Study
reveals the potentially large influences of fungi, one of the most biologically
diverse classes of organisms, on our energy supplies
A new study--which includes the first
large-scale comparison of fungi that cause rot decay--suggests that the
evolution of a type of fungi known as white rot may have brought an end to a
60-million-year-long period of coal deposition known as the Carboniferous
period. Coal deposits that accumulated during the Carboniferous, which ended
about 300 million years ago, have historically fueled about 50 percent of U.S.
electric power generation.
In addition, the study provides insights
about diverse fungal enzymes that might be used in the future to help generate
biofuels, which are currently among the most promising and attractive
alternatives to fossil fuels for powering vehicles.
The study, which was conducted by a team
of 71 researchers from 12 countries, appears in the June 29, 2012 issue of
Science and was partially funded by the National Science Foundation (NSF).
There are almost 1.5 million fungi
species on Earth. They perform essential ecological roles that include
decomposing organisms and serving as food for many insect species and larger
organisms.
However, only about five percent of
fungi species have, thus far, been classified. The new study is part of an
effort--supported by NSF's Assembling the Tree of Life and Partnerships for
Enhancing Expertise in Taxonomy programs--to resolve evolutionary relationships
between fungi species, define the diversity of fungi, and explain the early
evolutionary history of fungi. Information produced by this effort is integral
to the story of life on Earth and the evolution of its varied ecosystems.
The
end of a geologic era
Coal is composed of the fossilized
remains of plants--mostly lignin, which is a complex polymer that is an
important component of the cell walls of plants and helps give wood its
strength and rigidity. The study indicates that white rot fungi, which are the
only types of microorganisms that can break down lignin, evolved at the end of
the Carboniferous green period, and that the synchrony between the rise of
white rot fungi and the close of the Carboniferous was no coincidence.
According to the study, once white rot,
which breaks down lignin via enzymatic activity, became an ecological force, it
destroyed huge accumulations of woody debris that would have otherwise escaped
decay to ultimately be fossilized as coal.
So if not for the advent of white rot,
large coal deposits may have continued to form long after the end of the
Carboniferous period. This study supports a paper published in 1990 by Jennifer
M. Robinson that pegged the evolution of white rot as a potential contributing
factor to the end of the Carboniferous period.
The
matrix
Lignin exists in cell walls as part of a
tough matrix with cellulose, which is a carbohydrate composed of sugar
subunits. But once white rot attacks and destroys lignin, the matrix collapses,
and the cellulose is freed--to be devoured by the white rot as food.
The ability of white rot fungi to decay
lignin may ultimately be used to help conquer what is among the world's most
longstanding and vexing problems associated with the large-scale production of
biofuels: that is, obtaining plant carbohydrates that could be converted into
biofuels via fermentation processes.
It may ultimately be feasible to use
white rot to break down lignin to release cellulose from cell walls, which
could then be broken down into sugars. Next, the sugars would be fed to yeast
that would be fermented into alcohols that would provide the bases for new
biofuels.
In addition, because enzymes from white
rot fungi are able to break down complex organic molecules, they have been
investigated for use in bioremediation operations that involve breaking down
contaminants to remove them from the environment.
Genomic
comparisons
"Our study was designed to
reconstruct the evolution of lignin decay mechanisms in fungi, analyze the
distribution of enzymes that enable fungi to break down lignin, and better
define the evolution of the gene families that encode those enzymes," said
David Hibbett of Clark University, who led the study.
Hibbett and his team focused on a large
group of fungi known as Agaricomycetes, which include white rot fungi as well
as mushroom species that have the familiar cap-and-stem shape. The
Agaricomycetes group also includes brown rot fungi that can destroy wood by
breaking down cellulose and hemicellulose, which is another component of cell
walls--all the while without breaking down lignin.
The researchers compared 31 fungal
genomes--26 of which were sequenced at the Department of Energy's Joint Genome
Institute, including 12 that were sequenced at the DOE JGI specifically for the
study, and were then annotated and analyzed by NSF-funded researchers in
collaboration with JGI and other partners.
"The 12 new genome sequences could
serve as potential resources for industrial microbiologists aiming to develop
new tools for producing biofuels, bioremediation or other products, perhaps by
using recombinant DNA methods or by selecting new organisms for fermentation,"
said Hibbett.
"This study exemplifies the
tremendous gains we can make in understanding complicated biologic processes
such as lignin decomposition when we learn about the genealogical relationships
of organisms," said Charles Lydeard, an NSF program director.
The
evolution of white rot
The study also involved tracking the
evolution of lignin-decomposing enzymes back through time. This was done via
so-called "molecular clock analyses." Such analyses are based on the
assumption that genes accumulate mutations through evolution at fairly
predictable rates--similar to the way that the hands of a clock advance around
a clock at predictable rates. The ability to estimate these mutation rates
enables researchers to trace mutations back in time and estimate how recently
fungal lineages shared a common ancestor but then diverged from one another.
Results of molecular clock analyses
suggest that the oldest ancestor of the Agaricomcyetes was a white rot species
that possessed multiple lignin-degrading enzymes and lived roughly 300 million
years ago. Many surviving lineages of Agaricomycetes-including fungi species
known as wood-decaying polypores and bracket fungi-produce lignin-degrading
enzymes. "Our results suggest that the ability of fungi to break down lignin
evolved only once," said Hibbett.
In addition, Hibbett said, "This
study underscores the adaptability of fungi." This adapatability is
underscored by the fact that some Agaricomycete lineages have maintained their
lignin-degrading enzymes. By contrast, other Agaricomycete lineages, including
brown rot and mycorrhizal species, which survive via symbiotic relationships
with the roots of certain trees without decaying them, ultimately lost their
lignin-degrading enzymes as they developed alternative methods of obtaining
nutrition, said Hibbett.
Potential
payback
The economic value of fungi is already
almost incalculable: fungi currently impact diverse applied disciplines,
including agriculture, medicine and drug discovery. The more scientists learn
about these important organisms, the more likely they will be to identify
additional uses for them that will benefit the economy, the environment, and
human welfare, as well as to develop new ways to fight wood rot that, at great
costs, kills trees and destroys wood structures, including homes and ships.
Joseph Spatafora of Oregon State
University who is a co-author on the study said, "It's a really exciting
time in fungal biology, and part of that is due to the technology today that
allows us to address the really longstanding questions."
-NSF-
Posts
No comments:
Post a Comment