Nemergut et al. 2007

Nemergut DR, Anderson SP, Cleveland CC, Martin AP, Miller AE, Seimon A, Schmidt SK. 2007. Microbial community succession in an unvegetated, recently deglaciated soil. Microbial Ecology 53: 110-122.

These authors describe the partly-predictable patterns of succession among the soil microbes of a glacial foreland in Peru. Primary succession on new terrain, as found in front of a receding glacier, has been studied to some extent, especially regarding the vegetation. Studies of the microbial communities have been rarer, but the few that have been conducted have suggested that these communities also show predictable patterns of community assembly and turnover associated with soil age. Basic ecological theory has led to the nitrogen paradigm of primary succession in soils: nitrogen is absent from new mineral substrate, thus nitrogen fixing organisms have a competitive advantage and are therefore abundant. The presence of nitrogen fixers is tightly linked to the accumulation of soil nitrogen; hence these organisms may facilitate later successional stages.

The study site in this paper is in Peru, at a glacier that seems to be receding quickly. These authors sampled from 3 transects arranged parallel to the front of the glacier, located adjacent to the glacier on soil less than a year old, 100m away on soil about 4 years old, and 500m away on soil about 20 years old. This area receives very high inputs of pollen, leading to the hypothesis that heterotrophic, nitrogen-fixing organisms may be present, using the pollen as a carbon source but drawing nitrogen from the atmosphere because the C:N ratio of pollen is higher than that of microbial biomass. Surface soil samples were collected, kept at 0C, and analyzed in Colorado. Much of the analyses were detailed phylogenetic examination, including the P-test of Martin (2002); note that he is one of the authors of this paper. OTU and a range of sequence-data fine-tuning techniques were also employed.

Over the study area, autotrophic nitrogen fixers were abundant. The bacteria found were extremely diverse at the highest taxonomic levels, and many sequences identified were not closely related to existing sequences in public databases. Diversity increased rapidly from the youngest soils to the 4-year-old, then plateaued.

One very interesting group of bacteria found are the Comamodaceae; sequences reported in other studies were in many cases derived from glacial or ice-sheet ice. The Comamodaceae found in the youngest soils here may have persisted as viable populations in the glacier; differences between the two closely-examined youngest communities suggest physical and genetic isolation for hundreds to thousands of years, allowing speciation events to accumulate differences.

Other patterns among the sequences identified suggest that the earliest colonizers of new terrain may be cosmopolitan – some of the Comamodaceae sequences, for example, are similar to those derived from a glacier in Nunavut. Later colonizers may be more endemic, and displace the earliest colonizers as soils age. The trophic status of the first colonizers is not clear; these authors did not have a test for definite autotrophs or heterotrophs, as Comamodaceae are known to include both modes. At the macrobiological level, the earliest arrivals on new terrain are typically heterotrophs, insects that feed on deposited organic matter such as wind-blown pollen.

This paper is very useful to me, describing as it does a complete set of analytical procedures for my planned biogeographic / phylogenetic studies, as well as providing data in the form of publicly-accessible sequences and analyzed information on patterns of soil microbial community assembly.