Microbial Community Succession in an Unvegetated, Recently Degiaclated Soli

Prim ary succession is a fundam ental process in m acro­ ecosystems; however, if and how soil developm ent influences m icrobial com m unity structure is poorly understood. Thus, we investigated changes in the bacterial com m unity along a chronosequence o f three unvegetated, early successional soils (~20-year age gradient) from a receding glacier in southeastern Peru using molecular phylogenetic techniques. We found that evenness, phylo­ genetic diversity, and the num ber o f phylotypes were lowest in the youngest soils, increased in the interm ediate aged soils, and plateaued in the oldest soils. This increase in diversity was com m ensurate w ith an increase in the num ber o f sequences related to com m on soil bacteria in the older soils, including m em bers o f the divisions Acidobacteria, Bacteroidetes, an d Verrucom icrohia. Sequences related to the Com am onadaceae clade of the Betaproteobacteria were dom inant in the youngest soil, decreased in abundance in the interm ediate age soil, and were no t detected in the oldest soil. These sequences are closely related to culturable heterotrophs from rock and ice environm ents, suggesting that they originated from organisms living w ithin or below the glacier. Sequences related to a variety of nitrogen (N)-fixing clades w ithin the Cyanobacteria were abundant along the chronose­ quence, com prising 6-40% o f phylotypes along the age gradient. A lthough there was no obvious change in the overall abundance of cyanobacterial sequences along the chronosequence, there was a dram atic shift in the abundance o f specific cyanobacterial phylotypes, w ith the Correspondence to: D iana R. N em ergut; E-mail: nem ergut@ colorado. edu interm ediate aged soils containing the greatest diversity o f these sequences. M ost soil biogeochemical character­ istics showed little change along this ~20-year soil age gradient; however, soil N pools significantly increased with soil age, perhaps as a result o f the activity o f the N-fixing Cyanobacteria. O ur results suggest that, like m acrobial com m unities, soil m icrobial com m unities are structured by substrate age, and that they, too, undergo predictable changes through time.