How terrestrial runoff shapes aquatic microbiome composition and function

Raven L. Bier, Savannah River Ecology Lab, University of Georgia and Silke Langenheder, Department of Ecology and Genetics/Limnology, Uppsala University, Uppsala, Sweden

Background/Question/Methods
Aquatic microbiomes are shaped in part by transfers of microorganisms, resources, and stressors across terrestrial and aquatic boundaries. These transfer events can vary in both frequency and magnitude. Understanding how these events regulate the composition of microbial communities has important implications for ecosystem functioning because microorganisms implement most ecosystem processes that are involved in carbon and nutrient transformations. Dispersal is a key mechanism to understand and predict the diversity, composition, and functioning of bacterial communities. For microorganisms, this process is complex since it is not restricted to the immigration of a few external species into a local community but instead involves mixing of biotic communities and associated abiotic matrices. Thus, each community set is exposed to new environmental conditions. Using an 8-day microcosm experiment with a range of soil:lake bacteria ratios and two levels of dissolved nutrients, we tested the hypotheses that mixing of lake and soil bacteria 1) increases the taxonomic diversity of resulting lake bacterial communities and 2) increases the rates of ecosystem processes involved in carbon use.
Results/Conclusions
Our results showed that the primary influence on bacterial richness and composition was the initial concentration of dissolved nutrients, while the ratio of soil-to-lake water-derived taxa and time were less influential. Bacterial richness differed by percent soil bacteria inoculum and the composition of combined soil and lake bacteria communities shifted over time for ambient and elevated dissolved nutrients conditions. At ambient nutrient conditions, bacterial carbon production increased with the percent of soil bacteria inoculum, but the strength of this pattern decreased over time. Elevated nutrient conditions generated greater bacterial carbon production but the ratio of soil bacteria was less influential than with ambient nutrient conditions. Under both ambient and elevated nutrients, the potential activity rates of two extracellular enzymes involved in degrading plant cell walls and algal and bacterial exudates changed depending on the percent soil bacteria and time. These results may have implications for aquatic bacteria community composition and activity under fluctuating weather conditions where communities could experience elevated or reduced inputs from the terrestrial environment.