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By Peter Rejcek
Alison Murray studies tiny critters with a potentially
big role in the marine ecosystem of the Southern Ocean
surrounding Antarctica.
Murray is interested in how bacterioplankton, the bacterial
component of plankton communities, make their living in
the Antarctic winter waters, when the long austral night
presumably shuts down or slows many biological processes
that rely on solar energy.
Murray and her colleagues, who include several international
collaborators, as well as Joe Grzymski, a colleague at
the Desert Research Institute, and Hugh Ducklow, principal
investigator for the Palmer Long Term Ecological Research
(PAL LTER) program, will head to Palmer Station in July,
the middle of winter. From the U.S. Antarctic Program
(USAP) research station, they will collect and analyze
the marine microorganisms, which also include a form of
life distinct from bacteria called archaea.
“The concept for the project is trying to understand
what is unique about the organisms that are adapted to
Antarctic winter,” explained Murray during a phone
interview from her office at the Desert Research Institute
in Reno, Nev.
An International Polar Year (IPY) project, the study
will focus on one of the key themes of the two-year-long
polar science campaign: life in the cold and dark. Murray’s
proposal dovetails nicely with a series of field projects
she conducted earlier this decade from Palmer Station
to characterize bacterial diversity and study their gene
expression under a program focusing on extreme environments.
Genetic expression studies what genes for an organism
are “turned on” or “turned off” under
different environmental conditions.
Grzymski explained: “If an organism is really stressed
out, it ‘turns on’ stress-response genes and
‘turns off’ genes that detract from the stress
response.
“Gene expression studies monitor what the cells
are actually doing; it is one step beyond identifying
the key players in the Antarctic winter and summer bacterioplankton
communities,” he added.
For example, organisms that live in ice and subzero temperatures
might need to “turn on” genes that encode ice-binding
proteins during a particularly low-temperature day in
the sea ice, while simultaneously down-regulating, or
“turning off,” genes involved in growth and
metabolism to deal with the stresses due to ice crystal
formation, which can break cell walls.
The marine ecologists are particularly interested in
figuring out how gene expression varies from season to
season. From Murray’s previous work, they already
know that composition and diversity of these microorganisms
varies greatly from summer to winter.
Murray said the question then becomes, “What is
different between the organisms that dominate the plankton
during austral winter and summer?” The answer lies
in genome sequences of these organisms.
Ducklow noted Murray’s previous work was instrumental
in telling the researchers something about the composition
of bacterial communities. It’s only in the last 10
years, he said, that researchers have been able to make
such distinctions thanks to advances in molecular ecology
and genomics, the study of genes that reveal biological
diversity and the entire DNA sequence of an organism.
“We just worked on ‘bacteria’ without
any real attention to which ones were doing what,”
explained Ducklow, a marine ecologist and co-director
of The Ecosystems Center at the Marine Biological Laboratory
in Woods Hole, Mass. “With new genomic tools, we
can really probe the composition of that community.”
Ducklow used the following analogy to explain the point
further: Imagine walking in a forest. You see these things
you know are trees, but you can’t tell one from the
other — you can’t distinguish a pine tree from
a birch, for example.
“That’s the way we were with bacteria until
quite recently,” Ducklow said. “So now we’re
able to see what the changes are, and also be able to
start investigating processes of how they’re able
to change over the season and from place to place.”
Murray helped solve that problem using a molecular biology
tool called denaturing gradient gel electrophoresis. Basically,
she developed a genetic barcode for the bacteria in the
plankton community. Using samples collected over an entire
year, her team was able to compare the genetic barcodes
from summer and winter.
“There is really large turnover,” Murray said
of the seasonal comparison. “The barcode completely
changed over that annual cycle, more so than anywhere
else I’ve worked.”
They also learned that bacterial diversity declines in
summer, while the richest diversity occurs in winter.
The finding is interesting because microorganisms are
presumably more active in summer than winter, as they
participate in various biological processes, such as the
cycling of carbon and nitrogen. One might expect diversity
to increase, not decrease, in the summer.
“Maybe the summer conditions are a little rougher
on the bacterioplankton than we previously acknowledged,”
Murray said. “I think [increased winter diversity]
is counterintuitive and something we’re looking into.”
Microbes one key component of carbon, nitrogen cycles
in the ocean
In the spring and summer, bacteria are involved in an
incredible amount of metabolic activity, decomposing organic
matter produced through photosynthesis by phytoplankton,
and turning it back into carbon dioxide. Without the recycling
activities of bacteria and zooplankton, the bulk organic
matter would sink into the deep ocean, isolating the CO2
from the atmosphere.
“What’s opposing the sinking process is microbial
respiration, which is turning the organic matter back
into CO2 before it can sink out,” Ducklow explained.
“Part of the system is trying to put it into the
deep sea, while the other part is trying to burn it back
out into the atmosphere.
“Bacteria represent one of the most important compartments
at the basis of all food webs in marine ecosystems,”
said Jean-Francois Ghiglione, a researcher at the Institution
Centre National de la Recherche Scientifique in Paris.
He is one of the international collaborators for the IPY
project at Palmer Station.
“An estimated 20 to 50 percent of marine primary
productivity is channeled through bacteria,” he added.
“They respond clearly to environmental changes, and
participate in all biogeochemical cycles. Therefore, they
have many advantages to serve as a basis for any long-term
oceanic observation systems.”
The scientists also want to learn more about how archaea
are involved, particularly in the carbon and nitrogen
cycles. Archaea are single-celled microorganisms that
“look” like bacteria, but are actually more
closely related to complex, multi-cellular organisms such
as plants.
They were discovered in frigid coastal Antarctic waters
by scientist Ed DeLong in 1994. In the mid-1990s, as a
graduate student, Murray worked with DeLong at the University
of California, Santa Barbara, doing some of the first
molecular research on Southern Ocean microorganisms.
“At the time, it was quite surprising to see them
down there because we didn’t know their global distribution,
and they were commonly thought to inhabit extremely hot,
salty, and often anoxic [lacking oxygen] environments,”
Murray said. Now they know archaea are one of the most
abundant single-celled organisms in the oceans, their
sheer biomass accounting for upwards of 40 percent of
the cells in the deep ocean.
However, distribution of archaea in the Southern Ocean
is different from other marine environments, Murray said.
“It’s one of the only places in the world where
we find them in the surface waters.”
Again, it appears to be a seasonal thing, as the archaea
are found in the deeper parts of the ocean in the summer
and inhabit the upper water column from late fall to early
spring. “That gave us a clue that things really change
down there seasonally,” Murray said.
The seven-member science team will work from July through
September collecting their samples and analyzing the organisms
right at Palmer Station, including the genomic work. Ducklow’s
graduate student Kristin Myers and technician Matthew
Erickson also collected samples during this austral summer
for comparison.
Though their tools are innovative, there’s nothing
too high-tech about how they collect the bacteria and
archaea. The technique will depend largely on the condition
of the sea ice in the winter. Open ocean means the team
can use Zodiacs, inflatable boats, to collect samples.
The presence of sea ice will require team members to ski
to designated sites and drill holes through the ice to
reach the water.
“We have to be flexible because those conditions
change at Palmer, and they’re not exactly predictable,”
Murray said.
The scientists use big carboys, similar to what a homebrewer
uses to make beer, to collect seawater, which can contain
about 200,000 cells per milliliter. The team will work
in an environmentally controlled room at the station,
where the temperature will be that of seawater, about
minus 1.8 degrees Celsius.
A filtration system squeezes out the water, leaving all
the critters behind. “We get a kind of microbial
soup,” Murray said.
The researchers will share that soup, using samples for
different purposes “to crack the door open on processes
rarely studied in the high latitude waters of the Southern
Ocean,” Murray said, as they describe genomic diversity,
gene and protein expression between summer and winter,
as well as investigate the survival adaptations of wintertime
bacterioplankton.
“It will be a busy day each time we go out,”
she added. “You can’t quite understand the ecosystem
ecology down there without having a seasonal perspective.”
And given the international flavor of the project, the
scientists expect to learn something about how different
programs operate.
“The IPY project is a wonderful opportunity to work
together with our American colleagues, to share our experience,
and get new insight on this extreme and very fragile environment,”
Ghiglione said.
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