BERKELEY--A metabolic switch that triggers algae to turn sunlight into large quantities of hydrogen gas, a valuable fuel, is the subject of a new discovery reported for the first time by University of California, Berkeley, scientists and their Colorado colleagues. The news appears in this month's issue of the journal "Plant Physiology."
"I guess it's the equivalent of striking oil," said UC Berkeley plant and microbial biology professor Tasios Melis. "It was enormously exciting, it was unbelievable."
Melis and postdoctoral associate Liping Zhang of UC Berkeley made the discovery - funded by the U.S. Department of Energy (DOE) Hydrogen Program - with Dr. Michael Seibert, Dr. Maria Ghirardi and postdoctoral associate Marc Forestier of the National Renewable Energy Laboratory (NREL) in Golden, Colorado.
Currently, hydrogen fuel is extracted from natural gas, a non-renewable energy source. The new discovery makes it possible to harness nature's own tool, photosynthesis, to produce the promising alternative fuel from sunlight and water. A joint patent on this new technique for capturing solar energy has been taken out by the two institutions.
So far, only small-scale cultures of the microscopic green alga Chlamydomonas reinhardtii have been examined in the laboratory for their hydrogen production capabilities, Melis said.
"In the future, both small-scale industrial and commercial operations and larger utility photobioreactor complexes can be envisioned using this process," Melis said.
While current production rates are not high enough to make the process immediately viable commercially, the researchers believe that yields could rise by at least tenfold with further research, someday making the technique an attractive fuel-producing option.
Preliminary rough estimates, for instance, suggest it is conceivable that a single, small commercial pond could produce enough hydrogen gas to meet the weekly fuel needs of a dozen or so automobiles, Melis said.
The scientific team is just beginning to test ways to maximize hydrogen production, including varying the particular type of microalga used and its growth conditions.
Many energy experts believe hydrogen gas one day could become the world's best renewable source of energy and an environmentally friendly replacement for fossil fuels.
"Hydrogen is so clean burning that what comes out of the exhaust pipe is pure water," Melis said. "You can drink it."
Engineering advances for hydrogen storage, transportation and utilization, many sponsored by the U.S. DOE Hydrogen Program, are beginning to make the fuel feasible to power automobiles and buses and to generate electricity in this country, Seibert said.
"What has been lacking is a renewable source of hydrogen," he said.
For nearly 60 years, scientists have known that certain types of algae can produce the gas in this way, but only in trace amounts. Despite tinkering with the process, no one has been able to make the yield rise significantly without elaborate and costly procedures until the UC Berkeley and NREL teams made this discovery.
The breakthrough, Melis said, was discovering what he calls a "molecular switch." This is a process by which the cell's usual photosynthetic apparatus can be turned off at will and the cell can be directed to use stored energy with hydrogen as the byproduct.
"The switch is actually very simple to activate," Melis said. "It depends on the absence of an essential element, sulfur, from the microalga growth medium."
The absence of sulfur stops photosynthesis and thus halts the cell's internal production of oxygen. Without oxygen from any source, the anaerobic cells are not able to burn stored fuel in the usual way, through metabolic respiration. In order to survive, they are forced to activate the alternative metabolic pathway, which generates the hydrogen and may be universal in many types of algae.
"They're utilizing stored compounds and bleeding hydrogen just to survive," Melis said. "It's probably an ancient strategy that the organism developed to live in sulfur-poor anaerobic conditions."
He said the alga culture cannot live forever when it is switched over to hydrogen production, but that it can manage for a considerable period of time without negative effects.
The researchers first grow the alga "photosynthetically like every other plant on Earth," Melis said. This allows the green-colored microorganisms to collect sunlight and accumulate a generous supply of carbohydrates and other fuels.
When enough energy has been banked in this manner, the researchers tap it and turn it into hydrogen. To do this, they transfer the liquid alga culture, which resembles a lime-green soft drink, to stoppered one-liter glass bottles with no sulfur present. Then the culture is allowed to consume away all oxygen.
After about 24 hours, photosynthesis and normal metabolic respiration stop, and hydrogen begins to bubble to the top of the bottles and bleed off into tall, hydrogen-collection glass tubes.
"It was actually a surprise when we detected significant amounts of hydrogen coming out of the culture," Melis said. "We thought we would get trace amounts, but we got bulk amounts."
After up to four days of generating an hourly average of about three milliliters of hydrogen per liter of culture, the culture is depleted of stored fuel and must be allowed to return to photosynthesis. Then, two or three days later, it again can be tapped for hydrogen, Melis said.
"The cell culture can go back and forth like this many times," Ghirardi said.