Switching 100 percent of the algae’s photosynthesis to hydrogen might not be possible. “The rule of thumb is, if we bring that up to 50 percent, it would be economically viable,” Melis says. With 50 percent capacity, one acre of algae could produce 40 kilograms of hydrogen per day. That would bring the cost of producing hydrogen to $2.80 a kilogram. At this price, hydrogen could compete with gasoline, since a kilogram of hydrogen is equivalent in energy to a gallon of gasoline.
In 2000, Melis, working with researchers at the National Renewable Energy Laboratory (NREL), found that depriving the algae of sulfur nutrients forced the cells to make more hydrogen. The researchers were only able to deprive the algae of sulfur for a few days at a time, but during that time, about 10 percent of the algae’s photosynthetic capacity went toward making hydrogen.
Researchers at NREL are making progress in increasing hydrogen-production efficiency, according to lead researcher Michael Seibert. They can now force the algae to generate hydrogen for up to three months, as opposed to just a few days. Seibert expects that Melis’s chlorophyll-trimmed algae will be useful when the process is transferred to large bioreactors. Until the NREL researchers test the mutant algae, though, he says that it may be too early to tell.