Lives in a gram of soil: linking life cycle biology to soil ecology.

Soils are among the most diverse but least understood environments on the planet. Just below our feet, secluded from our sight, we find complex communities of bacteria, fungi, protists, and animals. These communities organize themselves in both time and space, from fast to slow growers, from decomposers to predators, from early colonizers to late arrivers. Despite a growing understanding of community dynamics, the lives of organisms within the soil often remain poorly understood: how do organisms grow, propagate, and disperse within their particle-packed environments? In this essay, I reflect on the microbial life cycles found in soils. Starting from a single gram of soil collected on our campus, I explore the conceptual building blocks underlying life cycle biology, how life cycles emerge within their environment, and how they are coupled between species. Microbes express widely diverse life cycle motifs with many unique life stages promoting growth and/or propagation. These life stages have a major impact on community ecology; they determine the fate of microbes within soils and affect how, when, and where microbes interact. A life cycle perspective is thus essential for deepening our understanding of the community dynamics that unfold underfoot.