Spatial microbial flows: hidden fluxes of detritus, diversity, and function in meta-ecosystems

Ecosystems are linked by spatial flows of energy, nutrients, and organisms across ecosystem boundaries, forming a meta-ecosystem. The study of spatial flows has typically focused on fluxes of materials (e.g., nutrients or organic matter) and conspicuous organisms (e.g., fish or insects). However, recent evidence from field studies has suggested that numerically significant spatial flows of microorganisms occur across ecosystem boundaries, particularly across the terrestrial-aquatic interface. These spatial microbial flows may be particularly relevant for meta-ecosystem dynamics because the potentially high dispersal rates of microorganisms could exert spatial controls on biodiversity and redistribute microbially mediated ecosystem functions. While understudied empirically, the magnitudes of spatial microbial flows could be quite large in some meta-ecosystems. Estimates of dispersal from terrestrial to aquatic systems have been on the order of 1020 bacterial cells per year in some lakes, but microbial spatial flows likely vary widely in strength and direction through time and space, raising questions about their ecological significance. Much of this uncertainty stems from our lack of understanding of the fate and activity of individual microbial dispersers, which are difficult to track and measure in situ. Microbial spatial flows could sustain biodiversity and ecosystem functions in recipient ecosystems if dispersers typically have high survival and metabolic activity (potentially sustained by co-transport of available material flows); or they may serve as a spatial flow of materials if survival is lower, primarily having indirect effects on diversity and functioning. The fate of microbial dispersers depends on properties of donor and recipient ecosystems, the biotic and abiotic flows that connect them, and the broader spatial and temporal variation across the landscape. Meta-ecosystem ecology can help further integrate microbial movement into our understanding of ecosystem dynamics at the landscape scale.