Christner et al., 2008

-BACK TO TOP-

Despite the integral role of ice nucleators (IN) in atmospheric processes leading to precipitation, their sources and distributions have not been well established. We examined IN in snowfall from mid- and high-latitude locations and found that the most active were biological in origin. Of the IN larger than 0.2 micrometer that were active at temperatures warmer than -7°C, 69 to 100% were biological, and a substantial fraction were bacteria. Our results indicate that the biosphere is a source of highly active IN and suggest that these biological particles may affect the precipitation cycle and/or their own precipitation during atmospheric transport.

Christner et al., 2006

-BACK TO TOP-

Subglacial Lake Vostok is located ,4 km beneath the surface of the East Antarctic Ice Sheet and has been isolated from the atmosphere for >15 million yr. Concerns for environmental protection have prevented direct sampling of the lake water thus far. However, an ice core has been retrieved from above the lake in which the bottom ,85 m represents lake water that has accreted (i.e., frozen) to the bottom of the ice sheet. We measured selected constituents within the accretion ice core to predict geomicrobiological conditions within the surface waters of the lake. Bacterial density is two- to sevenfold higher in accretion ice than the overlying glacial ice, implying that Lake Vostok is a source of bacterial carbon beneath the ice sheet. Phylogenetic analysis of amplified small subunit ribosomal ribonucleic acid (rRNA) gene sequences in accretion ice formed over a deep portion of the lake revealed phylotypes that classify within the b-, g-, and d-Proteobacteria. Cellular, major ion, and dissolved organic carbon levels all decreased with depth in the accretion ice (depth is a proxy for increasing distance from the shoreline), implying a greater potential for biological activity in the shallow shoreline waters of
the lake. Although the exact nature of the biology within Lake Vostok awaits direct sampling of the lake water, our data from the accretion ice support the working hypothesis that a sustained microbial ecosystem is present in this subglacial lake environment, despite high pressure, constant cold, low nutrient input, potentially high oxygen concentrations, and an absence of sunlight.

Priscu et al., 2006

-BACK TO TOP-

Glacial ice covers more than 15 x 10⁶ km² of our planet and has a role in driving all processes on Earth. Glacial ice also contains an important reservoir of information on past climatic events extending back at least one million years. Paleoclimatologists have used this information to determine past climate changes and to predict what changes may occur in the future. Despite the wealth of information trapped in ice cores, little information exists on the microorganisms immured in the ice. Recent discoveries over the past decade have shown that glacial ice contains an important record of microorganisms on our planet that can be used to assess biogeochemical processes and habitat types that occurred during past glacial and interglacial periods. This record may also contain important information on microbial evolution and physiology, and provide new biomedical information on pathogens. It is important that biologists be included in future ice coring efforts if comprehensive views of past conditions on Earth are to be obtained. Such information will benefit all sciences involved in deciphering the ice core record and will provide the necessary information in our search for life on other icy worlds.

Royston-Bishop et al., 2005

-BACK TO TOP-

The nature of microscopic particulates in meteoric and accreted ice from the Vostok ice core, is assessed in conjunction with existing ice core data to investigate the mechanism by which particulates are incorporated into refrozen lake water. Melted ice samples from a range of ice core depths were filtered through 0.2 µm polycarbonate membranes and secondary electron images were collected at ×500 magnification using a scanning electron microscope. Image analysis software was used to characterise the size and shape of particulates. Similar distributions of major axis lengths, surface areas and shape factors (aspect ratio and compactness) for particulates in all accreted ice samples suggest that a single process may be responsible for incorporating the vast majority of particulates for all depths. Calculation of Stokes settling velocities for particulates of various sizes implies that 98 % of particulates observed could float to the ice-water interface with upward water velocities of 0.0003 m s-1 where they could be incorporated by growing ice crystals at the ice-water interface, or by rising frazil ice crystals. The presence of particulates that are expected to sink in the water column (2 %) and the uneven distribution of particulates in the ice core further implies that periodic perturbations to the lakes circulation, involving increased velocities, may have occurred in the past.

Christner et al., 2005

-BACK TO TOP-

Evidence gathered from spacecraft orbiting Mars has shown that water ice exists at both poles and may form a large subsurface reservoir at lower latitudes. The recent exploration of the martian surface by unmanned landers and surface rovers, and the planned missions to eventually return samples to Earth have raised concerns regarding both forward and back contamination. Methods to search for life in these icy environments and adequate protocols to prevent contamination can be tested with earthly analogues. Studies of ice cores on Earth have established past climate changes and geological events, both globally and regionally, but only recently have these results been correlated with the biological materials (i.e., plant fragments, seeds, pollen grains, fungal spores, and microorganisms) that are entrapped and preserved within the ice. The inclusion of biology into ice coring research brings with it a whole new approach towards decontamination. Our investigations on ice from the Vostok core (Antarctica) have shown that the outer portion of the cores have up to 3 and 2 orders of magnitude higher bacterial density and dissolved organic carbon (DOC) than the inner portion of the cores, respectively, as a result of drilling and handling. The extreme gradients that exist between the outer and inner portion of these samples make contamination a very relevant aspect of geomicrobiological investigations of ice cores, particularly when the actual numbers of ambient bacterial cells are low. To address this issue and the inherent concern it raises for the integrity of future investigations with ice core materials from terrestrial and extraterrestrial environments, we employed a procedure to monitor the decontamination process in which ice core surfaces are painted with a solution containing a tracer microorganism, plasmid DNA, and fluorescent dye before sampling. Using this approach, a simple and direct method is proposed to verify the authenticity of geomicrobiological results obtained from ice core materials. Our protocol has important implications for the design of life detection experiments on Mars and the decontamination of samples that will eventually be returned to Earth.

Christner et al., 2005

-BACK TO TOP-

Fungi, algae, protists, bacteria and viruses have been detected and recovered from polar ice cores, but there are very few similar reports describing the microorganisms preserved in non-polar glacial ice of different age and from different locations. Fortunately, for such studies, we have access to ice cores archived at the Byrd Polar Research Center (BPRC) at The Ohio State University. These ice cores have been collected over many years, from globally-distributed sites, and many have already been subjected to extensive physical and chemical analyses. These, therefore, provide the opportunity to isolate and to characterize microorganisms from glacial ice formed at defined dates, under known climate conditions, at geographically very different locations.   Here we review the results of bacterial isolations from meltwater obtained from the interiors of non-polar and polar glacial ice cores of different vintage, and from Lake Vostok accretion ice. 

Priscu and Christner, 2004

-BACK TO TOP-

A review of microbiological  investigations on permanent Antarctic lake ice, glaciers (polar and temperate), and subglacial Antarctic lakes.  We argue that these ecosystems are relevant to discussions of the evolution of life on Earth or other icy bodies in the solar system, and estimate that the cell numbers and carbon in icy environments are comparable to those of freshwater ecosystems.  Therefore, Earth's frozen realms must be included within global carbon budgets and recognized as ecologically significant components of the biosphere. 

Christner et al., 2003

-BACK TO TOP-

Inhabitants of a cryoconite hole formed in the Canada glacier in the McMurdo Dry Valley region of Antarctica have been isolated and identified by small subunit (16S/18S) rDNA amplification, cloning and sequencing. The sequences obtained revealed the presence of members of eight bacterial lineages (Proteobacteria, Acidobacterium, Cytophagales, Planctomycetes, candidate division BD, Verrucomicrobia, Actinobacteria, and Cyanobacteria) and metazoan (nematode, tardigrade, and rotifer), truffle (Choiromyces), ciliate (Spathidium), and green algal (Pleurastrium) Eukarya. Bacterial recovery was ~20-fold higher at 4oC and 15oC than at 22oC, and obligately psychrophilic bacteria were identified and isolated. Several of the rDNA molecules amplified from isolates and directly from cryoconite DNA preparations had sequences similar to rDNA molecules of species present in adjacent lake ice and microbial mat environments. This cryoconite hole community was therefore most likely seeded by particulates from these local environments. Cryoconite holes may serve as biological refuges which, on glacial melting, can repopulate the local environments.

Christner et al., 2003

-BACK TO TOP-

Ice that forms the bottom 18 m of a 308 m ice core drilled from the Guliya ice cap on the Qinghan-Tibetan plateau in Western China is over 750,000 years old, and is the oldest glacial ice known to date. Fourteen bacterial isolates have been recovered from samples of this ice from ~296 m below the surface (mbs). Based on 16 S rDNA sequences, these are members of the α- and β-proteobacterial, actinobacterial and low-G+C gram positive bacterial lineages. 16S rDNA molecules have also been amplified directly, cloned and sequenced from the ice-core melt water. These originated from Pseudomonas and Acinetobacter (-proteobacterial species. These results demonstrate that bacteria can be recovered from water ice that has frozen for time periods relevant to biological survival through terrestrial ice ages or during inter-planetary transport. 

Christner, 2002

-BACK TO TOP-

DNA and protein precursors were incorporated into trichloroacetic acid-precipitated material by bacterial cell suspensions during incubation for 50 to 100 days at -15^oC. Incorporation did not occur at -70^oC and was inhibited by antibiotics. The results demonstrate that bacteria can perform macromolecular synthesis under conditions that mimic entrapment in glacial ice.

Christner et al., 2002

-BACK TO TOP-

Christner, B.C., E. Mosley-Thompson, L.G. Thompson, V. Zagorodnov, K. Sandman, and J.N. Reeve.  2002.  Isolation and identification of bacteria from ancient and modern ice core archives.  In:  The Patagonian Ice Fields. A unique natural laboratory for environmental and climate change studies, edited by G. Casassa, F.V. Sepúlveda, and R. Sinclair, Kluwer Academic / Plenum Publishers, New York.

Glacial ice traps and preserves soluble chemical species, gases and particulates including pollen grains, fungal spores and bacteria in chronologically-deposited archives. We have constructed an ice-core sampling system that melts ice only from the interior of cores, thereby avoiding surface contamination, and using this system we have isolated, cultured and characterized bacteria from ice cores that range from 5 to 20,000 years in age and that originating from both polar and non-polar regions. Low-latitude, high-altitude non-polar ice cores generally contain more culturable bacteria than polar ices, consistent with closer proximities to major biological ecosystems. Direct plating of melt-water from a 200-year old sample of ice from the Guliya ice cap on the Tibetan Plateau (China) generated ~180 bacterial colonies per ml [colony forming units/ml; (cfu/ml)], whereas melt water from late Holocene ice from Taylor Dome in Antarctica contained only 10 cfu/ml, and <10 cfu/ml were present ice of the same age from the Antarctic Peninsula and from Greenland. Based on their small-subunit ribosomal RNA-encoding DNA (rDNA) sequences many, but not all of the bacteria isolated are spore-forming species belong to Bacillus and Actinomycete genera. Non-chronological fluctuations are observed in the numbers of bacteria present, consistent with episodic deposition resulting from attachment to larger particulates.

Christner, 2002

-BACK TO TOP-

An extraction system has been constructed that melts ice from the interior of ice cores and collects the resulting melt water aseptically.  Using this system, bacteria entrapped in modern and ancient glacial ice from worldwide locations and in an ice core extending into accreted Lake Vostok ice have been isolated using enrichment culture and identified by amplification and sequencing of DNA-encoding 16S rRNA genes.  In general, ice cores from non-polar locations contained larger numbers and species of cultivable bacteria than samples from polar ices, presumably due to the closer proximity of terrestrial biological ecosystems and exposed landscape.  When compared with other polar locations, higher numbers of isolates were obtained from ices adjacent to the Dry Valley complex of Antarctica, consistent with the influx of airborne biological particles from local environments serving as the primary factor controlling the numbers of microorganisms present.  The numbers of recoverable bacteria did not correlate directly with the age of the ice, and isolates were recovered from the oldest samples examined (>500K years old).  The 16S rDNA sequences from bacterial isolates and amplicons obtained directly from samples position within 6 different bacterial lines of descent (a-, b-, and g-proteobacteria, high and low G+C gram positive bacteria, and the Cytophaga/Flavobacterium/Bacteroides).  Some of the isolated bacteria have close phylogenetic relationships with species originating from permanently cold environments, and other ice core sites or different portions (time periods) of the same core.  Macromolecular synthesis was demonstrated in bacteria frozen under conditions analogous to those in glacial ice, and the possibility exists that metabolic activity and repair may occur during extended periods of glacial entrapment.  Several of the species identified in Lake Vostok accretion ice are also related to glacial isolates and species from other cold environments.  These ice core studies have provided a glimpse of the microorganisms likely to inhabitant this potentially unique subsurface ecosystem.  Investigating microbial survival in ice and exploring potential habitats for activity within the glacial and subglacial environment has confirmed that these could have served as refuge environments for life during periods of global glaciation (Snowball Earth), and has provided data for extrapolations to the likelihood of microorganisms surviving frozen in extraterrestrial habitats or during interplanetary transport. 

Christner et al., 2001

-BACK TO TOP-

Lake Vostok, the largest subglacial lake in Antarctica, is separated from the surface by <4 km of glacial ice. It has been isolated from direct surface input for at least 420 000 years, and the possibility of a novel environment and ecosystem therefore exists. Lake Vostok water has not been sampled, but an ice core has been recovered that extends into the ice accreted below glacial ice by freezing of Lake Vostok water.  Here, we report the recovery of bacterial isolates belonging to the Brachybacteria, Methylobacterium, Paenibacillus and Sphingomonas lineages from a sample of melt water from this accretion ice that originated 3593 m below the surface. We have also amplified small-subunit ribosomal RNA-encoding DNA molecules (16S rDNAs) directly from this melt water that originated from a- and b-proteobacteria, low- and high-G+C Gram-positive bacteria and a member of the Cytophaga/ Flavobacterium/Bacteroides lineage.

Christner et al., 2000

-BACK TO TOP-

An extraction system has been constructed that melts ice from the interior of ice cores and collects the resulting water aseptically.  Using this system, bacteria entrapped in ice cores from different geographic locations, that range in age from 5 to 20,000 years old, have been isolated and characterized. Ice cores from the Guliya ice cap on the Tibetan Plateau (China) contained the highest number of colony-forming units per milliliter (ª180 cfu ml°1) and representatives of many different bacterial species. Much lower numbers of bacteria (>20 cfu ml°1) were recovered from Sajama (Bolivia) ice cores, although in general such nonpolar ice cores contained more culturable bacteria than samples of polar ice, presumably due to the closer proximity of major biological ecosystems. More bacteria were recovered from Late Holocene ice from the Taylor Dome region than from ice of the same age from the Antarctic peninsula or from Greenland. Bacterial isolates were identified, in terms of their closest phylogenetic relatives, by determining small-subunit ribosomal RNA-encoding DNA sequences (16S rDNAs), and most were related to spore-forming Bacillus and Actinomycetes species, or to nonsporulating Gram positive bacteria. The numbers of recoverable bacteria did not correlate directly with the age of the ice, indicating that most bacteria were deposited episodically in snowflakes and/or attached to larger particles of inorganic and organic debris.  By identifying the features that facilitate microbial survival within terrestrial ice, extrapolations to the likelihood of microorganisms surviving frozen in water ice on Mars, Europa, or within comets will be improved.