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| Leafcutter ants tending their fungal garden |
ScienceDaily, March 1, 2012
Leafcutter ants, the
tiny red dots known for carrying green leaves as they march through tropical
forests, are also talented farmers that cultivate gardens of fungi and
bacteria. Ants eat fungi from the so-called fungal gardens, but the bacteria's
role has been unclear until now.
New research shows
the bacteria help decompose the leaves and play a major role in turning the
leaves into nutrients that may be important for both ants and fungi. The
findings were published March 1 by The ISME Journal, a publication of the
International Society for Microbial Ecology.
"This research
provides some of the first tangible details about the fascinating symbiotic
relationship between leafcutter ants, fungi and bacteria," said Kristin
Burnum, a bioanalytical chemist at the Department of Energy's Pacific Northwest
National Laboratory. Burnum is a co-author on the paper and led the study's
protein analysis. "Understanding how bacteria turn plant matter into a
source of energy in ant fungal gardens could also help improve biofuel
production."
The gardens in
question are initially sowed by the ants, which bring leaf pieces into their
underground nests. From the leaves grow the fungus Leucoagaricus gongylophorus,
traditionally thought of as the ants' food. The relationship between leafcutter
ants and fungi has been known since 1874, but it wasn't until the late 1990s
that scientists started to also identify bacteria in the underground gardens.
Since then, a
lively debate has gone on about the bacteria's role. Because pure samples of
the garden fungi grown in laboratories don't easily degrade cellulose, a
molecule that gives plants structural stability, many scientists have argued
the bacteria help decompose the leaves. Other researchers have proposed
bacteria -- like the microscopic bugs in our guts -- help ants obtain nutrients
from the leaves.
Lead author Frank
Aylward of the University of Wisconsin-Madison, Burnum and their co-authors set
out to help resolve the debate by doing a comprehensive survey of the various
bacteria species that live in the gardens and examining the suite of proteins
those bacteria produce. They traveled to a Smithsonian Tropical Research
Institute site near Gamboa, Panama, and gathered samples of fungal gardens
tended by two ant species, Atta colombica and Atta cephalotes.
Aylward and several
others on the research team are part of the Great Lakes Bioenergy Research
Center, one of three Bioenergy Research Centers established by DOE's Office of
Science in 2007 to accelerate research toward the development of cost-effective
advanced biofuels from nonfood plant fiber. The University of Wisconsin-Madison
leads the Great Lakes center.
To produce results
that more accurately reflect the large diversity of real-world gardens, the
team collected large samples with bits of leaves, ants, fungi and bacteria
intermixed instead of just gathering samples of the bacteria they intended to
study. This allowed them to better examine the entire community of bacteria
that live in the gardens and prevented them from missing some bacterial
species. The team then studied the bacterial community's genes and proteins --
an approach known as metagenomics and metaproteomics.
The researchers
sequenced their genetic samples at Lawrence Berkeley National Laboratory's DOE
Joint Genome Institute. With the help of an extensive library of bacterial
genes developed by co-author Cameron Currie, team members at University of
Wisconsin-Madison identified thousands of bacterial genetic sequences from the
two ant gardens. More than two-thirds of the bacterial species found were from
just a few groups. More than half of those identified belong to the family Enterobacteriaceae,
whose members are known to ferment sugars and include the intestinal microbes
that help animals digest food.
From the bacteria,
Burnum and her PNNL colleagues in Richland examined proteins, the workhorses of
the cell that perform the tasks needed to keep organisms alive and well. They
used mass spectrometers at EMSL, the Department of Energy's Environmental
Molecular Sciences Laboratory at PNNL, to identify proteins in an A. colombica nest.
They found proteins
that were involved a surprising number of different metabolic pathways,
including:
•
Breaking down complex
sugars that make plants tough and durable, but difficult to digest.
•
Transporting sugars,
allowing broken-down sugars to be used for energy.
•
Making amino acids,
the buildings blocks of proteins.
•
Making vitamin B5,
which is needed to both break down proteins, carbohydrates and fats and to make
energy from nutrients.
When compared to
all other bacteria in Currie's large library of bacterial genes, very few --
just 0.2 to 0.6 percent -- of the garden bacteria were involved in breaking
down cellulose. Instead, most of the garden bacteria were involved in breaking
down simpler sugars, indicating that perhaps fungi initially breaks down
cellulose and the bacteria then turn the partially digested sugars that result
into a variety of nutrients that could promote the fungi's growth or even
nourish the ants themselves.
"Our results
show that calling these 'fungal gardens' is pretty misleading;
'fungus-bacterial communities' would be far more accurate," Burnum said.
"Bacteria are not only integral residents of these communities, but they
perform essential tasks that keep the communities -- and the ants that help
cultivate them -- living."
Next, the team
plans to analyze the fungi, lipids and various metabolic products found in the
gardens.
This study's
findings and future results could advance the work of scientists who are
looking at fungal enzymes to make biofuel out of plants. The enzymes, or
biological catalysts, of fungi are exceptionally talented at breaking down
cellulose in plants, making them a good model for large-scale biofuel
production.
"It's
apparent that neither fungi nor bacteria work in isolation when it comes to
leafcutter ant gardens," Burnum said. "It's possible that the same
goes for biomass conversion; perhaps both fungi and bacteria are needed to
efficiently turn plants into biofuel."
Story Source:
The above story is reprinted from materials provided byDOE/Pacific Northwest National Laboratory.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
- Frank O Aylward, Kristin E Burnum, Jarrod J Scott, Garret Suen, Susannah G Tringe, Sandra M Adams, Kerrie W Barry, Carrie D Nicora, Paul D Piehowski, Samuel O Purvine, Gabriel J Starrett, Lynne A Goodwin, Richard D Smith, Mary S Lipton, Cameron R Currie. Metagenomic and metaproteomic insights into bacterial communities in leaf-cutter ant fungus gardens. The ISME Journal, 2012; DOI: 10.1038/ISMEJ.2012.10
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