Subglacial Environments: Biological Features
CURRENT EVIDENCE FOR LIFE IN SUBGLACIAL AqUATIC ENVIRONMENTS
Biogeochemical Evidence for an Active Bacterial Flora in Lake Vostok
The total dissolved solids (TDS) in the lake water can be estimated by applying ice-water partition coefficients to the major ion concentrations in the accretion ice (Christner et al. 2006). These ice-water partition coefficients are assumed to be the same
EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS
as those calculated from the permanent ice cover and water column of Lake Bonney in the McMurdo Dry Valleys. In this way, Christner et al. (2006) estimated the TDS con-centrations of water in the embayment and the main lake to be 34 and 1.5 mmol L–1, respectively. The former value is well above typical concentrations in temperate lakes.
In the same way, the concentrations of non-purgeable organic carbon (NPOC), amino acids, and major ions can be estimated. In the 5G core the concentration of NPOC in the glacial ice ranged from 2 to 80 µmol L–1. In Lake Vostok, Christner et al. (2006) predicted that the NPOC concentrations would range from 17 to 250 µmol L–1. Total amino acids (dissolved free and combined amino acids plus protein) in glacial ice con-stituted 0.6-2 percent of the NPOC, while they made up 0.01 to 2 percent of NPOC in the accretion ice. Although there was a significant correlation between the NPOC and amino acid concentration in the accretion ice (r = 0.), no significant relationship existed between these parameters in the glacial ice (r = 0.0).
From these data plus information on the acid-extractable amino acid concentra-tions, Christner et al. (2006) concluded the following: relative to the main body of the lake, the waters near the shoreline represent a source of amino acids. This, together with the cell density data, implies that this region has elevated rates of heterotrophic biological activity and associated amino acid transformations.
Biological Evidence for an Active Bacterial Flora in Lake Vostok
Analysis of samples from the accretion ice of Lake Vostok by epifluorescence microscopy (Figure 3.2) and scanning electron microscopy has shown the presence of intact, DNA-containing cells; however there are conflicting reports on their concen-trations. Christner et al. (2006) reported that the number of bacterial cells counted in accretion ice depths between 3539 and 3572 m ranged from 98 ± 18 to 430 ± 23 cells mL–1. These data overlap with the range 200 to 3000 cells mL–1 reported for samples between 3541 and 3611 m (Karl et al. 1999; Priscu et al. 1999; Abyzov et al. 2001). Price (2007) notes that other reports indicate concentrations less than 10 cells mL–1, but suggests that these disparities may simply reflect sampling error if the microbes are located in veins between the large ice crystals. The bacterial abundance is two- to seven-fold higher in accretion ice than in the overlying glacial ice, implying that Lake Vostok is a source of bacterial carbon beneath the ice sheet. Scanning electron microscopy (Priscu et al. 1999) has shown that microbial cells are often associated with organic and inorganic particles.
When a sample of the accreted ice was melted, Karl et al. (1999) were able to measure 14CO2 produced from added 14C-glucose over incubations of 2, 11, and 18 days. They were careful to state, however, that the laboratory results were of potential activity only and in situ rates might be much lower.
Biological Evidence for Viability of the Bacterial Flora in Lake Vostok
Christner et al. (2006) report that differential cell staining revealed that the majority (75 to 99 percent) of cells in the accretion ice are potentially viable, which is comparable to the ~80 percent value reported by Miteva et al. (2004) from a single depth of “silty” basal ice in the Greenland GISP2 core. Positive rates of glucose respiration in four of the six samples tested by Christner et al. (2006) corroborated the presence of viable heterotrophic cells. When the respiration data were
normal-SUBGLACIAL ENVIRONMENTS: BIOLOGICAL FEATURES
FIGURE 3.2 Bacterial sample from the Lake Vostok accretion ice analyzed by epifluorescence microscopy. SOURCE: Karl et al. (1999).
3.02
ized to the lake temperature of –3°C, their measurements of glucose mineralization (0.005 ± 0.003 to 0.2 ± 0.005 nmol C L–1 day–1) were on the low end of the range reported by Karl et al. (1999) for an accretion ice core from 3603 m (0.3 ± 0.1 to 0.5 ± 0.4 nmol C L–1 day–1 at –3°C). However, Christner et al. (2006) found that in samples where respiration rates could be measured there were no significant levels of incorporation of substrate into macromolecules. On these same samples, Christner et al. (2006 p. 25) reported: “Our enrichment culturing studies on these samples have required up to 6 months of incubation in the dark at 4°C before growth is initiated (Arnold et al. 2005; Christner and Priscu unpublished). After this apparent period of resuscitation, the microbial (bacteria and yeast) species recovered and grew to visible densities on both liquid and agar-solidified media in <10 d, and many of the isolates were capable of growth to the stationary phase in 40 h at 22°C. When non-growing and sub-lethally injured cells are placed in a growth situation (e.g., Dodd et al. 1997;
Aldsworth et al. 1999), metabolism must first be initiated to repair incurred cellular damage before growth and reproduction can occur. Thus, if our incubations had been conducted over a longer time frame, we would expect such carbon-fed cells to eventu-ally initiate the growth phase.”
Evidence for an Autochthonous or Lake-Dwelling Microbial Flora
Microbiological and molecular-based studies of the accretion ice by six labora-tories indicate that this ice contains low but detectable amounts of prokaryotic cells and DNA (Karl et al. 1999; Priscu et al. 1999; Abyzov et al. 2001; Christner et al.
2001; Bulat et al. 2004). Christner et al. (2006) state that molecular identification of microbes in Vostok accretion ice by culturing and small subunit rRNA gene amplifica-tion, show close agreement with present-day microbiota. These were identified to lie within the bacterial phyla Proteobacteria (α, β, and γ), Firmicutes, Actinobacteria, and Bacteroidetes (Table 3.3). Further, they state (Christner et al. 2006; p. 2496):
EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS
TABLE 3.3 Microbial Diversity, Number of Cells, and Number of Inclusions Observed in Different Horizons of the Vostok 5G Deep Core Accretion Ice
Ice
0-5 (8.6) Abyzova Micrococci, short rods, pollen, many diatoms
< 200
3555 15 Abyzova Cytophaga spp. and many
Coccolithophoridae
600
3565 5-10 Abyzova Caulobacter spp. 250
3579 5-10 Abyzova Many micrococci, short rods,
and Coccolithophoridae
100
3585 0-5 Abyzova Cytophaga spp. 250
3587 0-5 Sambrottob Fungi, rod-shaped bacteria,
diatoms, pollen grains
Not quantified
3590 0-5 Priscuc Acidoorax sp., Comamonas
sp., Afipia sp., Actinomyces sp.
2800-36,000
3592 0-5 Abyzova Many different kinds 900
3593 Shallow
0-5 (4) Christnerd Brachybacterium sp., Paenibacillus sp., Methylobacterium sp., Sphingomonas sp.
Not quantified
3598 5-10 Abyzova Many Coccolithophoridae 500
3603 5-10 Karle Gram-negative (?) rods, cocci,
and vibrios
200-300
3606 5-10 Abyzova Many different kinds 700
3611 Open lake (3608-3623 m)
0 (0) Abyzova Mostly diatoms and cyanobacteria
aData from Abyzov et al. (2005). Techniques used included light microscopy and scanning electron micros-copy. Coccolithophoriade, cyanobacteia, actinobacteria, rod and coccus-shaped bacteria, and fungal conidia and hyphae were dectected in all ice horizons examined.
bData from Sambrotto and Burckle (2005). Techniques included culturing and microscopy.
cData from Priscu et. al. (1999). Techniques included microscopy and polymerase chain reaction (PCR) amplification.
dData from Christner et al. (2005). Techniques included microscopy and PCR amplification.
eData from Karl et al. (1999). Techniques included microscopy, flow cytometry, and 14C uptake.
fData from S.O. Rogers and S. Bulat, unpublished. Techniques included culturing and PCR amplification.
SOURCE: From Bell et al. (2005).
SUBGLACIAL ENVIRONMENTS: BIOLOGICAL FEATURES
sidering that metabolic inferences for the γ and δ-proteobacterial clones are based on distant phylogenetic relationships, confident predictions about physiology are unachiev-able. Although equivocal, the clustering of the identified small subunit rRNA gene sequences among aerobic and anaerobic species of bacteria with metabolisms dedicated to iron and sulfur respiration or oxidation implies that these metals play a role in the bioenergetics of microorganisms that occur in Lake Vostok.”
Likelihood of Growth
Rates of proliferation of introduced as well as native microbes are likely to be greatly reduced in subglacial aquatic environments by the low ambient temperatures and high pressure conditions. Growth rates may be suppressed further by the inhibitory effects of high oxygen concentrations and by low concentrations of inorganic nutrients and organic carbon substrates.
Christner et al. (2006) applied Price and Sower’s (2004) model to address the potential for survival, maintenance, and growth of microheterotrophs in Lake Vostok and concluded that carbon supply rates would be adequate for maintenance but not growth. It should be noted, however, that their upper estimate of in situ NPOC con-centrations of 250 µmol L–1 (3 mg C L–1) greatly exceeds the total organic carbon in offshore marine environments of around 75 µmol L–1 (0.9 mg L–1).
Furthermore, organic carbon may vary among aquatic environments and spatially within aquatic environments and could achieve higher values than the estimates derived from Vostok accretion ice. For example, the organic carbon supply from melting ice and subglacial waters entering at the northern end of Lake Vostok could be higher than at the accretion ice end, more than 200 km away, particularly if there are organic inputs from active microbial processes at the ice sheet-rock interface. Many of the aquatic environments may be stratified and organic carbon levels could be higher in the bottom waters of each lake associated with microbial decomposition and release from the sediments.
Lake sediments typically contain orders of magnitude higher bacteria, nutrient, and DOC concentrations. The upward plumes associated with normal geothermal heating could potentially transport these constituents from the sediments to the upper water column in some aquatic environments. Subglacial lake sediments may also provide a microbial refuge from high oxygen concentrations in the overlying lake water.
Another of the interesting features of subglacial aquatic environments that distin-guishes them from surface aquatic environments but is similar to deep marine envi-ronments is the absence of ultraviolet (UV) photochemistry. The exposure of natural surface waters to sunlight has multiple effects on DOC availability for microbial pro-cesses, including the photodegradation of higher-molecular-weight materials into more biologically reactive constituents, and the polymerization of certain autotroph-derived organic compounds into forms that are less available for microbial activity (Tranvik and Bertilsson 2001). This absence of UV effects will thus have both positive and negative implications for microbial growth in subglacial aquatic environments.
In summary, growth rates are unknown for bacteria in subglacial ecosystems, but they are likely to be slow and possibly negligible in some situations. Analogous systems include the deep subseafloor biosphere where bacterial turnover times have been esti-mated as up to 22 years (Schippers et al. 2005). These slow rates of metabolism and growth also have implications for the extent of genetic adaptation to the subglacial
EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS
environment given that rates of evolution tend to be slow under conditions of low temperature (Gillooly et al. 2005) and low productivity (Horner-Devine et al. 2003).
BIOLOGY—CONCLUSIONS