Polar Biol (2002) 25: 591–596 DOI 10.1007/s00300-002-0388-5
O R I GI N A L P A P E R
Christin Sa¨wstro¨m Æ Paul Mumford Æ William Marshall Andrew Hodson Æ Johanna Laybourn-Parry
The microbial communities and primary productivity of cryoconite holes in an Arctic glacier (Svalbard 79N) Accepted: 6 April 2002 / Published online: 15 May 2002 Springer-Verlag 2002
Abstract The microbial communities and photosynthetic capacity of cryoconite holes on the Midre Love´nbreen Glacier at 79N in Spitzbergen (Svalbard Archipelago) were investigated in July/August 2000 and July 2001. The constituents of the microbial assemblages were more abundant in material on the cryoconite bottoms than in the overlying water. Bacterial concentrations ranged from 1.00 to 4.50·104 ml–1 in the water and from 4.67 to 7.07·104 ml–1 in the bottom material; virus-like particles (VLP) ranged from 3.97 to 12.70·104 ml–1 in the water and from 27.5 to 37.59·104 ml–1 on the bottom. VLP: bacteria ratios ranged between 0.24 and 8.11, with highest ratios in the bottom assemblages. Heterotrophic nanoflagellate (HNAN) abundances were significantly lower than those of the autotrophic nanoflagellates (PNAN). Moreover, HNAN biomass was lower than bacterial biomass, indicating that the HNAN were exploiting other energy sources as well as bacteria, for example, VLP and dissolved organic carbon. The bottom material was dominated by cyanobacteria (mostly Phormidium sp.), while both the water and the bottom layer contained a small number of chlorophyte species (Chlorella sp., Cylindromonas sp. and Chlamydomonas nivalis). Ciliates were very sparse, only occurring on the bottom. On occasions, the glacier surface carried meltwater with well-developed biofilms, in which ciliates (Monodinium, Strombidium and Halteria) occurred. All of these species are found in nearby lakes. One to three rotifers were noted in the biofilm samples and in samples from three of the cryoconite holes. The assemblages of
C. Sa¨wstro¨m Æ W. Marshall Æ J. Laybourn-Parry (&) School of Life and Environmental Sciences, University of Nottingham, University Park, Nottingham, NG7 2RD, UK E-mail: J.
[email protected] P. Mumford Æ A. Hodson Department of Geography, University of Sheffield, Winter Street, Sheffield, S10 2TN, UK
the cryoconite holes were comparable to the truncated food webs seen in Antarctic lakes, but were even more simplified and sparse in terms of biomass. Photosynthesis in the meltwater on the glacier surface ranged between 0.60 and 8.33 lg C l–1 h–1. Within the cryoconites, photosynthetic rates were usually highest on the bottom (0.63–156.99 lg C l–1 h–1), while in the overlying water, rates ranged between 0.34 and 10.56 lg C l–1 h–1. Given the density of cryoconite holes (circa 6% of the glacier surface, or 12 holes m–2), there was significant carbon fixation and nutrient cycling occurring on the glacier, associated with cryoconite communities.
Introduction Although nutrient cycling in snow-covered catchments has received significant research attention over the last 10 years (Jones1999), there have been few studies of the ecology of catchments characterised by permanent glacier ice. While the microbial ice communities associated with sea ice and lake ice are proving to be of considerable interest (e.g. Felip et al. 1995; Vincent et al. 2000), the potential biological activity of glaciers has received only limited attention. However, liquidwater environments clearly exist in glaciers. In areas of bare glacier ice, perhaps the most common water stores are small supra-glacial melt depressions known as ‘‘cryoconite holes’’. These structures are found on glaciers in both the Arctic and Antarctic, and are notable because earlier, largely quantitative studies, have found that significant biological activity may be present (Steinbock 1936; Gerdel and Drouet 1960; Wharton et al. 1981). Cryoconites are relatively shallow, straight-sided holes with concave bottoms. They tend not to be more than around 50–60 cm in depth and are filled almost to the surface with water in summer, while they typically freeze in winter (Gerdel and Drouet 1960). The holes occur in the ablation regions of glaciers, where
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there is ice loss due to melting, wind erosion, evaporation and calving (Wharton et al. 1985). In certain cases, a direct positive relationship between cryoconite depth and elevation has been found (Gribbon 1979), suggesting that the decrease of sensible and latent heat inputs to the glacier surface with altitude may encourage deeper holes. However, the formation of cryoconite holes is related to other terms in the surface energy balance of glacier ice, because dark windblown organic and mineral material is first deposited on the surface and warms in the sun to melt a small depression on the glacier surface. Once formed, the depression grows into a cryoconite, due to a variety of physical processes, that may include absorption of solar radiation transmitted obliquely through the walls, a combination of direct or diffuse radiation reaching the bottom, and downwards convection of water warmed by solar radiation at the surface of the hole (Gribbon 1979; Wharton et al. 1985). However, Gerdel and Drouet (1960), who studied cryoconites of the Thula and Nuna ramps in Greenland, argued that due to the high albedo of the glacier, 80% of the solar radiation was reflected from the surface, leaving little energy to penetrate the ice surrounding the developing holes. They suggested that the only physical process that might play a role in the development of the cryoconites was the absorption of solar radiation by the water surface, enabling ice on the sides and bottom of the hole to melt. A problem with this hypothesis was that it did not explain the uniform, straight wall of cryoconite holes. Thus Gerdel and Drouet (1960) speculated that biothermal processes such as photosynthesis were also an important component of cryoconite thermodynamics. Later, McIntyre (1984) tested this hypothesis by chemically killing the biota of cryoconites and quantifying hole development with and without biological activity. His results suggested that biothermal effects were relatively small and could only contribute up to 10% of the heat in the holes. Typically, cryoconites contain a layer of material on their bottoms that is both organic and inorganic in nature. Qualitative studies have revealed cyanobacteria to be the major biotic constituent, as well as some green algae, diatoms, rotifers and fungi (Steinbock 1936; Charlesworth 1957; Gerdel and Drouet 1960; Wharton et al. 1981). Arctic cryoconite holes contained rotifers, whereas those of the Canada Glacier in Southern Victoria Land (Antarctica) were entirely dominated by bacteria and protozoa (Wharton et al. 1981). The present study was conducted to produce a quantitative and qualitative picture of cryoconite communities in a high-Arctic glacier and to determine levels of photosynthesis. The study was undertaken to provide pertinent data that might indicate the capacity of the surface of an Arctic glacier to support microbial life, which might assist in the development of a more appropriate energy balance model of cryoconite environments.
Materials and methods Fieldwork location and methodology Fieldwork was undertaken on the Midre Lovenbreen, a polythermal valley glacier in the Arctic archipelago of Svalbard (71–81N). Midre Lovenbreen is adjacent to the international research station in Ny A˚lesund. It is a well-researched glacier, having a mass balance record that extends back to the late 1960s (Hagen and Liestøl 1990). These data show that the winter accumulation rate on the glacier is low (average: 0.74 m water equivalent since records begun) and that the average net balance is significantly negative (–0.34 m), and has most probably been so since the late 1920s (Hagen and Liestøl 1990; Washington et al. 2000; J. Kohler, unpublished data). For this reason, the average accumulation area ratio is low (circa 36%) and much of the glacier becomes snow-free by the end of July. Surface meltwater, cryoconite water and cryoconite detritus were sampled from a zone in the mid-parts of the ablation area at around 200 m altitude during the summers 2000 and 2001. Here, cryoconites represented around 6% of the glacier surface (average 12 holes m–2). Water samples were taken from the upper 10 cm of water and the bottom of cryoconites, using a hypodermic syringe with a tube (5 mm inner diameter) attached to its end, during the summers of 2000 and 2001. In 2000, samples were collected on four occasions between 26 July and 22 August, and in 2001 a set of samples was collected on 15 July from a range of cryoconites. Aliquots (500 ml) were fixed in Lugol’s iodine for enumeration and identification of larger protozoa and meiofauna, and 50-ml aliquots were fixed in buffered glutaraldehyde to a final concentration of 4% for the enumeration of bacteria, viruses and flagellates. The depths and widths of the holes were measured with a ruler. Temperatures were taken with a digital thermometer with a 14-cm probe. In 2000, samples were also collected for inorganic nutrient analysis (PO4-P and NH4-N) in the dissolved phase, plus the particulate N and P content of cryoconite bottom organic material. On three of the four sampling occasions in 2000, the cryoconite holes were in hydrological contact with each other because of melt on the glacier surface. On these occasions, samples for biological analysis were collected from this surface water, which in places formed a clear biofilm.
Analysis of samples For bacterial and nanoflagellate counts, 5-and 10-ml aliquots were stained with DAPI (4¢6-diamidino-2-phenylindole, Sigma) and filtered onto either 0.2-lm polycarbonate filters (bacteria) or 2.0-lm polycarbonate filters (nanoflagellates). Analysis for abundances and biomass was undertaken using epifluorescence microscopy at ·1600, using UV excitation for the enumeration of bacteria, and both the UV filter and the blue filter for the separation of heterotrophic (HNAN) and phototrophic (PNAN) nanoflagellates. Biomass determinations were made by measuring 50 cells on each preparation and calculating mean cell volume using the nearest geometric shape. Conversion to carbon values was made using a conversion factor of 0.20 pg C lm3 for bacteria (Bratbak and Dundas 1984) and 0.22 pg C lm3 for nanoflagellates (Børsheim and Bratbak 1987). Virus-like particles (VLP) were also enumerated in 2001 using the fluorochrome SYBR green I on 0.02-lm Anodisc membrane filters, following the procedure of Noble and Fuhrman (1998). Lugol’s iodine-fixed samples were concentrated by settling, and counted in a Sedgewick-Rafter counting chamber under phase microscopy at ·230. Concentrations of inorganic nutrients (NO3 and PO4) were determined using the methods of Mackereth et al. (1989). These included standard colorimetric tests via reduction to NO2 and via the molybdenum-blue method (Murphy and Riley 1962), respectively. NH4 was determined colorimetrically following conversion to NH3, and passage through a gas-permeable membrane (Foss Tecator 2000). Total N and P of all water and organic bottom material were determined as NO3 and PO4, following microwave digestion.
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The mean air temperatures over the abalation zone of the glacier surface during the summers of 2000 and 2001 were 2.5C, varying from a minimum of –5.5C to a maximum of 10.3C. In 2000, incident radiation reached a maximum of 2.5·103 lmol m–2 s–1 and the overall daily average for summer 2000 was 7.2·102 lmol m–2 s–1. The depth of the cryoconites ranged from 10 cm to 30 cm (mean 20 cm±5.9) and from 5 cm to 50 cm in diameter. Volumes were calculated using the linear dimensions in the equation V=(p·depth·width2)/4 and varied from 0.59 l to 37.3 l. Water temperatures were remarkably uniform, and in all but one of the cryoconite holes was 0.1C. The only exception was the largest hole which had a temperature of – 0.1C. Inorganic nutrients in the water of the cryoconites (PO4-P and NH4-N) were low, with soluble reactive phosphorus ranging from 0.9 lg l–1 to 9.0 lg l–1 and ammonium from below the limit of detection (3 lg l–1) to 12.9 lg l–1, and nitrate-N from below the limit for detection (3 lg l–1) to 10.1 lg l–1 (Table 1). However, particulate N and P, representing in excess of 99.5% of the distribution of these nutrients in the holes, were present in significantly higher concentrations, ranging between 6,310 and 34,700, and 1,800 and 4,050 lg l–1, respectively (Table 1).
Bacterial concentrations showed considerable variation and ranged from 2.5 to 29.5·104 ml–1, but were typically between 2.5 and 5.70·104 ml–1 (Table 2). The high value (cryoconite 6 in 2001) was atypical because this hole was formed from a bird wing. VLP abundances were higher than bacterial numbers in each hole and ranged between 3.97·104 ml–1 and 7.04·104 ml–1 with VLP to bacteria ratios of between 0.24 and 2.11 (Table 2). The highest VLP abundances were again found in cryoconite hole 6. There was a significant correlation between VLP and bacterial concentrations over the four sampling dates in summer 2000 (r=0.606, P=0.013); however, there was no relationship apparent from the one sampling in 2001. Generally, in each hole, bacterial concentrations were higher in the bottom material (Table 2). Nanoflagellates, both PNAN and HNAN, occurred in all the cryoconites studied, as one would anticipate given that bacteria were present (Figs. 1, 2). During 2000, the samples were stored before analysis and there was some loss of autofluorescence by the PNAN, making it difficult to gain completely accurate values for PNAN; thus, for 2000 the total nanoflagellates are given (Fig. 1). In 2001, the PNAN significantly outnumbered the HNAN (Fig. 2a, b). There was no correlation between bacterial abundance and HNAN concentrations. As with bacteria, higher numbers of nanoflagellates were usually found in the bottom or benthic material compared with the water, or what may regarded as the plankton of the holes. The contribution of bacteria, PNAN and HNAN to biomass reveals an interesting anomaly in that bacterial biomass was lower than HNAN biomass, suggesting that the HNAN were possibly exploiting other food resources, e.g. VLP or dissolved organic carbon (Table 3). Within the benthic material, cyanobacteria were very common, particularly Phormidium with some Nostoc; the former reached abundances of 9.14·102 filaments ml–1. The chlorophytes in the water and benthos were represented by three genera: Chlorella sp., Cylindromonas sp. and the snow alga Chlamydomonas nivalis. These constituted the PNAN, and they were most abundant in the benthic material. The occasional ciliate was also seen in the benthic mass, usually hypotrichs, but ciliates were absent from the water or ‘‘plankton’’. On one of the occasions when there was free water on the glacier surface, the biofilm material was analysed and revealed a bacterial concentration of 6.12·104 ml–1,
Table 1. Mean concentrations of nitrogen and phosphorus in their various forms from three cryoconite holes in summer 2000 with SEM. NH4-N, NO3-N and PO4-P concentrations relate to the
water phase. TN and TP denote total nitrogen and phosphorus, respectively, and are given in concentrations of mg l–1 rather than lg l–1 and relate to the material on the bottom of the cryoconites
During the summer of 2000, photosynthesis in the cryoconites was measured by the incorporation of 14C bicarbonate (SteemanNielsen 1952). Water samples were taken from the top 10 cm and from the bottom of the hole on four occasions between 26 July and 22 August. On three occasions, the holes were in hydrological contact, so water was also taken from the glacier surface for photosynthetic determinations. Experiments were conducted immediately on return to the laboratory under a temperature and light regimen prevalent on the glacier surface at the time of collection. This was within 1.5 h of collection. Photosynthetically active radiation (PAR) was determined with a LICOR PAR sensor (LI192SA). Light readings were taken at the surface of the cryoconites as the light sensor was too large to fit into the holes. 14C sodium bicarbonate was added to 100 ml of sample from each depth to a final concentration of 1.0 lCi ml–1. Three aliquots were removed and acid-killed with 200 ll of 6 M HCl as controls. Three 7-ml aliquots were removed and treated with 200 ll of 1 M NaOH to ascertain the exact activity added. Three experimental aliquots were then incubated for 12 h and the experiment terminated by the addition of 200 ll 6 M HCl. Control and experimental samples were shaken for 3 h to drive off unincorporated 14C; 10 ml scintillation fluid was added and the samples counted. Measurements of total available CO2 in the water column were obtained by Gran titration.
Results
Date
NH4-N (lg l–1)
NO3-N (lg l–1)
TN (mg l–1)
PO4-P (lg l–1)
TP (mg l–1)
26.7.00 5.8.00 14.8.00 22.8.00
3.3±0.4 4.51±2.8 19.34±17.2 0.43±1.3
2.67±4.1 6.97±3.4 5.63±4.3 3.09±4.0
15.5±10.2 14.7±10.1 23.7±9.2 33.1±3.4
2.56±1.29 2.96±0.26 3.25±0.12 4.77±0.71
2.4±1.2 2.9±1.1 3.0±0.6 3.6±0.9
594 Table 2. Bacterial abundances, VLP abundances and VLP:bacterial ratios during 2000 and 2001. Water samples in 2001 were integrated samples from the top 10 cm of the cryoconite. VLP were not enumerated from bottom material
Date
Cryoconite number or position
Bacteria·104 ml–1
26.7.00 26.7.00 05.8.00 05.8.00 14.8.00 14.8.00 22.8.00 22.8.00 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01 15.7.01
Water Bottom Water Bottom Water Bottom Water Bottom 1 water 1 bottom 2 water 2 bottom 3 water 3 bottom 4 water 4 bottom 5 water 5 bottom 6 water 6 bottom 7 water 7 bottom
3.60 5.70 2.82 4.64 2.69 7.07 3.38 No sample 2.85 2.74 2.52 6.05 2.79 3.85 2.80 – 3.41 2.65 4.50 11.77 1.00 5.14
VLP· 104 ml–1
VLP:bacteria ratio
5.89
1.64
12.71
4.50
6.52
2.43
9.72 No sample 3.97
2.88 No sample 1.39
5.25
2.08
4.86
1.74
5.92
2.11
6.20
1.82
7.04
0.24
–
–
Fig. 1. Nanoflagellate concentrations in cryoconites during 2000 (filled columns water; unfilled columns bottom, with SEM)
VLP of 9.24·104 ml–1 and nanoflagellates of 12·102 ml–1. Notably there were ciliates and heliozoans in this material. Paraphysomonas was one of the dominant HNAN seen in the holes and on the glacier surface. Among the ciliates, the haptorid Monodinium and the oligotrichs, Halteria and Strombidium, were the only species, giving a mean ciliate abundance of 10 cells ml–1. Rotifers were seen in the surface samples and also occasionally in the holes. Two species occurred, Polyarthra and an unidentified soft-bodied species. The photosynthetic activity of the cryoconites and the glacial surface water ranged between 0.60 lg C l–1 h–1 and 156.99 lg C l–1 h–1 (Table 4). Apart from 26 July, photosynthesis on the glacier surface was low. On 26 July, the glacier was clothed in fog, which caused a significant decrease in PAR. On only one occasion did photosynthesis in the water exceed that in the benthos. Generally levels of photosynthesis in the water of the cryoconites were low. Given that the albedo of water is much less than the ice, it is likely that photosynthetically
Fig. 2. a Phototrophic nanoflagellates in cryoconite holes in summer 2001(filled columns water; unfilled columns bottom material) b Heterotrophic nanoflagellates in cryoconite holes in summer 2001 ( filled columns water; unfilled columns bottom material)
active radiation levels in the holes were high. We were unable to measure them because our light meter was too large to fit into the holes. Primary production on the bottom of the holes reached a high level in late August, equivalent to 3.76 mg C l–1 day–1. Given the abundance
595 Table 3. Mean biomass (lg C l–1) for bacteria, HNAN and PNAN in seven cryoconites in July 2001 Microbial Top 10 cm component
Maximum and Bottom Maximum and Minimum Minimum
Bacteria HNAN PNAN
2.44–44.16 0.37–2.70 1.70–122.00
9.37 1.19 33.80
7.41 12.21 86.00
2.75–20.40 0.35–34.33 4.70–224.90
Table 4. Photosynthetic rates in cryoconite holes and on the glacier surface during July and August 2000. Values are lm C l–1 h–1 with SEM Date
Glacier surface
Top 10 cm
Bottom
26 July 5 August 11 August 22 August
8.33±0.51 – 0.60±0.08 0.65±0.14
10.56±1.41 0.24±0.09 0.34±0.20 1.16±0.27
0.63±0.03 24.59±0.75 31.22±2.01 156.99±4.00
of cryoconites per unit area of the glacier, there is a significant level of carbon fixation occurring each summer.
Discussion The community of Arctic cryoconites closely resembled the plankton and algal-mat communities of Antarctic lakes (Hawes et al. 1992; Vincent et al. 1993; LaybournParry 1997; James et al. 1998), with a few notable differences. The cryoconite assemblages had few ciliated protozoans, and the benthic material lacked significant meiofauna like that found in benthic algal mats (Cathey et al. 1981). Cyanobacteria, particularly Phormidium and Nostoc, dominated the mat communities of Antarctic lakes and ponds (Hawes et al. 1992; Vincent et al. 1993). They also dominated the benthic material of the cryoconites in Svalbard, as they did in the holes on the Canada Glacier in Antarctica (Wharton et al. 1981). Other Arctic glaciers had cryoconite communities containing Calothrix, Plectonema and Schizothrix (Gerdel and Drouet 1960). The green algal assemblages of cryoconites have low species diversity. Typical species included Chlamydomonas nivalis and Chloromonas nivalis in the Canada Glacier. Both are snow algae adapted to living in high-light environments (Wharton et al. 1981). Desmids have also been recorded in Arctic cryoconites (Steinbock 1936; Gerdel and Drouet 1960). Generally, the species found in the cryoconites reflect what occurs in the local terrestrial and aquatic ecosystems, suggesting that this is the source of the colonising propagules. This was the case for the Canada Glacier (Wharton et al. 1985) and was also true for the Midre Love´nbreen Glacier. To our knowledge, the plankton and ‘‘benthic’’ bacterial, viral and nanoflagellate communities of cryoconite holes have not previously been investigated. Viruses occurred in relatively high concentrations in the plankton of Antarctic lakes (1.01–36.0·106 ml–1) (Kep-
ner and Wharton 1998; Laybourn-Parry et al. 2001a). The concentrations of VLP in cryoconites were 2 orders of magnitude lower than those of Antarctic freshwater lakes (1.01–3.28·106 ml–1). Viruses play an important role in recycling carbon in aquatic environments. They attack and cause lysis of bacterial, algal and protozoan cells, thereby short-circuiting the carbon cycle by recycling carbon to the pool before it can be passed up the food chain (Fuhrman 1999). Viral lysis can significantly reduce both photosynthesis and bacterial production. The VLP to bacterial ratios seen in the cryoconites of the Midre Love´nbreen Glacier were similar to those seen in the extremely oligotrophic freshwater lakes of the Vestfold Hills, Antarctica (Laybourn-Parry et al. 2001a). Given these ratios, it is likely that viruses play a significant role in the carbon dynamics of the microbially dominated cryoconite communities. Bacterial abundances were within the range seen in Antarctic lakes. For example, in Lake Hoare, fed by the Canada Glacier, concentrations ranged between 0.09 and 0.119·104 ml–1 (Roberts and Laybourn-Parry 1999), and in the two large freshwater lakes in the Vestfold Hills, between 1.19 and 25.0·104 ml–1 (Laybourn-Parry et al. 1995; Laybourn-Parry and Bayliss 1996). Bacteria form the main food source for HNAN in aquatic ecosystems. The concentrations found in Midre Love´nbreen were lower than those typically found in Antarctic lakes. Among polar lacustrine PNAN communities, the phenomenon of mixotrophy is a widely used survival strategy (Laybourn-Parry et al. 2000a). The species found in cryoconite holes are not recorded as having mixotrophic capabilities; however, among Antarctic mixotrophs, there are species that are not known to be mixotrophic in lower-latitude lakes, for example, Pyramimonas (Bell and Laybourn-Parry 1999). The growth rates of bacteria and nanoflagellates are likely to be low in cryoconites where temperatures hover just above freezing. Temperatures in freshwater Antarctic lakes range between 0.5 and 4C, whereas those of Arctic lakes can reach 18C (Laybourn-Parry 2002). Thus cryoconite holes rank among the coldest polar freshwater ecosystems. In Antarctic systems, bacterial production ranged between 0.067 and 10 lg C l–1 day–1 (Laybourn-Parry et al. 1995, 2001b; Takacs and Priscu 1998). Based on Q10, one would anticipate that bacterial production in the cryoconites would be at least 40% lower. Using Antarctic HNAN specific growth rates (Laybourn-Parry et al. 2000b) adjusted for temperature and bacterial biomass, the estimated specific growth rates of the cryoconite HNAN would be around 0.004 h–1 with a doubling time of 173 h. Photosynthesis in the cryoconite plankton is probably not limited by inorganic nutrient concentrations, as on most occasions there were measurable levels of inorganic N and P. In the benthic communities, the major part of photosynthesis is undoubtedly achieved by the cyanobacteria, and this reached quite high levels on occasions. The highest rates of photosynthesis seen in the Midre Love´nbreen Glacier exceeded those recorded for both
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Arctic and Antarctic lakes (O’Brien et al. 1992; Bayliss et al. 1997; Markager et al. 1999; Laybourn-Parry et al. 2001b). A much more quantitative assessment of carbon fixation per unit area of the glacier is required. Given the high concentration of cryoconites on Midre Love´nbreen and production in the surface melt layer of the glacier, these ice-dominated aquatic environments may be important contributors to carbon fixation in terrestrial polar environments, particularly in the Antarctic and Arctic polar deserts. They possess simple truncated microbial food webs that are even more reduced than those of ultra-oligotrophic Antarctic lakes, in that they lack a ciliated protozoan component. Cryoconites are one of the most extreme polar communities, offering an analogue for life on Mars, as do other extremophile microbial communities (Horneck 2000). Acknowledgements This work was an opportunistic study undertaken while A. Hodson and J. Laybourn-Parry were working in Spitzbergen on Natural Environment Research Council funded projects. We are grateful to Peter Wynn who assisted with fieldwork, and Nick Cox, the manager of the N.E.R.C. Ny A˚lesund Research Station.
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