Turning algae into a biofuel that can readily replace petroleum looks promising, but one of the big roadblocks to mass-producing the algae is the need for fertilizer. Although algae have trouble reusing the nitrogen and phosphorus left in the water after their predecessors have been converted to bio-oil, the group of Nina Lin, an assistant professor of chemical engineering, found that bacteria can survive on those algae remnants.
The group of Phil Savage, an Arthur F. Thurnau professor of chemical engineering, has shown that 90 percent of the chemical energy in the algae can be captured in biocrude oil, and biocrude can already be refined to 97 percent purity. But algae need nitrogen and phosphorus in their environment, and fertilizer is a major cost, said Lin.
Because the nitrogen and phosphorus tend to stay in the watery byproduct when the algae has been “pressure-cooked” into oil, many researchers had assumed that it could fertilize the next batch of algae.
“Most of the nitrogen and phosphorus is in the form of ammonia and phosphate, which should be readily available for the algae,” said Michael Nelson, a PhD student in Lin’s group and first author on the paper to appear in the May issue of Bioresource Technology.
But the field hadn’t considered what happens to the carbon compounds at temperatures around 660 degrees Fahrenheit and pressures of nearly 200 times that of Earth’s atmosphere.
“It’s like charred meat – the surface contains compounds that are thought to be carcinogenic. Nasty compounds form at unnaturally high temperatures,” said Nelson.
Researchers could get algae to grow in the presence of the watery byproduct and its toxic carbon compounds, “but they had to dilute the heck out of it,” said Nelson. If the algae-to-water ratio was 20:80 before the conversion to oil, fresh algae couldn’t handle the byproduct in concentrations of more than half a percent.
Instead of writing off the watery leftovers, Lin’s team tried to grow microbes with the stuff. They tested the well-known Escherichia coli bacteria and Saccharomyces cerevisiae (brewing yeast) as well as the hardy Pseudomonas putida bacteria, a research candidate for remediating oil spills.
The brewing yeast didn’t do so well, but the team was pleased to find that E. coli and P. putida could grow well by feeding on the byproduct at 20 percent concentration, again with a starting algae-water ratio of 20:80. The bacteria continued to grow at concentrations of up to 40 percent.
Nelson made sure that the remainder of the solution only controlled the pH of the fluid and provided the trace amounts of metals that are missing from the watery byproduct. “I was very careful not to cheat,” he said, either with expensive chemicals or extra nutrients.
Now, the team is pressure-cooking the bacteria like the algae to find out what quality of oil it makes. They have also been growing many generations of bacteria, feeding them only on the watery product from algae reactions, also known as the “aqueous phase”.
“Because we don’t know exactly what’s in this aqueous phase or how to rationally engineer our species, we decided to use natural selection to evolve and isolate strains that are proved to have better fitness in this toxic and demanding environment,” said Lin. “We’ve been seeing some very encouraging results.”
In the future, they would like to explore whether the algae respond better to the watery byproduct after the bacteria have eaten some of the carbon compounds. Since algae often coexist with other microbes, and some can degrade certain molecules that are toxic to algae, Lin said, “We’re hoping this might actually happen in our system.”