Researchers at Ann Arbor’s University of Michigan today announced findings that a diverse mix of species improves the stability and fuel-oil yield of algae biofuel systems, as well as their resistance to invasion by outsiders.
U-M scientists grew combinations of freshwater algal species in 80 artificial ponds at U-M’s E.S. George Reserve near Pinckney. This was the first large-scale, controlled experiment to test the idea that biodiversity can improve the performance of algal biofuel systems in the field.
The experiment used funding from the National Science Foundation and the U-M Energy Institute. Bradley Cardinale, a professor at the U-M School for Environment and Sustainability.
Overall, researchers found that diverse mixes of algal species, known as polycultures, performed more key functions at higher levels than any single species, or were better at multitasking. However, researchers also found that polycultures did not produce more algal mass, or biomass, than the most productive single species, or monoculture.
“The results are key for the design of sustainable biofuel systems because they show that while a monoculture may be the optimal choice for maximizing short-term algae production, polycultures offer a more stable crop over longer periods of time,” says Casey Godwin, study lead author and a postdoctoral research fellow at U-M’s School for Environment and Sustainability.
The team’s findings are scheduled for publication June 18 in the journal Global Change Biology-Bioenergy.
Algae-derived biocrude oil is being studied as a potential renewable-energy alternative to fossil fuels. Algae grows quickly and can be converted into bio-oil, giving it the potential to generate more fuel from less surface area than crops such as corn. However, it is technically challenging to grow vast amounts of algae in large outdoor culture ponds, where algal biofuel cultivation systems face fluctuating weather conditions, diseases, pests, and invasions by nuisance species of algae.
Decades of ecological research have demonstrated that plant and animal communities containing a mix of species are, on average, more productive and more resistant to pests and diseases than less-diverse communities. The experiment to determine whether this is true for algal biofuel cultivation had two parts.
The first phase involved growing different combinations of six North American lake algal species in 180 aquarium-like tanks indoors. All six species are commonly used in biofuel systems. The second phase involved field-testing the four most promising algal species and species mixtures by growing them outdoors inside 290-gallon cattle tanks at the reserve. That work was done in summer 2016.
In both phases of the study, colleagues at the U-M College of Engineering used a technique called hydrothermal liquefaction to convert the algae into combustible oils, or biocrude, which can be refined to make transportation fuels such as biodiesel.
“First, we evaluated different combinations of algae in the lab, and then we brought the best ones out to nature, where they were exposed to fluctuating weather conditions, pests, disease, and all the other factors that have plagued algae-based fuel research efforts for 40 years,” says Godwin.
Researchers compared the ability of monocultures and polycultures to do several jobs at once: grow algal biomass, yield a high-quality biocrude, remain stable, resist population crashes, and repel invasions by unwanted algal species.
Polycultures delayed invasion, yielded significantly more biocrude, and were more stable. Polycultures performed more of the jobs at higher levels than any of the monocultures, a trait called multifunctionality.
However, the best monocultures produced more biomass, and polycultures had no significant effect on the magnitude and timing of population crashes.
“Our findings suggest there is a fundamental tradeoff when growing algal biofuel,” says Cardinale. “You can grow single-species crops that produce large amounts of biomass but are unstable and produce less biocrude. Or, if you are willing to give up some yield, you can use mixtures of species to produce a biofuel system that is more stable through time, more resistant to pest species, and which yields more biocrude oil.”
