When growing algae in outdoor ponds as a next-generation biofuel, a naturally diverse mix of species will help reduce the chance of crop failure, according to a federally funded study by University of Michigan researchers.

Algae-derived bio-crude oil is being studied as a potential renewable energy alternative to fossil fuels. University of Michigan ecologist Bradley Cardinale and his colleagues found that growing multiple algal species in 180 aquarium-like tanks helped stabilize bio-crude production and made the system more reliable and efficient.

While the experiment was conducted indoors, its findings have relevance for outdoor cultivation, said Cardinale, co-author of a paper published online recently in the journal Environmental Science & Technology.

"These findings are important because one of our greatest challenges in making algal biofuel affordable is improving the efficiency of outdoor growth ponds and preventing crashes that ruin crops. Companies spend large amounts of money to repeatedly set up these ponds. If species diversity increases efficiency while also reducing the chance of a crash, then it reduces costs as well," said Cardinale, a professor at the university's School of Natural Resources & Environment and director of the Cooperative Institute for Limnology & Ecosystems Research.

Surprisingly, the researchers also found that growing a mix of algal species did not increase bio-crude production compared to species grown alone as monocultures. In fact, competition among species often caused mixtures to produce less bio-crude than each species did individually.

This finding contrasts with decades of ecological research showing that communities containing more diverse sets of plants and animals are, on average, more productive and more efficient at using resources. Because of this prior work, the University of Michigan algae researchers had hypothesized that a mix of algal species would be more productive than single species, but that was not the case.

"Our results suggest there is a fundamental trade-off when growing algal biofuel," said Anita Narwani, a former Michigan postdoctoral researcher in the School of Natural Resources & Environment and lead author of the study. "You can grow single-species cultures that produce high yields but have a high risk of crop failure, or you can use mixtures of species that produce lower yields but are much less likely to crash and are more sustainable through time."

The research was funded by a $2 million, four-year grant from the National Science Foundation. Awarded in July 2013, the grant paid for a two-part study to identify and test naturally diverse groups of green algae that can be grown together to create a high-yield, environmentally sustainable and cost-effective system to produce next-generation biofuels.

The first phase of the study involved growing various combinations of six North American lake algal species in 180 aquarium-like tanks at a laboratory in the School of Natural Resources & Environment. Each 10-liter (2.2 gal.) tank contained either one, two, four or six species of algae. Half of the tanks were maintained at a constant temperature of 71.6°F; the other half were assigned to a variable-temperature treatment to simulate outdoor conditions. The temperature fluctuated between 62.6°F and 80.6°F at weekly intervals.

The experiment lasted for seven weeks and showed that mixtures of algal species were, on average, more stable and reliable in the face of temperature fluctuations.

The second phase of the project involved field-testing the most promising algal species and species mixtures by growing them outdoors in 80 fiberglass cattle tanks at the university's E.S. George Reserve, a 1,300-acre biological station near Pinckney, Mich. That work was conducted in the summer of 2016, and the results are being analyzed now.

In both phases of the study, colleagues at the University of Michigan College of Engineering used a technique called hydrothermal liquefaction to measure the quantity and quality of the combustible oils, or bio-crude, produced by the various algal combinations. They are also comparing the ability of single and multi-species systems to reuse and recycle wastes for additional growth.

Limited fossil fuel supplies, growing global demand for energy and increasing atmospheric levels of heat-trapping carbon dioxide gas have recently brought algae-derived biofuels to the forefront of renewable energy research programs.

However, a number of hurdles must be overcome to make industrial-scale production of algal bio-crude oil feasible and economically competitive with fossil fuel-based crude oil. For instance, when grown in large outdoor ponds, algae suffer from crop instabilities due to variations in sunlight and temperature as well as disturbances from pests, diseases and other unwanted invaders.