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The green algae Chlamydomonas can be made to be a producer of hydrogen, a clean fuel. © R. Kessel—G. Shih/Visuals Unlimited.

By exploiting an algal survival technique that generates hydrogen through a metabolic salvage pathway, researchers have taken a small but significant step toward developing an energy source that is both figuratively and literally green. "It’s an exciting breakthrough," says biochemist Anastasios Mellis of the University of California at Berkeley, noting that researchers have known for more than 60 years that many green algae can produce hydrogen, but no one had figured out a way to generate more than trace amounts from them.

Melis and his collaborators at the National Renewable Energy Laboratory (NREL) in Golden, Colo., now can reproducibly coax measurable quantities of hydrogen gas from the ubiquitous freshwater green alga Chlamydomonas reinhardtii. The key discovery is that, by depriving the algae of the sulfur which they need to build proteins while keeping them in sunlight, the cells enter a secondary metabolic state, reversing photosynthesis. Like a well-fed person suddenly forced onto a crash diet, the cells begin burning nonessential proteins and fuel that they have built up, yielding hydrogen as a byproduct.

This metabolic switch most likely evolved to help the algae survive in sulfur-poor conditions, Melis says. In his system, the algae sustain the secondary metabolic state for up to 150 hours at a stretch, and can be refreshed and recycled if given sulfur and allowed to revert to normal photosynthesis for a while. The researchers have run cultures through three continuous cycles of metabolic switching.

Hydrogen offers one of the cleanest, renewable energy sources available if it can be generated efficiently. The only "waste" products that result from burning hydrogen are excess heat and pure water. "If the development of hydrogen as an alternative fuel is successful, we are talking about changing the course of our civilization from one that depends on [finite] fossil fuels to one that relies on a clean and renewable form of energy," Melis says.

John Turner, a senior scientist at NREL also developing renewable approaches to hydrogen production, calls the Melis team’s achievement an important step, but its significance is confined to the field of photobiology and perhaps to the futuristic vision of renewable hydrogen generation, he says. "In terms of something that can contribute to the commercial hydrogen economy, it is a long way away."

Melis and his colleagues agree with that assessment. While three milliliters of hydrogen per liter of culture an hour is a huge leap forward over previous attempts to collect this gas from algae, the researchers are achieving only a small percentage of the algae’s potential capabilities, he says, and they are far from commercially viable production levels at this point. "What is most important at this stage is to elucidate the details of the mechanism that leads to hydrogen production," he says. "We need to find out the electron transport pathways, the catabolism of substrates, and the role of light in hydrogen production. My feeling is that as we study the mechanism, we will find ways in which to improve it."

Even so, photobiology is about 10 years behind other approaches such as photovoltaic systems and wind energy, Turner notes. These approaches lag behind the current predominant means of generating hydrogen, in which steam is used to treat natural gas. Still, all agree that at this early stage of research and development, every promising hydrogen production technology should be pursued. "We don’t know yet which approach or approaches are going to work or be economically viable," says Michael Siebert of NREL, one of Melis’s colleagues. Perhaps a range of methods will be employed one day, Melis adds.

There may be certain advantages to photobiology as a means for generating useful fuels, researchers note. It may prove more efficient than wind or solar arrays, which could reduce capital costs, Turner says. Theoretically, the hydrogenase enzyme in green algae that controls the metabolic switch should be almost three times as efficient as the nitrogenase enzyme in cyanobacteria, which also are being explored as a potential source of hydrogen energy, Siebert says.

Melis and his colleagues acknowledge that their algal technology ultimately may never come to commercial fruition. However, with their discovery, the concept of algae-stocked ponds dotting the landscape and generating hydrogen fuel for homes and cars is no longer in the realm of pure science fiction.

Christine Stencel
Christine Stencel is a science writer and manager in the ASM Communications Department.