Can dormancy account for patterns of microbial biogeography?

Background/Question/Methods

Dormancy is a bet-hedging strategy that allows organisms to enter a reversible state of reduced metabolic activity. This process lowers the risk of local extinction through recruitment of individuals from seed banks and increases the probability of colonization by allowing immigrants to avoid mortality. Together, dormancy should enhance taxa richness at the local scale (α-diversity), while decreasing compositional turnover over at larger spatial scales (β-diversity). We tested these predictions using a combination of simulation modeling and a spatial survey of soil bacterial communities. The model was used to construct taxa area relationships (TAR) while controlling for dispersal and dormancy. We compared these simulated TARs with those constructed from the spatial survey for the total (active + dormant) and active soil bacteria via sequencing of 16S rRNA genes and transcripts, respectively.

Results/Conclusions

Compared to macroorganisms (plants and animals), the spatial turnover of bacterial communities is extremely low.  It is often assumed that this is due to the high dispersal capacity of microorganisms. Our simulations support this view: the z-value (slope) of the TAR relationship declines with increasing dispersal ability. However, a similar effect can be achieved by increasing the persistence of individuals within a patch by allowing for dormancy. Our empirical data suggest that dormancy affects microbial diversity and local and regional scales. Compositionally, soil microbial communities look very different depending on whether they are characterized using DNA- or RNA-based approaches. For example, our results suggest that up to 40% a community may be comprised of inactive taxa. Together, these results lend support to the view that spatial patterns of biodiversity are strongly influenced by dormant seed banks.