A rendering of the M. hungatei, showing a granule at one end, represented by the green sphere. |
Archaea
are among the oldest known life-forms, but they are not well
understood. It was only in the 1970s that these single-celled
microorganisms were designated as a domain of life distinct from
bacteria and multicellular organisms called eukaryotes.
Robert
Gunsalus, a UCLA professor of microbiology, immunology, and molecular
genetics, developed an interest in Archaea because of their ability to
thrive in harsh environments. Now, using state-of-the-art imaging
equipment at the California NanoSystems Institute (CNSI) at UCLA, he has
shown for the first time that a type of Archaea known as
Methanosprillum hungatei contains incredibly efficient energy-storage
structures.
The findings are published in Environmental Microbiology.
M.
hungatei is of considerable environmental significance because of its
unique ability to form symbiotic relationships with syntrophic bacteria
to break down organic matter and produce methane gas. Yet while their
important role in the food chain has been studied, little has been known
about how they generate and store energy.
Gunsalus
has researched anaerobic organisms like M. hungatei—microbes that
thrive in oxygen-depleted environments where energy is often extremely
limited—for a number of years. And when Hong Zhou, a professor of
microbiology, immunology, and molecular genetics, arrived at UCLA in
2006, Gunsalus saw an opportunity to delve further into their mysteries.
“When
Hong came to UCLA, his reputation in imaging nanoscale structures was
already well established,” says Gunsalus, who is also a member of the
UCLA–Department of Energy Institute for Genomics and Proteomics. “His
arrival on campus brought together the expertise to do what no one had
yet done—a detailed study of the sub-cellular structures in M.
hungatei.”
Much
of the actual imaging work for the study was performed by Dan Toso, a
graduate student in Zhou’s lab, using equipment from the Electron
Imaging Center for Nanomachines (EICN), a core lab at the CNSI directed
by Zhou. When Toso and the rest of the team produced the most detailed
images yet made of the M. hungatei interior, they were surprised by the
appearance of granules, structures measuring approximately 150
nm in dia. that store energy.
“Once
we imaged the M. hungatei, we noticed how dark the granules appeared,”
says Zhou, a researcher at the CNSI. “The darkness arises from their
density, and by studying this density, we discovered their
energy-storage capacity.”
The
group was able to determine the granule density—about four times that
of water—by using a Titan scanning transmission electron tomography
(STEM) microscope, cryo-electron microscopy, and energy-dispersive X-ray
spectroscopy, all part of the EICN lab’s extensive tool set.
The
tiny granules, which account for less than 0.5% of the cell, are
so efficient that they each store 100-fold more energy than the entire
rest of the cell. Each M. hungatei produces two granules, one at each
end of the cell. Because all M. hungatei produce granules in the same
location, and typically at the same time in their life-cycle, it is
likely that their DNA contains specific genetic instructions for the
creation and positioning of the granules.
The
researchers hope to utilize knowledge gained from the recent sequencing
of the M. hungatei genome by the U.S. Department of Energy Joint Genome
Institute to further study the structures. If the specific genetic
instructions for creating granules can be found in the genome, it might
be possible to use the granules as a sort of chemical battery for
engineered synthetic cells.
Beyond
their energy-storage capacity, M. hungatei still have more secrets to
reveal, according to the researchers. They also produce a distinct
nanostructure sheath around their cell membrane that might serve as a
sort of protection, or “cell armor,” against the harsh environments in
which they are typically found. Though the sheaths were discovered in
the 1970s, the technology necessary for studying them in detail had yet
to be developed at that time.
“M.
hungatei have evolved unique features in order to survive in very harsh
and low-energy environments,” Gunsalus says. “The presence of
cutting-edge equipment and world-class experts at UCLA allows us to
closely study them, hopefully revealing their myriad of secrets.”
The
researchers’ next goals are to elucidate the exact biological function
of the granules and sheaths in M. hungatei. Many functions have been
proposed for the granules, including as energy sources for cell
division, or to power flagella that move the cells, or even as a
protection against metal toxicity from heavy metals like iron or copper.
