Лёд и Снег • 2012 • № 4 (120)
УДК 573.4+573.52+910.3
Prospects for life in the subglacial Lake Vostok
© 2012 г. S.A. Bulat1, D. Marie2, J.-R. Petit3
Petersburg Nuclear Physics Institute, NRC Kurchatov Institute, Gatchina, Russia; 2Biological Station CNRS, Roscoff, France;
3Laboratoire de Glaciologie et Geophysique de l'Environnement, CNRS/UJF Saint-Martin-d'Heres, France
bulat@omrb.pnpi.spb.ru
Статья принята к печати 25 августа 2012 г.
Accretion ice, Antarctic ice sheet, chemolithoautotrophs, extreme environment, Hydrogenophilus thermoluteolus, life detection, subglacial aquatic environments.
Антарктический ледниковый покров, конжеляционный лёд, обнаружение жизни, подледниковые водные среды, хемолитоавтотрофы, экстремальная среда, Hydrogenophilus thermoluteolus.
The objective was to estimate the genuine microbial content of ice samples from refrozen water (accretion ice) from the subglacial Lake Vostok (Antarctica) buried beneath the 4-km thick East Antarctic ice sheet as well as surface snow nearby Vostok station. The lake ice samples were extracted by heavy deep ice drilling from 3764 m below the surface reaching the depth 3769.3 m by February 2012 (lake entering). High pressure, an ultra low carbon and chemical content, isolation, complete darkness and the probable excess of oxygen in water for millions of years characterize this extreme environment. A decontamination protocol was first applied to samples selected for the absence of cracks to remove the outer part contaminated by handling and drilling fluid. Preliminary indications showed the accretion ice samples to be almost gas free with the very low impurity content. Flow cytometry showed the very low unevenly distributed biomass in both accretion (0-19 cells per ml) and glacier (0-24 cells per ml) ice and surface snow (0-0.02 cells per ml) as well while repeated microscopic observations were unsuccessful meaning that the whole Central East Antarctic ice sheet seems to be microbial cell-free.
We used strategies of Ancient DNA research that include establishing contaminant databases and criteria to validate the amplification results. To date, positive results that passed the artifacts and contaminant databases have been obtained for a few bacterial phylotypes only in accretion ice samples featured by some bedrock sediments. Amongst them are the chemolithoautotrophic thermophile Hydrogenophilus thermoluteolus of beta-Proteobacteria, the actinobacterium rather related (95%) to Ilumatobacter luminis and one unclassified phylotype distantly related (92%) to soil-inhabiting uncultured bacteria. Combined with geochemical and geophysical considerations, our results suggest the presence of a deep biosphere, possibly thriving within some active faults of the bedrock encircling the subglacial lake, where the temperature can be as high as 50 °C and in situ hydrogen is probably present. Our approach indicates that the search for life in the subglacial Lake Vostok is constrained by a high probability of forward-contamination. Our strategy includes strict decontamination procedures, thorough tracking of contaminants at each step of the analysis and validation of the results along with geophysical and ecological considerations for the lake setting. This may serve to establish a guideline protocol for studying extraterrestrial ice samples.
The subglacial Lake Vostok is the largest, deepest, and most studied lake among 386 subglacial water features inventoried until now through airborne radio-echo sounding and satellite altimetry surveys [24]. It is located beneath the Russian Vostok Antarctic research station, and it was entered February 5, 2012 at the depth 3769.3 m but has not yet been sampled. The refrozen lake water, or accretion, ice collected by deep drilling and used as a proxy for the lake water has proved to be exceptionally clean, though much diluted, leading to controversial biological and chemical analyses. Nevertheless, the likely indigenous DNA fingerprint of a chemolithoautotrophic thermophile seems to fit with the lake's extreme environment.
Subglacial lakes are water bodies formed at the base of the 3-4 km thick Antarctic ice sheet due to the geo-thermal heat flux, which melts basal ice layers. The melt water accumulates in bedrock troughs with long residence
times [5], and may be periodically discharged through a subglacial hydrologic network [26], depending on geological and bedrock relief settings.
The subglacial Lake Vostok was discovered by radi-oechosounding in the late 1960s [18]. It was mapped in detail by radar-altimetry in the 1990s [23] and defined as a lake after seismic ground studies in 1996 [12]. It is the largest and deepest subglacial lake located beneath Vostok station (78 S, 106 E, elev. 3488 m, ~1,300 km from the coast, mean surface annual temperature -55.1 °C) under the ~3,750-4,200 m thick and rather ancient [19] central East Antarctic ice sheet. The subglacial Lake Vostok is the best-studied water feature in Antarctica [20]. It is crescent shaped and extends northward from Vostok station for more than 275 km in a deep trough and has a surface area of ~15,000 km2, similar to that of Lake Ontario in North America. It is 65 km wide and ~400 m deep on
average with a volume of ~6,100 km3 [1]. There are two rather separate basins, one with a trough as deep as 1,650 m in the southern region [2, 15, 25]. Lake Vostok is a tectonically controlled subglacial lake with minor recently recorded tectonic activity [25]. The lake has likely been isolated from the surface for about 14 million years, the time of the major East Antarctica glaciation [6].
The overlaying glacier flows at ~2 m/year across the lake. From South (Vostok region) to North, the ice thickness increases from 3,750 to 4,200 m because of the deepening bedrock trough. This imposes a tilted interface between the lacustrine water and the overlying ice. As the melting point changes with pressure, the base of the glacier melts on the thick side (Northern region) supplying the lake. Water also refreezes in the upper layers of the lake in the Southern region, and the so-called accretion ice is exported by the glacier's movement out of the lake. This leads to replacement of the lake water every ~40-80 ka [5, 20]. The subglacial lake environment is characterized by high pressure (~400 bars) and in-situ temperatures close to the freezing point of water (-2.6 °C). Energy resources are seriously limited as there is complete darkness and an ultra-low dissolved organic carbon content (DOC < 13 mkg C/L [21], S. Preunkert, pers. comm.). The lake water is likely highly saturated with air, particularly molecular oxygen, released over long periods of time from the continuous ice melting (~800 mg/L [1]) (upper limit 700 and 1300 mg/L for dissolved O2 concentration) [14, 17, respectively]. The accretion ice is also enriched in radiogenic 4He originating from the bedrock and degassed through deep faults [20].
The accretion ice is composed of two distinct layers: the uppermost ice (from 3,539 to 3,608 m) containing millimeter size mica-clay inclusions (accretion ice I) (Fig.) and the deeper ice (below 3,608 m), which is transparent and very clean (accretion ice II). Various groups have studied the biological content of accretion ice samples. The
nature of the samples, the integrity of microbes, the possibility of contamination from the drilling and recovery process, introduction of non-indigenous materials, and the sparse microbial populations all suggest that caution should be taken when interpreting the results. Not unexpectedly, these limitations have led to differing opinions and, in some cases, contradictory results [7, 9, 22]. For example, measured microbial populations in ice range from thousands of cells per ml (up to 36000 [3, 9, 22]) to only a few (1-10) cells/mL [7, 8]. Such a large variation (three orders of magnitude) may be caused by differences in methodology used to decontaminate, retrieve, and process the samples, and some microbes may originate from external contamination during ice coring with drilling fluids [4].
From the most recent studies, for which chemical and biological contamination is better documented and controlled, the Lake Vostok ice appears very clean and much diluted with respect to chemical and biological content and the lake water (at least the upper layer which seems to be enriched with glacier melt water due to insufficient mixing [1]) appears almost entirely lifeless. Only a few bacterial phylotypes were identified in accretion ice, which were not related to contamination [7]. The confidently recorded DNA fingerprint of the chemolithoauto-trophic thermophile Hydrogenophilus thermoluteolus suggests the presence of a deep biosphere within the bedrock [25], while the expected high oxygen stress of the water may be very restrictive for biology [7].
Stringent decontamination procedures have to be applied in order to remove the outer part of the ice core contaminated by heavy drilling operations with a kerosene-based drilling fluid. The protocol consists of scraping off the outer surface, washing off the kerosene with special solutions followed by ozone treatment, and a series of washes with ultrapure low-DOC content water in certified clean room conditions. The chemical composition of the ice has to first meet the reference profiles imposed by clean working condi-
3608 m deep Vostok mono-crystalline accretion ice segment containing the biggest mineral inclusion called «Big Kamina». Magnified images of the inclusion are given as inserts to the left (with size denoted in mm) as well as to the right on a picture. Artificial (due to temperature change during ice transportation) cracks can be seen inside the ice crystal Сегмент монокристаллического льда керна Восток с глубины 3608 м, содержащий наибольшее из известных минеральных включений и получившее наименование «Большая Камина». Увеличенное изображение включения для разных режимов съёмки приведен
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.