Exploring C substrate and soil moisture effects on bacterial diversity through 13C- and 18O-DNA stable isotope probing in polar deserts

Background/Question/Methods

One of the overarching goals of microbial ecology is to understand what key abiotic and biotic environmental factors are responsible for maintaining high levels of microbial diversity. This is complicated by the fact that soil bacteria experience a high degree of resource variability, and a large fraction of the bacterial diversity is often dormant or metabolically inactive at any given time. Recent advances in stable isotope probing (SIP) are helping to identify bacteria that respond to environmental fluctuations and beginning to directly link biological diversity with ecosystem function. In soils, C substrate and moisture resources are primary factors stimulating bacterial metabolic activity. Despite this, our understanding of how C substrates and soil moisture individually influence bacterial diversity is limited. C substrate-mediated effects on diversity are necessarily confounded by soil moisture since periods of high moisture mobilize C substrates within the soil matrix. In an attempt to disentangle substrate and moisture effects on diversity, we performed ¹³C-DNA SIP with ¹³C-labeled mannitol, a sugar alcohol, and ¹⁸O-DNA SIP with H₂¹⁸O, at the McMurdo Dry Valleys Long-term Ecological Research site located in Antarctica.

Results/Conclusions

We have successfully performed both ¹³C-DNA and ¹⁸O-DNA SIP in our Antarctic soils and separated the DNA of bacteria that responded to resource influxes of ¹³C-labeled mannitol from H₂¹⁸O. Based on quantitative PCR results of 16S rRNA gene copies, bacterial DNA showed evidence of ¹³C- and ¹⁸O-labeled DNA with bacteria incorporating both labels within days of application. However, ¹³C-DNA SIP demonstrated better separation between labeled and unlabeled DNA than ¹⁸O-DNA SIP, suggesting that C substrates, instead of moisture, stimulated bacteria and may have the strongest influence on diversity. We are currently using tag pyrosequencing of the 16S rRNA gene of the labeled and unlabeled SIP fractions to identify specific C substrate and moisture effects on bacterial taxa and diversity. Our results suggest that C substrates influence bacteria and, possibly, diversity more than moisture. Additionally, our study represents an intensive research endeavor to combine multiple forms of SIP to identify how essential bacterial resources regulate specific taxa of bacteria and diversity.