2.1. Battleship Promontory: Correlation Between Biology and Snow
On Battleship Promontory (76°55′S, 161°58′E, elevation 1294 m), in the Convoy Range, sandstone rocks vary widely in size and shape, from outcrops tens of meters across, to boulders a few meters high, and to small stones forming a part of the rubble field (
Figure 1). An opportunity to observe the effective snow condition presented itself during a field trip in late January, 2005. Following a significant snowfall, the snow covering the rocks was drastically re-arranged by wind.
Direct contact with snow is not always necessary for a rock to be colonized, and the presence of moisture is not the sole criterion for colonization. For instance, the feet of boulders and stones in loose rubble fields—kept moist by contact with damp soil—are uniformly colonized. Endolithic organisms can also exist within the lower surface of a thin overhang, apparently sustained by downward movement of moisture penetrating the upper surface. In the Dry Valleys, where winds frequently gust up to 15 meters per second, mostly from the southeast [
1], some sandstone surfaces are heavily abraded and undergo grain-by-grain disintegration [
7]. Under these conditions, slow-growing endolithic organisms are unable to establish a foothold.
Figure 1.
Relationship between rock slope, orientation, and biological activity on Battleship Promontory, Convoy Range, Antarctica. North is to the lower left corner of the photograph (note shadow on the ground). Vertical surfaces are devoid of organisms, as is evident from their dark red coloration, because they cannot trap snow. Sloped surfaces are colonized and show exfoliation, regardless of orientation (boulders in foreground), because they trap some snow, but excess snow is removed by gravity and wind. Heavily-scoured surfaces are devoid of colonization. The rate of scouring is such that the organisms are unable to establish a foothold.
Figure 1.
Relationship between rock slope, orientation, and biological activity on Battleship Promontory, Convoy Range, Antarctica. North is to the lower left corner of the photograph (note shadow on the ground). Vertical surfaces are devoid of organisms, as is evident from their dark red coloration, because they cannot trap snow. Sloped surfaces are colonized and show exfoliation, regardless of orientation (boulders in foreground), because they trap some snow, but excess snow is removed by gravity and wind. Heavily-scoured surfaces are devoid of colonization. The rate of scouring is such that the organisms are unable to establish a foothold.
These special situations aside, contact with snow is essential for colonization. Hence, vertical cliffs, which cannot trap snow, are devoid of organisms. This is true for both north- and south-facing cliffs (
Figure 1). These “abiotic” surfaces are covered by a relatively uniform dark red coating [
7]. This coating is the consequence, not the cause, of the rock′s abiotic condition. Where such surfaces have access to moisture, for example, if they lie next to a colonized corner, the coating is destroyed by biological activity and recedes (
Figure 1).
Moderately-sloped surfaces at the tops of boulders have the ideal effective snow condition. They can trap some snow, but excess snow either falls off or is blown away by strong winds. As a result, snow covers on these surfaces are thin, especially around the edges (
Figure 2). North- and south-facing slopes are equally well colonized, suggesting that snow, not temperature, controls where the organisms can or cannot exist. The presence of microorganisms under these surfaces was confirmed both in the field (
Figure 3) and by examining returned samples using scanning electron microscopy (
Figure 4). These relatively dry surfaces are colonized primarily by the lichen-dominated community, while the permanently moist rocks on the ground generally harbor cyanobacteria.
Figure 2.
Snow melting on a moderately-sloped boulder top on Battleship Promontory. Melting occurs in summer around noon, when insolation is maximal. (Scale 10 cm).
Figure 2.
Snow melting on a moderately-sloped boulder top on Battleship Promontory. Melting occurs in summer around noon, when insolation is maximal. (Scale 10 cm).
Figure 3.
Fractured sandstone showing the presence of microorganisms just below the surface. The leaching of iron from the surrounding sandstone improves photosynthetic access to sunlight. (Scale 5 cm).
Figure 3.
Fractured sandstone showing the presence of microorganisms just below the surface. The leaching of iron from the surrounding sandstone improves photosynthetic access to sunlight. (Scale 5 cm).
Figure 4.
Scanning electron micrograph showing endolithic lichens in sandstone, with soredia, characteristic reproductive structures consisting of small groups of algal cells surrounded by fungal cells (arrows). (Scale 10 µm).
Figure 4.
Scanning electron micrograph showing endolithic lichens in sandstone, with soredia, characteristic reproductive structures consisting of small groups of algal cells surrounded by fungal cells (arrows). (Scale 10 µm).
Figure 5.
Evidence that persistent thick snow covers are detrimental. (a) Partially-covered sandstone surface in the lee of a boulder (red arrow) on Battleship Promontory after winds cleared the snow off the area around the edge. (b) View of the exposed area from above, showing that it is heavily colonized. (c) View after the snow was deliberately removed, showing that the covered area (below the added black line) is largely devoid of biological activity. (Scale 20 cm).
Figure 5.
Evidence that persistent thick snow covers are detrimental. (a) Partially-covered sandstone surface in the lee of a boulder (red arrow) on Battleship Promontory after winds cleared the snow off the area around the edge. (b) View of the exposed area from above, showing that it is heavily colonized. (c) View after the snow was deliberately removed, showing that the covered area (below the added black line) is largely devoid of biological activity. (Scale 20 cm).
Perhaps the strongest evidence that snow, not temperature, controls colonization on Battleship Promontory comes from flat, horizontal surfaces. Despite uniform insolation, these surfaces are not always uniformly colonized. Where the colonization is not uniform, it is correlated with the distribution of snow. It seems that the organisms prefer less, not more snow. An example of this observation is shown in
Figure 5. This sandstone slab, situated in the lee of a boulder, was partially covered by 8–10 cm of snow (
Figure 5a). The photographs in
Figure 5b and 5c show a bird’s-eye view of the slab before and after the snow was deliberately removed for observation. The snow-free outer edge is actively colonized, but the snow-covered area shows little evidence of biological activity.
The colonized sandstone rocks on Battleship Promontory can be divided into two categories. First, there are surfaces elevated above the ground. Due to gravity- and wind-assisted snow removal, the effective snow condition of these rocks stays relatively constant and optimal regardless of snowfall volume. These communities appear to be all viable. Second, there are surfaces at ground level and surfaces in a topographic low (e.g., gullies), where snow removal cannot occur and drift snow accumulates. On these surfaces, the effective snow condition can vary considerably and change through time. A surface that is favorable in a climate with low annual precipitation may become unfavorable in a climate with high annual precipitation, and vice versa. In an extreme environment, where the organisms grow slowly [
11], extinction occurs relatively quickly, but re-colonization would be slow. As a result, repeated episodes of colonization, death, and re-colonization may occur in these rocks.
2.2. Cause of Death on Mount Fleming and Horseshoe Mountain: Climate Cooling or Fluctuations in Precipitation?
Mount Fleming and Horseshoe Mountain, in the Asgard Range, are two sites where sandstone outcrops contain mostly dead and entirely dead microbial communities, respectively. Relative to Battleship Promontory, these sites are located farther south, slightly more inland, and at a higher elevation (2200 m). Accordingly, they have a colder climate. The mean January air temperature on Battleship Promontory is −16.6 °C. In contrast, the values for Mount Fleming and Horseshoe Mountain are −18.7 °C and −22.4 °C, respectively [
8]. Based on these data, Friedmann and his colleagues concluded that the cold limit separating hostile from life-supporting environments runs roughly through the area of Mount Fleming [
8]. This conclusion was based on the assumption that, at those locations, the communities went extinct because the temperatures no longer rose sufficiently to melt whatever snow there was. In light of the effective snow condition, it is possible that the Mount Fleming and Horseshoe Mountain communities went extinct from changes in annual precipitation, not from low temperature. The landscapes on Mount Fleming and Horseshoe Mountain are relatively flat, with little opportunity to trap snow. During periods of relatively high precipitation, it is possible that exposed surfaces were covered by a thin film of snow that permitted the rocks to warm up to a degree sufficient to produce meltwater. During drier periods with less snow, however, whatever snow there was could have been more effectively removed by the wind, thereby reducing the overall period of metabolic activity and increasing the probability of the death of the community. In this case, the isolated occurrence of colonies on Mount Fleming could be a sign of recovery of the ecosystem during what is now a wetter period.