|
Overturning two long-held misconceptions about oil production in
algae, scientists at the U.S. Department of Energy’s Brookhaven National
Laboratory show that ramping up the microbes’ overall metabolism by feeding
them more carbon increases oil production as the organisms continue to grow.
The findings—published online in Plant and Cell Physiology—may point to
new ways to turn photosynthetic green algae into tiny “green factories” for
producing raw materials for alternative fuels.
“We are interested in algae because they grow very quickly and
can efficiently convert carbon dioxide into carbon-chain molecules like starch
and oils,” said Brookhaven biologist Changcheng Xu, the paper’s lead author.
With eight times the energy density of starch, algal oil in particular could be
an ideal raw material for making biodiesel and other renewable fuels.
But there have been some problems turning microscopic algae into
oil producing factories.
For one thing, when the tiny microbes take in carbon dioxide for
photosynthesis, they preferentially convert the carbon into starch rather than
oils. “Normally, algae produce very little oil,” Xu said.
Before the current research, the only way scientists knew to tip
the balance in favor of oil production was to starve the algae of certain key
nutrients, like nitrogen. Oil output would increase, but the algae would stop
growing—not ideal conditions for continuous production.
Another issue was that scientists didn’t know much about the
details of oil biochemistry in algae. “Much of what we thought we knew was
inferred from studies performed on higher plants,” said Brookhaven biochemist
John Shanklin, a co-author who’s conducted extensive research on plant oil
production. Recent studies have hinted at big differences between the microbial
algae and their more complex photosynthetic relatives.
“Our goal was to learn all we could about the factors that
contribute to oil production in algae, including those that control metabolic
switching between starch and oil, to see if we could shift the balance to oil
production without stopping algae growth,” Xu said.
The scientists grew cultures of Chlamydomonas reinhardtii—the “fruit fly” of algae—under a variety of nutrient conditions, with and without
inhibitors that would limit specific biochemical pathways. They also studied a
mutant Chlamydomonas
that lacks the capacity to make starch. By comparing how much oil
accumulated over time in the two strains across the various conditions, they
were able to learn why carbon preferentially partitions into starch rather than
oil, and how to affect the process.
The main finding was that feeding the algae more carbon (in the
form of acetate) quickly maxed out the production of starch to the point that
any additional carbon was channeled into high-gear oil production. And, most
significantly, under the excess carbon condition and without nutrient deprivation, the microbes
kept growing while producing oil.
Confocal image of the algae Chlamydomonas showing the accumulation of oil droplets (golden dots). Red represents chlorophyll autofluorescence. |
“This overturns the previously held dogma that algae growth and
increased oil production are mutually exclusive,” Xu said.
The detailed studies, conducted mainly by Brookhaven research
associates Jilian Fan and Chengshi Yan, showed that the amount of carbon was
the key factor determining how much oil was produced: more carbon resulted in
more oil; less carbon limited production. This was another surprise because a
lot of approaches for increasing oil production have focused on the role of
enzymes involved in producing fatty acids and oils. In this study, inhibiting
enzyme production had little effect on oil output.
“This is an example of a substantial difference between algae
and higher plants,” said Shanklin.
In plants, the enzymes directly involved in the oil biosynthetic
pathway are the limiting factors in oil production. In algae, the limiting step
is not in the oil biosynthesis itself, but further back in central metabolism.
This is not all that different from what we see in human
metabolism, Xu points out: Eating more carbon-rich carbohydrates pushes our
metabolism to increase oil (fat) production and storage.
“It’s kind of surprising that, in some ways, we’re more like
algae than higher plants are,” Xu said, noting that scientists in other fields
may be interested in the details of metabolic switching uncovered by this
research.
But the next step for the Brookhaven team will be to look more
closely at the differences in carbon partitioning in algae and plants. This
part of the work will be led by co-author Jorg Schwender, an expert in
metabolic flux studies. The team will also work to translate what they’ve
learned in a model algal species into information that can help increase the
yield of commercial algal strains for the production of raw materials for
biofuels.
Source: Brookhaven National Laboratory
