Cockell CS, Stokes MD. 2004. Widespread colonization by polar hypoliths. Nature 431: 414.
These authors present a short report about the colonization of the undersides of rocks in the polar deserts by cyanobacteria and unicellular green algae. Where rocks have a protected but accessible underside, these organisms colonize, forming a pale green band a few centimetres across, between the part of the rock too exposed and dry, and too dark for photosynthesis. In polygon terrain, where the ground is sorted by frost heave (“periglacial processes”) and even large rocks are periodically jostled, areas of finer texture form in the centers of polygons, with larger rocks around the edges. The edge rocks were 100% colonized, while central areas were 5% colonized.
Showing posts with label Antarctica. Show all posts
Showing posts with label Antarctica. Show all posts
Tuesday, October 6, 2009
Friday, September 18, 2009
Barrett et al. 2006
Barrett JE, Virginia RA, Wall DH, Cary SC, Adams BJ, Hacker AL, Aislabie JM. 2006. Co-variation in soil biodiversity and biogeochemistry in northern and southern Victoria Land, Antarctica. Antarctic Science 18: 535-548.
These authors examined soil biota at three sites across about 7 degrees of latitude in the drier part of Antarctica. The latitudinal gradient here covers a range of different ecosystems, from relatively wet, more northern and coastal systems to extremely arid and barren southern systems. Here, latitude is not studied for its effects on ecosystems; its effects on ecosystems are exploited to cover the widest available range of conditions. Within each of the three sites, one wet and one dry location were chosen a priori based on obvious surface features such as meltwater drainage channels and the presence of moss beds. At each location, a set of transects were laid out and samples were collected. To quote them directly:
Soil invertebrates were investigated using both morphological and molecular techniques. There are only four metazoan phyla with any significant presence in Antarctica’s soils (Arthropoda, Nematoda, Rotifera, and Tardigrada), all of which are difficult to identify to species using standard morphological methods. While these authors were able to identify nematodes to species, rotifers, tardigrades, and mites were handled as MOTUs, molecular operational taxonomic units (Floyd et al. 2002). These MOTUs, based on ribosomal DNA sequences, were also used to generate a series of cladograms used to assess biodiversity at the research sites. Unfortunately, while the text descriptions of the methods and results are reasonably clear, the figures relating to the biodiversity and metazoan-molecular work are confusing and poorly described.
The results of the microbial analyses are broadly similar to the metazoan dataset. These authors were able to examine biodiversity at the level of microbes, and compare this diversity to both soil chemical characteristics such as water content and C:N ratio, and to the metazoan diversity. Perhaps surprisingly, they found no evidence to support the hypothesis of top-down control on bacterial populations by metazoan predators, as there was no correlation between DGGE and FAME-derived estimates of bacterial population size and species richness and cladogram and sugar-extract-derived estimates of nematode biodiversity. Bacterial diversity is described as not varying across sites, though community composition does. I take this to mean that while total species richness of bacteria (as measured by DGGE) was constant across sites, species turnover (Beta-diversity) was high. This is interesting to me, though not mentioned in the paper, because it seems to directly contradict the microbial-biogeography hypothesis of “everything is everywhere”. There is a mention in the description of this comparison of the importance of geography in structuring biodiversity, which reads to me like an opportunity to apply explicit geospatial techniques to their dataset.
Overall, while it is true that Antarctic soil ecosystems are extremely simple relative to other systems, there are a great many complex and variable interactions between even the few components of these systems, creating a great deal of complexity. This paper will be very useful to me in structuring some of my own investigations in the Arctic.
These authors examined soil biota at three sites across about 7 degrees of latitude in the drier part of Antarctica. The latitudinal gradient here covers a range of different ecosystems, from relatively wet, more northern and coastal systems to extremely arid and barren southern systems. Here, latitude is not studied for its effects on ecosystems; its effects on ecosystems are exploited to cover the widest available range of conditions. Within each of the three sites, one wet and one dry location were chosen a priori based on obvious surface features such as meltwater drainage channels and the presence of moss beds. At each location, a set of transects were laid out and samples were collected. To quote them directly:
"We investigated the structure (bacterial and metazoan diversity) and functioning (soil respiration) of soil communities and the influence of soil biogeochemical properties (organic matter, inorganic nutrients, physicochemical properties) on habitat suitability."This is quite similar in many respects to my planned investigations in the Arctic polar desert. The list of molecular techniques used in this paper, for example, serves as a useful guide or checklist of the procedures I intend to use.
Soil invertebrates were investigated using both morphological and molecular techniques. There are only four metazoan phyla with any significant presence in Antarctica’s soils (Arthropoda, Nematoda, Rotifera, and Tardigrada), all of which are difficult to identify to species using standard morphological methods. While these authors were able to identify nematodes to species, rotifers, tardigrades, and mites were handled as MOTUs, molecular operational taxonomic units (Floyd et al. 2002). These MOTUs, based on ribosomal DNA sequences, were also used to generate a series of cladograms used to assess biodiversity at the research sites. Unfortunately, while the text descriptions of the methods and results are reasonably clear, the figures relating to the biodiversity and metazoan-molecular work are confusing and poorly described.
The results of the microbial analyses are broadly similar to the metazoan dataset. These authors were able to examine biodiversity at the level of microbes, and compare this diversity to both soil chemical characteristics such as water content and C:N ratio, and to the metazoan diversity. Perhaps surprisingly, they found no evidence to support the hypothesis of top-down control on bacterial populations by metazoan predators, as there was no correlation between DGGE and FAME-derived estimates of bacterial population size and species richness and cladogram and sugar-extract-derived estimates of nematode biodiversity. Bacterial diversity is described as not varying across sites, though community composition does. I take this to mean that while total species richness of bacteria (as measured by DGGE) was constant across sites, species turnover (Beta-diversity) was high. This is interesting to me, though not mentioned in the paper, because it seems to directly contradict the microbial-biogeography hypothesis of “everything is everywhere”. There is a mention in the description of this comparison of the importance of geography in structuring biodiversity, which reads to me like an opportunity to apply explicit geospatial techniques to their dataset.
Overall, while it is true that Antarctic soil ecosystems are extremely simple relative to other systems, there are a great many complex and variable interactions between even the few components of these systems, creating a great deal of complexity. This paper will be very useful to me in structuring some of my own investigations in the Arctic.
Wednesday, September 2, 2009
Niederberger et al. 2008
Neiderberger TD, McDonald IR, Hacker AL, Soo RM, Barrett JE, Wall DH, Cary SC. 2008. Microbial community composition in soils of Northern Victoria Land, Antarctica. Environmental Microbiology 10: 1713-1724.
These authors present an analysis of a large collection of data regarding both microbial and metazoan biodiversity at relatively small scales in one part of Taylor Valley, Antarctica, one of the famous Dry Valleys. This contributes to both the Latitudinal Gradient Project, an international effort to characterize Antarctica, and to the biogeographical debate regarding the distribution and community assemblages of microbes and soil microfauna.
Biodiversity was higher than expected based on the physical characteristics of this extreme environment, and was much more variable at small (~200m) spatial scales. While the microbes identified by 16s sequences were not particularly surprising, the changes in community composition between study sites was high. This supports the hypothesis that extreme environments “select for” particular microbial physiologies, and that differences in soil physical features such as moisture and temperature are highly important, in distinct contrast to the “everything is everywhere” hypothesis of microbial biogeography.
NB October 1 2009: the “everything is everwhere” hypothesis (Beijerinck 1913) includes the second clause “the environment selects”, which implies my earlier impressions, above, are incorrect. This paper’s demonstration that extreme environments select for particular soil communities, and that local-scale variables such as moisture and temperature, rather than regional-scale variables such as climate factors, actually supports Beijerinck’s (1913) hypothesis, rather than countering it.
These authors present an analysis of a large collection of data regarding both microbial and metazoan biodiversity at relatively small scales in one part of Taylor Valley, Antarctica, one of the famous Dry Valleys. This contributes to both the Latitudinal Gradient Project, an international effort to characterize Antarctica, and to the biogeographical debate regarding the distribution and community assemblages of microbes and soil microfauna.
Biodiversity was higher than expected based on the physical characteristics of this extreme environment, and was much more variable at small (~200m) spatial scales. While the microbes identified by 16s sequences were not particularly surprising, the changes in community composition between study sites was high. This supports the hypothesis that extreme environments “select for” particular microbial physiologies, and that differences in soil physical features such as moisture and temperature are highly important, in distinct contrast to the “everything is everywhere” hypothesis of microbial biogeography.
NB October 1 2009: the “everything is everwhere” hypothesis (Beijerinck 1913) includes the second clause “the environment selects”, which implies my earlier impressions, above, are incorrect. This paper’s demonstration that extreme environments select for particular soil communities, and that local-scale variables such as moisture and temperature, rather than regional-scale variables such as climate factors, actually supports Beijerinck’s (1913) hypothesis, rather than countering it.
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