Functional basis of primary succession: Traits of the pioneer microbes

Primary succession describes the establishment of new ecosystems through the colonization of barren substrates (Odum, 1969). Bacteria are typically the first colonizers, which initiate key processes that enable ecosystem establishment and provide resources for fungi, plants, and animals to colonize later. The functional roles of these pioneer microbes are varied; for example, they can weather and detoxify environments to increase habitability, provide organic carbon and bioavailable nitrogen through carbon and nitrogen fixation, and form mutualistic relationships with plant species (Abdulla, 2009; Richardson & Simpson, 2011). As taxonomic and functional diversity increases over annual to decadal scales, communities in successional ecosystems transition from a collection of pioneer microbial species towards a robust and complex ecosystem (Álvarez-Molina et al., 2012; Brown & Jumpponen, 2015; Frouz et al., 2016). Much literature has explored plant community succession, resulting in a strong understanding of the nature, traits, and interactions of early and late colonizers and the development of strong ecological theory to explain these processes. However, understanding of pioneer microbial communities remains rudimentary, with most literature focusing on taxonomic composition rather than functional traits of colonizing bacteria. Numerous key questions remain, for example: How do the first microbes survive and grow in a barren and often hostile environment with minimal nutrients? How do microbes modify their environment for succeeding generations and species? How do these microbes interact with each other, plants, and animals as the new ecosystem develops? We propose here a series of study systems, methodological approaches, and ecological frameworks to address these questions. In turn, a better understanding of primary succession has broad implications for understanding ecological theory, biogeochemical processes, and responses to climate change. Moreover, by understanding how life colonizes barren and hostile locations, we can better predict how life evolved on Earth and might exist on other planets.