In nature, most organisms live in unpredictable environ- ments and typically experience conditions that are sub- optimal for growth and reproduction. A very common response to environmental stress is for organisms to enter a reversible state of reduced metabolic activity, or dormancy 1 . By doing so, these organisms can dras- tically lower their energetic expenditures and evade unfavourable conditions that would otherwise reduce the fitness of the population. Dormancy is not a cost- free strategy, however. Organisms must invest resources into resting structures and the machinery that is needed for transitioning into and out of a dormant state 2,3 . In addition, dormant organisms must be able to inter- pret and respond to signals associated with favourable conditions, otherwise they will miss opportunities for growth and reproduction 4–6 . Despite these general trade- offs, microbial species from all domains of life have evolved the ability to use dormancy during periods of environmental stress 7–9 (FIG. 1). Dormancy generates a seed bank, which we define as ‘a reservoir of dormant individuals that can potentially be resuscitated in the future under different environmen- tal conditions’. Seed banks can prolong the persistence of genotypes and populations, and also have important consequences for community- and ecosystem-level processes. Ecologists have developed theory and have empirically demonstrated the importance of seed banks for the diversity and functioning of communities of macroscopic organisms (that is, plants and animals). For example, the accumulation of long-lived organ- isms in a seed bank allows competing species to coexist via the storage effect, thus maintaining biodiversity 10 . Seed banks also determine how communities recover from disturbance 11 and potentially stabilize ecosystem processes under variable environmental conditions 12 . Because microbial dormancy is common in a range of ecosystems, it seems likely that the ecology and evolu- tion of microbial communities are strongly influenced by seed bank dynamics. Dormancy is a well-documented trait that helps microorganisms contend with environmental variability. Although it is common and phylogenetically widespread, dormancy is achieved through a diverse set of genetic and cellular mechanisms (FIG. 1). Dormancy has received considerable attention from microbiologists, in part because of its role in human diseases such as anthrax, cholera and tuberculosis 13 . As a result, microbiologists have unravelled the mechanistic underpinnings of dor- mancy in a handful of clinically and environmentally important strains of bacteria. Collectively, this infor- mation provides a strong foundation for exploring the prevalence and implications of dormancy in natural ecosystems, but we are still challenged by the complex- ity of environmental systems, which typically contain thousands of potentially interacting species. With recent innovations in molecular techniques and computational analyses, we are now poised to test theory about how dormancy influences the structure and function of microbial communities in complex natural settings. In this Review, dormancy is broadly defined as “any rest period or reversible interruption of the phenotypic development of an organism” (REF. 14). From the outset, *W.K. Kellogg Biological Station, Michigan State University, 3700 East Gull Lake Drive, Hickory Corners, Michigan 49060, USA. ‡ Department of Microbiology and Molecular Genetics, Michigan State University. § Present address: Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA. Correspondance to J.T.L. e-mail: lennonja@msu.edu doi:10.1038/nrmicro2504 Storage effect An ecological hypothesis stating that environmental fluctuations drive temporal variations in population growth that produce long-lived individual organisms, thus promoting multispecies coexistence. Microbial seed banks: the ecological and evolutionary implications of dormancy Jay T. Lennon* ‡ and Stuart E. Jones* § Abstract | Dormancy is a bet-hedging strategy used by a wide range of taxa, including microorganisms. It refers to an organism’s ability to enter a reversible state of low metabolic activity when faced with unfavourable environmental conditions. Dormant microorganisms generate a seed bank, which comprises individuals that are capable of being resuscitated following environmental change. In this Review, we highlight mechanisms that have evolved in microorganisms to allow them to successfully enter and exit a dormant state, and discuss the implications of microbial seed banks for evolutionary dynamics, population persistence, maintenance of biodiversity, and the stability of ecosystem processes. REVIEWS NATURE REVIEWS | Microbiology VOLUME 9 | FEBRUARY 2011 | 119 © 2011 Macmillan Publishers Limited. All rights reserved