Changes In Soil Bacterial Community Structure and Diversity Along a Coastal Sand Chronosequence.

Organism succession during ecosystem development has been relatively well studied for aboveground microbial communities while the associated pattern of change in microbial communities remains largely unknown. We initiated a study along a developmental sand-dune chronosequence bordering northern Lake Michigan at Wilderness State Park with the hypothesis that the soil bacterial communities will follow a pattern of change that is associated with soil, plant, and ecosystem development. This study site included 5 replicate sites along 9 dunes ranging in age from 95 to 4000 years since deposition.  The microbial composition and diversity in the soil was studied using PLFA and bacterial tag-encoded FLX amplicon pyrosequencing of the 16S rRNA gene. As hypothesized Bray-Curtis ordination indicated that bacterial community assembly changed along the developmental gradient. However, there was an effect of season in the younger but not older dunes. Soil Ca, Mg and pH changed across the chronosequences were found to significantly correlate to this pattern of bacterial community change.  Bacterial diversity represented by Simpson’s reciprocal index (Simpson’s 1/D) showed a steady decline from youngest to the oldest dunes with largest decline during the initial stages of soil development. The diversity declined from 212 to 49 (Simpson’s 1/D) during 95 to 440 years of soil development. The phyla Actinobacteria, Proteobacteria and Acidobacteria accounted for 75 % of the sequences, on average, across the chronosequences for both the seasons. The phylum Acidobacteria showed significant increase (r ~ 0.7) in relative abundance across the developmental chronosequences following the pH gradient from 7.8 to 3.4.  The phyla Actinobacteria and Cyanobacteria showed significant decrease (r ~ -0.80) in relative abundance with soil development while Proteobacteria abundance was relatively stable. The results suggest that gradients in soil pH and soil nutrients could be a strong driving force in shaping microbial community assembly across a developing soil ecosystem.