Experimental manipulation of selfish genetic elements links genes to microbial community function

Microbial communities underpin earth’s biological and biogeochemical processes, but their complexity hampers understanding. Here we draw upon predictions from the theory of selfish genetic elements (SGEs), combined with approaches from experimental evolution, comparative metagenomics and biochemistry, and show how naturally occurring SGEs can be used to manipulate genes that underpin community function. Communities comprising hundreds of bacterial genera established from garden compost were propagated with bi-weekly transfer for one year in nitrogen-limited minimal medium with cellulose (paper) as sole carbon source. At each transfer, SGEs from one set of independent communities were collected, pooled and redistributed among this same set of communities (horizontal treatment). A control was included in which SGEs never moved beyond the community in which they were originally present (vertical treatment). SGEs along with genes of ecological relevance were rapidly amplified across horizontal communities and their dynamics tracked. Enrichment of genes implicated in nitrogen metabolism, and particularly ammonification, led to biochemical assays that showed a significant functional difference between communities subject to horizontal versus vertical treatments. This simple experimental strategy offers a powerful new approach for unravelling dynamical processes underpinning microbial community function.