REVIEW OF LAKE VOSTOK STUDIES
Robin E. Bell
Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964,
p (914) 365-8827; f (914) 365-8179,
robinb@ldeo.columbia.edu
The identification of Lake Vostok in 1996 by Russian and British
scientists (Kapitsa et al., 1996) represented the culmination of
decades of data acquisition with a broad range of techniques
including ground based seismics, star observations, and airborne ice
penetrating radar supplemented by spaceborne altimetric
observations.
These measurements were the result of a long history
of investment in Antarctic research by the international science
community.
The initial discovery was subsequently complemented by
results from the Russian-French-American Vostok ice coring program
and the Russian Antarctic program. This review outlines the general
characteristics of the Lake, beginning with a description of the
overlying ice sheet, continuing to the lake itself and on into the
sedimentary deposits (Figure 3).
The horizontal extent of the Lake is estimated from the flat surface
(0.01 degrees) observed in the ERS-1 ice surface altimetry. The 4 km
thick ice sheet floats as it crosses the lake, just as ice sheets
become floating ice shelves at the grounding line. The flat ice
surface associated with Lake Vostok extends 280 km in the
north-south direction and 50-60 km in the east-west direction. Over
the lake the ice surface slopes from 3550 m above sea level in the
north to 3480 m above sea level in the south. The ice surface is ten
times flatter over Lake Vostok than in the surrounding regions.
The
regional ice flows in from an elevated feature known as Ridge B-C to
the west down the slope to the east. The presence of water may
significantly alter this flow (Robin, 1998). The flow rates across
Lake Vostok have been estimated from star sights at Vostok Station
in 1964 and 1972 (Kapitsa et al., 1996) and synthetic aperture radar
(SAR) interferonmetric methods (Kwok et al., 1998).
The star sights
at Vostok Station suggest primarily an easterly ice flow (142
degrees) at 3.7 m/yr . The SAR results indicate a significant
component of flow (2.22 m/yr) along the lake axis (Kwok et al.,
1998). As the overlying ice sheet is probably the major source of
sediments, microbes and gas hydrates in the lake, understanding the
trajectory of the ice across the lake will be critical to
understanding the lake as a system.
The present understanding of the 3750-4100 m of ice sealing Lake
Vostok comes from limited
airborne ice penetrating radar data acquired by a joint U.S.-British
program in the 1970’s, and
from the deep ice core drilling at the Russian Vostok Station by an
international team of scientists
from 1989 - 1998. The radar data, collected as part of a
reconnaissance survey of Antarctica,
provides cross-sectional images of the bedrock surrounding the lake,
the internal layering within
the ice, and the base of the ice over the lake for six flight lines.
Across the lake the reflection from
the base of the ice sheet is strong and very flat. In contrast,
reflections from portions of the ice
sheet over bedrock are characterized by rugged reflections of
varying strength that are dominated
by reflection hyperbolas. Radar data indicate that water within the
northern half of the lake may
be very shallow (~10-30 m) and that several bedrock islands protrude
through the lake into the ice
sheet.
The ice thickness is 4150 m in the north thinning to 3750 m
in the south beneath Vostok Station.
The ice core at Vostok Station was drilled to recover a record of
global climate changes over the past 400,000 years which is
preserved in distinct ice layers. Near the bottom of the core,
beginning at a core depth of 3311 m, the ice first shows signs of
disruption of the layering by ice dynamics. Generally ice layers
become tilted and geochemical climatic signals become difficult to
interpret (Petit et al., 1998, Duval et al., 1998).
This layer
between 3311 m and 3538 m has been interpreted as ice which was part
of the continuous ice column but has been disrupted by deformation
processes as the ice sheet moves over the underlying bedrock. The
randomly distributed moraine particles in the base of this section
are interpreted as an active shear layer. Below this layer, changes
in ice character are significant with a dramatic increase in crystal
size (to 10-100 cm), a decrease by two orders of magnitude in the
electric conductivity, the stable isotopic content of the ice and
the gas content.
These physical and chemical changes continue
through the base of the Vostok ice core at 3623 m and is interpreted
to represent ice accreted to the base of the ice sheet as it passed
over Lake Vostok. The upper 70 m of this large crystal ice includes
numerous mud inclusions approximately 1 mm in diameter. These 70 m
of “muddy” ice are interpreted to be ice accreted during a repeated
melting and freezing cycle along the lake’s margin.
Below the 70 m
of ice containing mud (i.e. below 3608 m) the ice is very clear and
is believed to have been formed as accreted ice as the ice sheet
floated over Lake Vostok. In this interpretation, the base of the
ice sheet consists of a layer of 227 m of disrupted ice, 70 m of ice
with mud inclusions and approximately 150 m of clear accreted ice.
A
freezing rate of several mm per year is required to generate these
layers of accreted ice.
Figure 3:
Cartoon of Lake Vostok indicating the ice flow over the
Lake near Vostok Station.
The melting and accreting processes are
indicated at the base of the ice sheet.
Arrows also indicate the
potential circulation within the lake.
The accretion ice is the
light blue layered material at the base of the ice sheet.
The
sediments (orange lined pattern) and hypothesized gas hydrates
(pebble pattern) on the lake floor are shown.
The Russian seismic experiments, led by Kapitsa in the 1960’s and by
Popkov in the 1990’s
(Popkov et al., 1998), provided insights into the depth of the lake
at the southern end of the Lake
and the presence of sediments. Interpretation of Kapitsa’s 1960’s
data is that 500 m of water exist between the base of the ice sheet
and the underlying rock (Figure 3).
These seismic experiments show
the base of the lake is 710 m below sea level.
This level is close
to the estimated level of 600 m below sea level for the northern
portion of the lake. Recent seismic experiments have confirmed the
early measurement of ~500 m of water beneath Vostok Station and
deeper water (670 m) several kilometers to the north.
These new
experiments also identified 90-300 m sediment layers close to Vostok
Station. Sediments were absent 15 km to the southwest. Leichenkov
used very limited gravity data to infer as much as 4-5 km of
sediments in the central portion of the lake (Leichenkov et al.,
1998).
Russian scientists (Kapitsa et al., 1996) have suggested that
Lake Vostok results from extensional tectonics, inferring that the
Lake has an origin similar to Lakes Malawi (Africa) and Baikal
(Russia) (Figure 4).
Figure 4:
Satellite images of several large lakes shown at the same
scale.
(a) An ERS-1 image of Lake Vostok (R. Kwok, JPL).
Lake Vostok
shows as the flat featureless region. In this image north is to the
right,
and Vostok Station is on the left of the image.
Both (b) and
(c) are AVHRR false color composite images.
Red indicates regions of
high thermal emittance, either bare soil or urban areas.
Green
represents vegetation, Blue primarily indicates clouds and black is
water.
(b) An AVHRR image of Lake Ontario, a glacially scoured lake
in North America.
Toronto is the red area at the western end of the
lake (left side of image).
An AVHRR image of Lake Malawi, an
active rift lake from the East African Rift system.
North is to the
right in this image.
This interpretation is based on the long narrow
nature of the lake and the bounding topography in some profiles. If
the extensional origin is correct, the lake may have thick sequences
of sediment, elevated heat flow, and hot springs.
Conceptual models of circulation within the lake have been advanced
by Zotikov (1998) and Salamatin (1998). These models are based on
the density differentials associated with variable ice thickness
across the lake. The poor understanding of the size of the lake, the
distribution of the melting and freezing regions and the geothermal
flux, limits the applicability of these models.
Finally, in terms of understanding microbes within the lake, the
overlying Vostok ice core contains a diverse range of microbes
including algae, diatoms, bacteria, fungi, yeasts and actiomycetes
(Ellis-Evans and Wynn-Williams, 1996). These organisms have been
demonstrated to be viable to depths as deep as 2400 m (Abyzov,
1993).
In summary, these data provide us with a general sense of the
horizontal scale of the lake and hints of the nature of the Lake’s
structure and origin, but many questions remain unanswered.
THE OVERLYING ICE - MELTING AND FREEZING
Martin J. Siegert
Bristol Glaciology Centre, School of Geographical Sciences,
University of Bristol, Bristol BS8 1SS, UK,
p. 44-117-928-7875; f. 44-117-928-7878,
m.j.siegert@bristol.ac.uk
The location and extent of Lake Vostok have been determined from
ERS-1 altimetry and radar sounding (Kapitsa et al., 1996).
The ice
thickness over the lake is 3740 m at Vostok Station and 4150 m at
the northern extreme of the lake. The ice-sheet surface elevation
decreases by ~40 m from north to south, whilst the base of the ice
sheet increases by ~400 m. The water depth is about 500 m at Vostok
Station (from seismic information) and a few tens of meters at the
northern end (from VHF radio-wave penetration through water).
The basal ice-sheet conditions that prevail over the lake have not
been previously identified. However, this information is required in
order to establish the environment within the lake and, from this,
the likelihood of life in the water.
A new interpretation of
internal ice-sheet layering from existing airborne 60 and 300 MHz
radar indicates that as ice flows across the subglacial lake,
distinct melting and freezing zones occur at the ice-water
interface. These events suggest a major transfer of water between
the ice sheet and lake, inducing circulation in the lake and the
deposition of gaseous hydrates and sediments into the lake.
The position of one airborne radar line (Fig. 5) is approximately
parallel to the direction of ice flow as derived from InSAR
interferometry and steady-state ice flow considerations (Siegert and
Ridley, 1998). Three individual radar layers, extracted from the raw
60 MHz radar data, were continuously traced across the lake.
The
change in ice thickness between the top two internal layers, and the
change in ice thickness between the lowest layer and the ice-sheet
base, were then calculated (Fig. 6).
Figure 5:
The position of one airborne radar line is approximately
parallel
to the direction of ice flow as derived from InSAR
interferometry and steady-state ice flow considerations.
Generally, over grounded sections of ice sheets, internal layers are
observed to converge and diverge in vertical sections as ice gets
thinner and thicker, respectively.
In contrast, if the grounded
ice-sheet base is flat, the internal layers tend to be flat in
response. Along a W-E transect across the middle of Lake Vostok, the
ice thickness is relatively constant and the ice-sheet base is very
flat (Fig. 6).
However, along this line, internal radar layers from
60 Mhz radar are (1) approximately parallel to each other and (2)
non-parallel to the ice base (Fig. 6). Any loss or gain in thickness
between the ice base and the lowest internal layer along the
flow-parallel transect probably reflects accumulation or ablation of
ice at the ice-water interface. In contrast, 300 MHz radar indicates
that compression of layering occurs in the top layers of the ice
sheet, where ice density changes cause internal reflections.
Other possible explanations for the pattern of internal radar
layering observed in the transect can be discounted. For example,
decoupling within the ice sheet (so that ice flow above the internal
layers is different from that below) is unlikely because of
negligible basal shear stress between ice and water.
Further,
convergent and divergent flow around the bedrock island (Fig. 6) is
not observed in the ice-surface velocity field derived from InSAR
interferometry.
Figure 6:
Calculation of the change in ice thickness between the top
two internal layers,
and the change in ice thickness between the
lowest layer and the ice-sheet base.
Divergent flow around the island in lower ice layers
would only cause ice thickening in adjacent regions.
However,
thickening of the ice sheet on either side of the island is not
observed in radar data. Furthermore, the internal layers do not
reflect ice flow around bedrock upstream of the lake because radar
data show that such ice structure involves deeper internal layers
diverging with increasing ice depth, whereas the layering in our
transect maintains a steady separation of internal layers across the
lake.
Assuming that ice does not accelerate across the lake (e.g. Mayer
and Siegert, submitted), the
ice velocity will be steady at around 2 m yr-1 across the transect
from west to east (left to right in Fig. 6).
The processed 60 MHz radar data can then be used to
determine rates of change of ice thickness between the lowest layer
and the subglacial interface. Assuming that there is neither lateral
flow nor compression of ice in the lower layers, these rates of
change of ice thickness may be related directly to rates of
subglacial melting or freezing (Fig.6).
Using this method, melting of up to 15 cm yr-1 occurs across the
first ten kilometers of the ice-water interface (Fig. 6d).
This zone is followed by a thirty
kilometer-long region of net freezing with an accumulation rate of
up to 8 cm yr-1 (Fig. 6d). These data, therefore, indicate
significant release of water from the ice sheet to the lake over the
first 10 km of the transect, which is followed by net refreezing of
lake water to the ice base.
Using these estimates approximately 400 m of basal ice will be
accreted to the base of the ice sheet as it traverses the central
portion of Lake Vostok. This compares to the 200 m of refrozen ice
observed 100 km to the south at Vostok Station in the narrow portion
of the lake (Fig. 5).
The melting of the ice sheet as it first
encounters the lake provides a supply of water, gas hydrates,
biological debris and sediments to the lake. The sediments and gas
hydrates will be deposited at the base of the lake, while the water
will be refrozen in the base of the ice sheet in the accretion zone.
The refrozen or accreted ice appears to be derived from freshwater
(J. R. Petit, pers. comm.).
This investigation indicates how basal ice-sheet conditions may be
identified from analysis of airborne radar data. However, the
present radar dataset is too sparse to provide a detailed analysis
of ice-sheet basal melting and freezing for the entire 14000 km2
area of the lake.
New radar data are therefore required to extend
this investigation over the full extent of Lake Vostok. Analysis of
new surveys will quantify the total volume of water involved in the
exchange between the ice sheet and the lake, and allow calculation
of the input of non-ice material to the lake.
This volume estimate
will supplement the glaciological parameters that radar measurements
will provide.
EVIDENCE FROM THE VOSTOK ICE CORE STUDIES
J. R. Petit
LGGE-CNRS, BP 96, 38402 St. Martin d’Hčres Cedex, France,
p. +33 (0)4 76 82 42 44, fax +33 (0)4 76 82 42 01,
petit@glaciog.ujf-grenoble.fr
As part of the long term Russian-American-French collaboration on
Vostok ice cores, started in 1989, the drilling of hole number 5G
was completed during the 97-98 field season. Ice coring reached 3623
m depth, the deepest ice core ever obtained.
The drilling operations
stopped 120 m from the ice/water interface to prevent contamination
of the underlying lake by kerosene based drilling fluid.
The ice core continuously sampled for paleoclimate studies and
discontinuous sections have been sent to selected laboratories in
three countries. Below 3350 m depth, one half of the main core was
cut as a continuous archive for future studies, and stored at -55°C
in an ice cave at Vostok station.
The very good quality and
transparency of the retrieved deep ice allowed for continuous visual
inspection of the ice inclusions, studies of ice crystals, and
measurements of electrical conductivity. Preliminary isotopic
measurements of the ice, (deuterium, dD), and analyses of the gas
and dust content have be performed on selected deep ice samples.
The upper 3000 m of the ice core (88% of the total ice thickness)
provides a continuous paleoclimatic record of the last 400,000
years. The preservation of this paleoclimatic record is due to the
slow velocities of the glacier ice and the low accumulation rates at
Vostok Station (presently 2 cm water equivalent per year).
Preliminary studies of the ice have yielded information on;
a) the
local temperature and precipitation rates (from isotopic composition
studies)
b) aerosol fluxes of marine volcanic, and terrestrial
origin (from chemical, ECM and dust content analyses)
c)
atmospheric trace gases (in particular the greenhouse gas content
[CO2 and CH4] and the isotopic composition of this “fossil” air)
d) the physical properties of the ice, including air hydrates,
ice crystals
The preliminary results of these studies indicate that
the main patterns of the Vostok temperature are well correlated to
global ice volume from deep sea sediments, back to the marine stage
11 (circa 400,000 BP) (Petit et al., 1999).
The record shows four
complete climatic cycles, including four ice age or glacial periods
associated with the development of large ice sheets over the
Northern Hemisphere, and four transitional warmer interglacial
periods (Petit et al., 1998).
Between depths of 3300 m and 3538 m, the layering is disturbed by
ice sheet dynamics. For
example, at 3311 m depth, three volcanic ash layers 10 cm apart are
tilted in opposite directions. Moreover, 10 m deeper, at 3321 m,
stable isotope content, gas composition and dust concentrations of
the ice, display very sharp and significant variations which cannot
be of climatic origin.
In these deep layers, the geochemical
parameters interpreted as climatic proxies can no longer be
interpreted as the glacial-interglacial cycles.
The observed values
are intermediate between glacial and interglacial levels, suggesting
the layers have been mixed. At the base of this ice there is
evidence of disruption due to ice sheet dynamics (3460 - 3538 m).
The ice contains randomly distributed moraine particles with
particle sizes up to a few millimeters in diameter, indicative of an
active shear layer.
Beneath these disturbed and apparently mixed layers, (below 3538 m)
the ice character
changes dramatically: ice crystals are very large (10-100 cm),
electrical conductivity drops by two
orders of magnitude, stable isotope content of ice shifts, and gas
content becomes two orders of
magnitude lower. These drastic and related changes, indicate that
the basal ice at this location is
re-frozen lake water. The accreted ice at the base of the Vostok
core is about 220 m thick, or 6% of
the total ice thickness.
The ice from the Vostok basin originates from the Ridge B area and
flows over the lake in a manner similar to an ice shelf. Temperature
in the ice sheet and melting or freezing events at the base are
linked to ice sheet dynamics and lake and bedrock heat fluxes.
Whilst Lake Vostok exhibits evidence of large scale melting, the
flow line passing through Vostok site indicates a significant
refreezing event. This provides a constraint that must be taken into
account when modeling the ice paths and dating the climatic record.
Sampling the lake and underlying sediments is necessary, but will
require the development of “clean” sampling techniques.
A
continuation of geophysical measurements in the existing bore hole,
and complementary studies of deep ice from Vostok, may provide
important insights into the ice sheet, regional geology and the
lake.
TECTONIC SETTING OF LAKE VOSTOK
Ian Dalziel
Institute for Geophysics, University of Texas, Austin, 4412
Spicewood Springs Rd.,
Bldg. 600, Austin TX 78759-8500,
p (512) 471.0431, f (512) 471-8844,
ian@ig.utexas.edu
Lake Vostok is located at 77°S, 105°E within the East Antarctic
Precambrian craton, remote (>500 km) from both the Neoproterozoic
rifted Transantarctic margin and the Mesozoic rifted margin south of
Australia and India. Its specific geologic setting is completely
unknown.
It has been suggested on the basis of limited geophysical data that
the Lake occupies a structural depression such as a rift (Kapitsa et
al., 1996).
Assuming this to be correct, several plausible scenarios
can be developed that would explain the tectonic setting of such a
depression in central East Antarctica:
Intracratonic Rift associated with Extensional Processes:
Given the
presence of the extensive Lambert-Amery aulacogen along the Indian
Ocean margin of the craton at 69°45’S, 71°00’E, Lake Vostok could
occupy an intracratonic rift valley comparable to the lakes of the
East African rift.
An aulacogen is a rift system penetrating a
craton from its margin. This could be either an active rift system,
as suggested by Leitchenkov et al. (1998) or an ancient and
tectonically inactive rift.
Despite the presence of a young volcanic edifice at Gaussberg, also
on the Indian Ocean margin at
66°48’S, 89°11’E, there is nothing to directly indicate present
tectonic activity in the Lake Vostok
area. Gaussberg is >1000 km distant and located at the termination
of the Kerguelen oceanic plateau.
The Antarctic continent is
anomalously aseismic, and only proximity to the Gamburtsev
Subglacial Mountains with their unusual 4 km of relief at 80°30’S,
76°00’E might be taken to indicate any local tectonic or magmatic
activity. These mountains, which do not crop out, could be like the
Cenozoic Tibetsi or Hoggar volcanic massifs of North Africa.
Again,
however, there is no direct evidence of recent, let alone active,
volcanism or tectonism in central East Antarctica.
Evidence from
sedimentary strata within the Lambert-Amery system suggests that
this aulacogen is of Paleozoic age, and may be the southern limb of
a rift in India that predates Mesozoic opening of the Indian Ocean
basin (Veevers et al., 1994).
Rift Resulting From a Continental Collision: A depression containing
Lake Vostok and the Gamburtzev Subglacial Mountains could be in a
setting similar to Lake Baikal and the Tien Shan Mountains or
Mongolian Plateau, i.e. a rift and intracratonic uplift associated
with transmission of compressive stress thousands of kilometers into
a continental interior as a result of collision with another
continent.
Unlike Lake Baikal, however, Lake Vostok is not situated
within a craton that has undergone Cenozoic collision like that of
Asia with India. Veevers (1994) has suggested that the Gamburtzevs
may have resulted from far-field compressive stresses associated
with the amalgamation of Pangea at the end of Paleozic times along
the Ouachita-Alleghanian-Hercynian-Uralian suture.
Alternatively,
uplift and rifting within the East Antarctic craton could have been
generated in the latest Precambrian “Pan African”
continent-continent collision of East and West Gondwanaland along
the East African orogen (Dalziel, 1997).
The early Paleozoic Ross orogen along the Transantarctic Mountain margin was a subduction
related event which is not likely to have transmitted compressive
stress far into the cratonic interior. Consideration of subduction-generated
Andean uplifts, however well to the east of the present Pacific
margin of South America, demands that this possibility also be kept
open.
Hot Spot or Mantle Plume Driven Depression: Plate tectonic
reconstructions maintaining the present day positions of the
Atlantic and Indian ocean basin “hot spots” such as Tristan da Cunha
and Reunion islands, indicate that several of these (notably
Crozet-Heard and Kerguelen) could have been beneath East Antarctica
prior to the opening of the Southern Ocean basins.
The Gamburtzev
Subglacial Mountains and an associated Lake Vostok depression could
owe their origin to such activity.
Glacial Scour possibly Eroding an Older Feature: An erosional origin
for the Lake Vostok depression, i.e. a Lake Ontario-type scenario,
is possible, but could also have its origin in tectonism. For
example, several of the Great Lakes occupy depressions formed during
the development of the North American mid-continent rift system at
1100 Ma that was excavated by the Laurentide ice sheet during
Cenozoic glaciation of that continent.
Meteor Impact: Circular depressions in the interior of cratons can
form as a result of meteor impact. Even the elongate depression
indicated by the shape of Lake Vostok could result from a bolide
impact scar modified by subsequent tectonism, as in the case of the
elliptical Sudbury basin in Ontario, Canada.
Hence the age of the depression that Lake Vostok appears to occupy
could have resulted from a variety of tectonic causes, and could
range in age from Precambrian to Recent. At present, there is no
evidence to indicate that the setting is tectonically or
magmatically active.
Several lines of investigation should be
undertaken to clarify the tectonic setting, and hence the likely
history and possible present activity of the feature:
1. Airborne geophysical survey
of the region surrounding the lake
2. Seismic refraction profiling to ascertain the deep crustal
structure beneath the lake
3. Seismic reflection profiling to determine the shallower
structural setting, nature of the sedimentary fill, and relation to
overlying present ice sheet and its base 3 Comparable geophysical studies of the Gamburtzev Subglacial
Mountains
4. Sampling of the Gamburtzev Subglacial Mountains by
drilling - evidence of a young volcanic construct locally would
dramatically change the geologic picture.
EXPLORING MICROBIAL LIFE IN LAKE VOSTOK
James M. Tiedje
Center for Microbial Ecology, Michigan State University,
540 Plant and Soil Science Building, East Lansing, MI 48824-1325,
p (517)-353-9021, f (517)-353-2917,
tiedjej@pilot.msu.edu
Microorganisms have been on Earth at least 3.7 billion years and
during this evolutionary history have developed incredible
biochemical, physiological and morphological diversity.
Members of
the microbial world encompass the three domains of life, the
Bacteria, the Archaea, and the lower Eukarya.
This diversity
encompasses organisms with novel redox couples for production of
energy; adaptations to extremes of temperature, salt, and pH; novel
energy acquisition mechanisms as well as strategies for withstanding
starvation. About 4,200 prokaryotic species have been described out
of an estimated 105 to 106 prokaryotic species on Earth. Many of the
extant microorganisms have not been cultured in the laboratory and
hence remain unknown because we apparently cannot reproduce their
environment in the laboratory.
Conditions in Lake Vostok are not so severe as to make microbial
life impossible. Hence, at least some forms of microorganisms should
exist in Lake Vostok water and sediment.
The founding populations
(original inoculum) could come either from the rock or sediment
prior to ice cover, or from microbes trapped in the ice that are
slowly transported through the ice to the water. In either case,
Lake Vostok microbes would have been isolated from their global
relatives for at least 1 million years.
Some changes in genotype and
even phenotype could have occurred during this time, presumably
making the organisms more adapted to this cold, dark, oligotrophic
environment. The time scale of 1 million years, however, is not long
in terms of prokaryotic evolution when compared to their 3.7 x 109
year history. As points of reference, the E. coli-Salmonella
enterica genospecies, which are closely related organisms but
differentiated because of their health importance, are considered to
have diverged only in the last 100 million years (Lawrence and
Ochman, 1998).
Hence, species level differentiation may take at
least 10-100 million years. Secondly, changes due to mutation
(silent mutants) occur at the rate of approximately 5 x 10-10 per
base pair (bp) per replication (Drake et al., 1998). Assuming an
average gene size of 103 bp and 10 generations per year, one would
expect on average a change in only one base pair per gene in the 1
million years since Lake Vostok microbes have been isolated from
their relatives.
Other mechanisms of genetic change, especially
recombination and mutator genes, could have altered organism
phenotype more rapidly allowing for adaptation to Lake Vostok
conditions.
The above discussion is based on the conservative
estimate of biological isolation by the ice cover of 1 million
years. If the original inoculum were derived from rocks or sediments
that had been sealed from surface microbial contamination pre-Lake
Vostok, their age of isolation would have been longer, probably
35-40 million years. It should be noted that this form of isolation
is not unique to Lake Vostok rocks.
The major biological questions to be addressed in Lake Vostok would
appear to be the following:
-
Who (what taxonomic groups) lives there?
-
How different are the Lake Vostok organisms from what we already
know?
-
Who are the Lake Vostok organisms related to and from what
habitats do these related
organisms arise?
-
Which of the Lake Vostok organisms are metabolically active?
-
How do these organisms live in this unique environment?
-
Where do
they get their
energy (geothermal?, clathrates [gas hydrates]?, other?), and do
Lake Vostok natives
have special adaptive strategies for this environment?
Microbial exploration of a new ecosystem such as Lake Vostok should
include three complementary approaches since each gives unique and
vital information: nucleic acid-based methods, microscopy, and the
isolation-cultivation approach.
The nucleic acid-based methods
provide much more comprehensive information on the community than
culture-based methods and, through sequencing of small subunit
ribosomal RNA genes (SSU rRNA), provide information on the
organism’s identity.
rRNA-based methods such as sequencing of clone
libraries, fluorescent terminal restriction fragment length
polymorphism (T-RFLP) analysis, denaturing gradient gel
electrophoresis/temperature gradient gel electrophoresis (DGGE/TGGE),
fluorescent in situ hybridization (FISH), and quantitative
hybridization by phylogenetic group probes, are well proven methods
for exploring the microbial community of new habitats such as Lake
Vostok.
Other phylogenetically important genes such as 23S rRNA,
intergeneric spacer regions and gyrB may also be useful. Once pure
culture isolates are obtained, reverse sample genome probing (RSGP)
can be used to quantify the importance of isolated organisms in the
total community.
Microscopy remains a powerful exploratory approach because it is the
best method for comprehensive observation and quantification of the
microbial community.
New forms of microscopy such as confocal laser
scanning and environmental scanning electron microscopy, as well as
coupling microscopy with the use of fluorescent probes of various
types can reveal key information both on organism’s identity as well
as on their activity.
Isolation and cultivation of pure cultures remains the primary means
to fully characterize a microorganism, including its metabolic
capacity, unique physiology, confirming its taxonomy and for studies
at the molecular level. An example of the latter could be to
identify genes responsible for adaptation to cold, genes potentially
useful to making plants more winter hardy.
Strategies that might be
useful for cultivating Lake Vostok organisms would be to minimize
the shock of warming, matching the ion composition of the medium to
the lake water, maintaining oligotrophic nutritional conditions yet
stimulating growth, and planning for a long incubation period.
Special challenges for the study of Lake Vostok microbes would
likely include the following. Very low densities of microbes, which
is probably the case in Lake Vostok, always requires special
methodologies to concentrate cells. Furthermore, risk from
contamination from outside microbes is more problematic.
Determination of the metabolically active cells versus resting or
dead forms, is especially difficult at low temperatures because of
the low metabolic rate. Isolation and cultivation of oligotrophic
microbes is always difficult. The more interesting microbes are
likely to be the ones most difficult to cultivate and isolate. It
may be difficult to determine whether what is found is really new
and unique since so many of the world’s microbes remain unknown. To
answer this question one may have to seek “Lake Vostok-like”
relatives outside of Lake Vostok once the former are characterized.
Abyzov and colleagues have studied microbes in the Vostok ice core
by microscopy and cultivation (Abyzov et al., 1998).
They find low
densities (103 cells/ml) of microbes in the ice core extending to
ages of 240,000 years, the oldest period on which they have
reported. Microbial density fluctuated with ice core age, being
higher when the dust particle density is high, which also
corresponds to periods of greater atmospheric turbulence.
Bacteria
were the most prevalent microbial cells, but yeast, fungi,
microalgae, including diatoms, were also seen. Thawed ice samples
assimilated 14C-amino acids establishing that some of the cells were
alive.
Most of the organisms that were isolated from the ice core are
spore-formers, e.g. Bacillus. Attempts to isolate more oligotrophic
types apparently have not been made. Organisms from the ice core
could be one source of inoculum to Vostok Lake.
Studies on the microorganisms of Antarctica and buried Arctic
permafrost soils have relevance to Lake Vostok questions. Culturable
strains from 1 million year old buried arctic permafrost soil belong
to the Planococcus, Psychrobacterium, Arthrobacter, and
Exigobacterium groups. It is interesting that the closest relatives
of some of these strains are found in Antarctica.
Some of the
ancient arctic isolates grow relatively rapidly at -4.5°C. Hence,
growth rate at the Vostok temperature of -3.2°C would not appear to
be a limitation. The major limitation to microbial density in Lake
Vostok would be a renewable supply of energy.
If clathrates (gas
hydrates) were present, the potential microbial use of this energy
source would be particularly intriguing.
LAKE VOSTOK PLANETARY ANALOGS
Frank Carsey
California Institute of Technology Jet Propulsion Laboratory
JPL ms 300-323, 4800 Oak Grove Dr., Pasadena CA 91109,
p (818) 354-8163, f (818) 393-6720,
Frank.D.Carsey@jpl.nasa.gov
About the time that the true scale of Lake Vostok was generating
excitement in the Earth Science community, spacecraft images and
other data of the Galilean satellites of Jupiter similarly
electrified the Planetary Science community, and for a similar
reason:
in both cases strong evidence was suddenly provided for
large, previously unknown bodies of water which might well be home
to unique life forms.
As of this writing, large, old, subsurface
oceans are suspected on both Europa and Callisto, and water ice is
known or speculated to occur in a great number of other sites,
including Earth’s moon.
Meanwhile, the microbiologists are
revolutionizing the picture of biodiversity of life on Earth and
repeatedly astounding the scientific community and the public with
information on microbes thriving in sites long considered untenable
for life.
These developments are obviously interrelated; it is clear
that explorations of Lake Vostok and Europa/ Callisto have much in
common, including the scientific excitement of exploring a new
place.
The chief similarity is in the primary scientific goals at Lake
Vostok and the Jovian satellite oceans, the search for life. In the
Jovian system, this search must be carried out robotically, and the
robotic approach has much to offer in various sites on Earth where
such issues as contamination prevention and remoteness make sample
removal challenging.
Lake Vostok, in particular, is a site in which
low temperatures, high pressures, low salinity, isolation, and great
age indicate an oligotrophic environment.
This suggests that life
could occur in highly specialized microbial communities with low
populations.
This situation may not be representative of
Europa or
Callisto, as these sites may be prebiotic. However, the exercise of
locating and examining life in small numbers is clearly excellent
preparation for sites which may have no life forms at all. The
scientist will be testing a system trying to establish a negative,
which is demanding. Similarly, at both Earth and planetary sites,
the issue of evaluating habitat and bioenergy sources will be
crucial.
In addition to the physical and scientific similarities, the
technologies required for accessing
and studying the liquid water domains at Lake Vostok and Europa/Callisto
have numerous
elements in common, many of them quite challenging.
Both sites
require vehicles that can move
through great distances of ice, 4 to 10 km vertically; both sites
require communication of data
through the ice and water; both sites require sophisticated
instrumentation to locate and describe
life and evaluate habitats; and both sites call for exploration with
little basic data on site
characterization as they are unknown places.
In addition, it is
worth noting that when a NASA
mission goes to a planetary site it can take only the smallest
quantity of equipment, yet it must do
a sophisticated job. These kinds of capabilities could greatly
benefit Earth-bound science,
especially in polar regions, as the investment in on-site support
could be dramatically reduced, and more of the agency resources
could go into science.
Additional sites exist on Earth with key
similarities to both the deep ice sheet and the oceans of Europa/Callisto,
e.g., the deep ocean. Timely and interesting projects that promise
multi-use developments for all three sites include observations of
clathrates, high pressure habitat characteristics, and
microbiological studies.
There is clear benefit in collaborative efforts of U.S. and foreign
agencies concerned with cold-region science and operations, aqueous
instrumentation and robotics, high pressure/low temperature
processes in water and sediment, and extremophile biology. There are
programmatic vehicles in place to initiate and coordinate these
collaborations, NSF, NASA, the Polar Research Board and the
Scientific Committee for Antarctic Research.
Communications with and
among these agencies should be encouraged.
IDENTIFICATION OF LIFE
David C. White
University of Tennessee, ORNL, JPL, 10515 Research Dr., Suite
300Knoxville TN 37932-2575,
p (423) 974-8001, f (423)974-8027,
Milipids@aol.com
Lake Vostok as a pristine, cold, dark, high-pressure, and large lake
provides a new extreme environment in which to search for indigenous
microorganisms that have been isolated from the rest of the
biosphere for a long time.
Thus it is of paramount importance to
prevent contamination of the lake by organisms from the overlying
ice or contaminants introduced by the sampling device during the
assessment process.
The parallels to the detection of life on the Jovian moon Europa with a thick ice layer provide an excellent venue
for monitoring the Planetary Protection technologies’ life detection
through a thick ice cover. The technologies discussed below were
derived for use within the space program, but are applicable to the
Lake Vostok exploration project.
The cleaning, sterilization, and validation technologies for
extraterrestrial life detection require extraordinary “instrument”
protection.
Since the life forms that might be encountered may not
conform to the rules of life as currently understood, the JPL
Astrobiology team under Ken Nealson has defined the criteria for
life as having some essential characteristics that form the basis
for life detection:
1. Life detection technology will require mapping those localized
areas of heterogeneities in the distribution of biomarkers between
the putative life forms and the background matrices. These localized
areas of putative life forms must also show concentrations of
biomarkers and state conditions far from chemical equilibrium in the
components of cells, macromolecules, smaller molecules, and/or
elements. The system requires mapping in space and time to
demonstrate localization of these heterogeneities and their
metabolic activities.
2. The system must have an exploitable energy
source and this source for extraterrestrial life may be
non-traditional. Non-traditional energy sources could be tidal,
radiation, heat, wind, or magnetic, not typical of the visible solar
or chemosynthetic redox driven energy systems currently understood.
3. Whatever the system, the basic chemistry must be
thermodynamically feasible.
These broad constraints indicate that these missions will require
much more comprehensive
“instrument” cleaning than the Viking standard of 300 viable
spores/m2.
This was considered
adequate twenty years ago when the entire spacecraft was held at
112° C for a long period so that
no cells known on Earth were known to survive the treatment. This
was prior to the discovery of
the hyperthermophilic Archaea from the deep oceanic hydrothermal
vents.
The sterilization technologies currently under examination at JPL
utilize hydrogen peroxide under pressure (oxidative sterilization)
and low temperature non-oxidative use of supercritical fluid or
other solvents that result in cell lysis, leaving no organic
residues. The hydrogen peroxide yields water and oxygen.
Not only
must the critical areas of the spacecraft be sterile they must be
cleaned of biomarkers that could interfere with the detection of
life. Life detection will be based in part on detection spatial
heterogeneities in concentrations of biomarkers.
The JPL efforts in “instrument” cleaning are currently exploring in
situ destruction techniques utilizing ultra-violet with photodynamic
activation and deep ultra-violet delivered in a vacuum.
This is used
in combination with various types and recovery techniques more
effective than the previously employed cotton swab with 70% aqueous
alcohol at room temperature and pressure. Whatever the technology
utilized for cleaning, the residue left on the “instrument” after
cleaning must be analyzed quantitatively, structurally identified,
and mapped.
Validation of the cleaning will require detection of
biomarkers in cells, macromolecules, and small molecules. Cells will
be detected and mapped microscopically and live/dead determinations
made. These are currently compared to traditional viable culture
methods that are required for flight. Nucleic acid macromolecules
will be determined by polymerase chain reaction (PCR) of various
nucleic acid polymers and enzymes that detect their activity.
Small
molecule detection will exploit diagnostic lipids. Lipids can
quantitatively indicate viable biomass by differentiating the polar
phospholipids, which are lysed by endogenous phospholipases during
cell stress forming diglycerides.
The nutritional/physiological
status, as well as the community composition, can be determined by
analysis of specific lipid components, which with HPLC/electrospray
ionization/tandem mass spectrometry can be detected at the subfemtomolar levels (approaching detection limits of a single
bacterial cell).
Spores can be detected in this system by their dipicolinic acid content. Lipid analysis has the potential for
automation and speed by the application of enhanced solvent
extraction at high pressure saved temperatures. Components like
amino acids, carbohydrates, nucleotides can be detected at
subfemtomolar concentrations by capillary electrophoresis which has
great potential for miniaturization.
There is a possibility of using
tracer biomolecules labeled with several isotopes at unusual
concentrations that can be clearly identified. These techniques
would provide a direct estimate of the degree of contamination after
the cleaning procedures have been completed.
The JPL program currently utilizes modifications of extant
analytical detection methods and equipment to analyze “coupons”
exposed on the “instrument”. (Coupons or “witness plates” are
recoverable surfaces on or around the spacecraft that are exposed
and then removed for analysis; they can also be used to test various
cleaning methods by putting a known contaminant mixture on them and
then analyzing the biomarkers after treatment.)
Alternative recovery
methods of solvent or adhesive polymers like the Scotch tape 5414
used in forensic investigations are being explored. A proposed
second level of analysis would involve direct detection from the
“instrument” using soft X-rays, Raman, infrared, or fluorescent
detectors that could be mapped on a virtual “instrument” and
successively cleaned.
The next level would be on-line reporting of
in situ biosensors built into the “instrument”. These would be
developed into the in situ life detection systems that monitor the
extraterrestrial site and validate planetary protection.
Significant research remains to be done and adequate methods need to
be in place by 2000 if the new methods are to be used during sample
return missions.
International collaboration with industries,
academia and the government will be required to fulfill the
responsibility to protect Lake Vostok from contamination.
MICROBIAL CONTAMINATION CONTROL
Roger G. Kern
Technical Group Lead/Planetary Protection Technologies
Mail Stop: 89-2, Jet Propulsion Laboratory, 4800 Oak Grove Drive,
Pasadena, CA 91109
p (818) 354-2233, f (818) 393-4176,
Roger.G.Kern@jpl.nasa.gov
The Jet Propulsion Laboratory’s Planetary Protection Technologies
Group is currently assessing the feasibility of entering Lake Vostok
without introducing new types of microorganisms into the lake.
Since
the inception of robotic missions to the Mars surface, Viking
Landers 1 and 2 in the mid 1970’s, JPL has had an interest in this
specialized type of microbial contamination control.
The objective
of the Vostok microbial contamination protection research is to
prevent contamination of Lake Vostok with viable microbes from the
Earth’s surface while enabling the robotic exploration of the lake.
The Vostok contamination control challenge is composed of three
parts:
1) delivery of a clean and sterile probe to the ice surface 4
km above the lake
2) preventing contamination of the probe as it is
lowered down a warm water drilled bore hole to within a few hundred
meters of the lake surface
3) performing a sterilization event
upon entering the ice at the base of the bore hole to enable the melter probe to proceed without introducing viable surface
microorganisms into the lake
Microorganisms present in the ice
immediately above the lake are constantly raining into the lake as
the ice melts, at an estimated rate of 1 to 2 mm per year, and are
therefore not considered contamination in this approach.
An
environmentally benign chemical sterilization is being tested that
could take place at the base of the bore hole and would permit entry
into the ice above the lake without entraining viable microbes from
the surface.
JPL is currently adapting methods under development by the Mars
Exploration Technology Program for application to aqueous
environments such as Lake Vostok and the suspected Europan ocean.
For future exploration of the surface of Mars, JPL is currently
evaluating basic decontamination approaches for the efficacy against
microbial cells and molecular cell remnants; proteins, nucleic
acids, lipids, and carbohydrates.
These initial studies have focused
on hardware surface cleaning to remove materials of biological
origin from all surfaces both inside and outside the probe. Cleaning
techniques being evaluated at JPL include: hydrogen peroxide plasma
sterilization; 70% sterile ethanol wash; and existing precision
cleaning methods.
Sterilization techniques being evaluated at JPL
include: hydrogen peroxide plasma; gamma irradiation; and a dry heat
procedure developed for the Viking mission to Mars.
At present four methods for characterizing biological contamination
are being evaluated for use in verifying the level of cleanliness of
hardware. The first of these is a viable count assessment based on
the ability to remove and culture a single viable organism on
tryptic soy agar.
The second method does not require microbial
growth since it is widely recognized that less than 1% of the total
microbial world is currently culturable. Epifluorescent microscopy
is being adapted for validating microbial cleanliness. Microbes
sampled are transferred to a 0.2 micron filter where cells are
suitably stained to enumerate the total population as well as
confirm the absence of viability. This allows the assessment of the
microbial population independent of ability to culture in the
laboratory.
PCR techniques are being employed to detect the presence
of trace amounts of DNA associated with the sampled surfaces.
Recently capillary electrophoresis has been added to JPL’s list of
approaches for determining the presence or absence of trace
biological molecules associated with hardware. This research into
cleaning and sterilization methods, as well as techniques to
validate cleanliness is ongoing, and new approaches are constantly
being evaluated to achieve and assure a level of cleanliness and the
absence of viable microbes.
These ongoing planetary protection efforts can be applied to the
NASA Vostok Probe (consisting of a cryobot and hydrobot) and
instrumentation, and the overall mission design.
The current
planetary protection technologies research effort will influence the
selection of materials compatible with cleaning and sterilization
procedures. Recommendations are awaiting results that are expected
in 1999. Materials compatibility studies could lead to the
co-location of components with similar cleaning and sterilization
constraints (i.e. electronics, optics, chemical sensors).
The protection of Lake Vostok presents challenges new to NASA, since
the probe does not transverse sterile space, but rather a water
column containing viable surface organisms and ice containing a very
low level of viable spore forming microbes.
The mission sequence
will be determined by unique forward biological contamination
constraints.
The current mission approach calls for a sterile biobarrier capable of permitting pressure equalization, to deliver
the probe to the base of a warm water drilled bore hole. Prior to
further descent of the probe by ice melting, an antimicrobial
oxidizing agent would be employed to kill organisms present at the
base of the bore hole.
At present, experiments are underway to assess the application of
30% concentrated hydrogen peroxide (H2O2) to sterilize both the
water at the base of the borehole as well as the ice surfaces around
the probe. Freezing point suppression caused by the release of H2O2,
results in the melting of ice at the bore hole base and formation of
a sterile slush as the H2O2self dilutes to a concentration
permitting the solution to freeze.
Using this approach it may be
possible to execute a surface sterilization event at the base of the
bore hole in the ice, hundreds of meters above the lake. A straight
forward experimental design to test the efficacy of ice formation in
situ with respect to H2O22 concentration, temperature and time, is
planned to evaluate this approach.
The ability to enter the Lake without contamination that could
impact either the environment or the scientific goals of the
mission, will require stringent cleaning, sterilization and
verification methods.
The proposed mission sequence for the Vostok melter probe calls for a sterilization event to occur at the base of
the bore hole that will enable the already sterile probe to leave
its biobarrier, pass through sterilized ice, and proceed to the
lake’s surface entraining only those living microorganisms that
naturally rain into the lake as the glacial ice melts.
The only
organisms recovered by culture to date from deep drill cores at
Vostok station are spore-forming bacteria and actinomycetes although
others may be present and as yet not detected.
