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INTRODUCTION
The
McMurdo Dry Valleys (MCM) represent the driest and coldest ecosystem known
and have, until relatively recently, been thought to harbor little life.
This ecosystem is comprised of a mosaic of glaciers, glacial streambeds,
exposed soils, and permanently ice-covered lakes; the only permanently
ice-covered lakes on Earth. The permanent ice eliminates wind-driven
mixing resulting in vertical transport at the level of molecular
diffusion, gas exchange between liquid water and the atmosphere, and
reduces light penetration. The lakes present the only habitat in this
ecosystem that contains permanent liquid water and supports year-round
metabolic activity in an environment that would normally appear to be
inhospitable to life. Lake levels in the MCM have changed dramatically in
response to climate variability since the last glacial maximum. The most
recent low stands of the lakes occurred ~1000 y ago when the lakes were
thought to be ice-free and nothing more than brine ponds. During the
recent evolution of these lakes, freshwater flowed over the salt brines
producing present day chemical stratification. Isotopic dating indicates
that the deep-water brines are more than 100,000 years old. The food web
of the lakes is dominated by prokaryotes and protists; few metazoans have
been observed. Biogeochemical studies on these lakes have revealed many
biogenic chemical gradients (e.g., N2O, CH4, DMS, DMSO) that lack simple
biochemical and thermodynamic explanations. These data beg one to ask if
the microorganisms present in the water columns today are responsible for
the geochemical gradients we now observe.
MO Venn Diagram
A
primary reason for establishing a MO for the dry valley lakes is to
understand not just how the environment controls the diversity of
organisms, but also how diversity itself controls the functioning of
ecosystems. Given the lack of metazoans, and the evolutionary history and
resultant geochemistry of these lakes, they offer a unique experimental
arena to search for novel microorganisms and study the interplay of
microbial diversity and ecosystem function. We will focus on prokaryotic
organisms within MCM lakes with the objective of elucidating those aspects
of their genome and metabolism that are critical to understanding their
role in biogeochemical cycles. Given the sensitive and relatively simple
systems in the dry valleys, our integrated approach will point the way
towards a broader integration of the biogeosciences.
STUDY
SITE
The
valleys harbor the only permanently ice-covered lakes on Earth. The lakes
occupy closed basins and vary in surface area (1-6 km2), depth (20-85 m),
and ice-cover thickness (3-5 m). The permanent ice covers greatly reduce
several aspects of normal lake physical chemistry including (1)
wind-driven mixing, resulting in vertical transport at the level of
molecular diffusion; (2) direct gas exchange between liquid water and the
atmosphere; (3) light penetration; and (4) sediment deposition into the
water column. The long mixing times mean that gradients of conserved
constituents exist in the water column for at least 20,000 years before
being dissipated by diffusion. Ecosystem properties in the water columns
of the lakes are also controlled by the seasonal uncoupling of
photoautotrophic and heterotrophic processes resulting from the unusual
solar cycle: 4 months of darkness followed by 4 months of continuous light
with twilight in between.
Overarching
Theme: To understand both how the environment controls the diversity
of organisms and how diversity itself controls the functioning of these
ecosystems.
Specific
Hypotheses:
1. Microbial communities in dry valley lakes will be
unlike those found in temperate lakes owing to demographic isolation,
constant low temperatures, decoupled light/dark cycles, an unmixed water
column, and strong legacy-derived geochemical gradients within the lakes.
2. Psychrophiles (Bacteria and Archaea)
isolated from the lakes will have unique phylogenetic lineages relative to
psychrotolerant species; the latter will be more closely related to
temperate prokaryotes.
3. Application of novel culturing methods will yield
organisms with unique physiological properties that allow for survival in
the dark, cold and saline environments that have existed in these lakes
for tens of thousands of years.
4. The major active nutrient cycles in the MCM lakes will
be revealed from the diversity and physiology of cultured representative
species obtained.
5. The application of thermodynamic principles to
known lake geochemistries can be used to predict the distribution of
different metabolic groups within the lakes.
6. In situ studies of extracellular
enzyme activity and cell viability can be used as a measure of microbial
and geochemical diversity in the lakes.
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