Evidence for Bacterial Oligotrophy in An Incubated Mollisol and Oxisol.

Experimental evidence for oligotrophic survival strategies in soil bacterial communities remains elusive. This study provides evidence for oligotrophy in soil bacterial populations by measuring ratios of bacterial rRNA:DNA extracted from soils incubated for one year. This ratio was calculated for individual taxa, and serves as a proxy for cellular activity. Copiotrophic groups are expected to have variable rRNA:DNA ratios under different conditions of moisture and carbon availability, as they may enter resting states when stressed. Conversely, oligotrophs are predicted to have constant rRNA:DNA ratios regardless of external conditions. Samples of a mollisol and oxisol were incubated at varying moisture contents, with and without added carbon. After one year, DNA and rRNA were extracted from subsamples of incubated soils. A portion of the 16S rRNA gene was amplified using polymerase chain reaction and sequenced using 454 pyrosequencing. Sequences were analyzed and classified using the Quantitative Insights Into Microbial Ecology (QIIME) software. Copy numbers of bacterial DNA were not significantly different from each other for any of the samples, while rRNA copy numbers were higher in the dry oxisols than in other treatments. Bacterial community structures differed by soil order, but not incubation treatment. Members of phylum Chloroflexi dominated the DNA-based communities in oxisols, while Planctomycetes and Acidobacteria were most prevalent in mollisols. Ratios of rRNA:DNA were higher for Proteobacteria and Chloroflexi in mollisols compared to oxisols. Ratios for Acidobacteria and Verrucomicrobia did not differ between soil orders or incubation conditions, suggesting oligotrophy. These results provide stronger evidence for oligotrophy in soil bacteria than have been obtained in observational studies.