram of chlorophyll per hour—is the most ever recorded in natural cyanobacteria under normal atmospheric conditions, says Pakrasi. If the bacteria behaved in the same way in a liter of culture medium as they did in the 25 milliliters of medium used in the experiment, they would make just over 900 ml of hydrogen in 48 hours.
The work shows what an unmodified cyanobacterium is capable of, says Pakrasi. There are at least 10 other strains of Cyanothece, and Pakrasi expects these to work in a similar way.
One can—and we have—enhance the rate by making genetic modifications to the system.— Himadri Pakrasi
Anindita Bandyopadhyay, Jana Stöckel, Hongtao Min, Louis A. Sherman, Himadri B. Pakrasi (2010) High rates of photobiological H2 production by a cyanobacterium under aerobic conditions. Nature Communications 1, 139 doi: 10.1038/ncomms1139