PS 19-181 - A trait-based modeling approach to understand niche differentiation and succession in the rhizosphere of a Mediterranean grassland

Background/Question/Methods

Multiple studies indicate that microbial populations in the rhizosphere change reproducibly during plant growth. In fact, root exudation itself might be a generalizable trait with rates and chemical composition that can be predicted from plant functional traits, and consistent temporal patterns across plant development stages. The chemical composition of root exudates interacts with metabolite preferences that are predictable from genomic traits. In particular, rhizosphere bacteria isolated from a Mediterranean grassland were found to differ from bacteria in the surrounding bulk soil in genomic traits related to resource acquisition, namely their metabolic potential to utilize organic acids as well as their potential for plant polymer degradation by glycoside hydrolase enzymes (GH). Contrary to conventional wisdom, the authors found that rhizosphere isolates had slower growth rates and hypothesized that substrate preference along with substrate utilization efficiency confer a selective fitness advantage for bacteria in the rhizosphere. Here, we used a genome-informed, trait-based dynamic energy budget model (DEBmicroTrait) to test the trade-off hypothesis between growth rate and carbon-use efficiency (CUE) for 39 soil isolates.

Results/Conclusions

Our results show that resource availability selects for different life history strategies. For metabolites that are assimilated at high rate and yield (sugars, amino acids), a significant inverse relationship exists between growth rate and CUE. Resource allocation to maintenance, GH production and metabolite uptake are nearly independent axes of variation, while growth production correlates with higher biomass turnover. At low growth rates, for growth on organic acids and auxins, the relationship between growth rate and CUE is positive. Maintenance requirements, including GH production, explain most of the variability in growth strategies of isolates at low growth rates. Furthermore, substrate preferences of isolates interact with CUE: we found significant differences in CUE between rhizosphere and bulk soil isolates for growth on metabolites that are preferentially consumed by rhizosphere bacteria. These results suggest that rate-efficiency trade-offs shape bacterial succession patterns in the rhizosphere. High rate is favored over efficiency during early plant growth, while the community will eventually converge on an overall enhancement of efficiency. To simulate succession patterns, DEBmicroTrait was coupled with a plant model (DEBplant). On-going work is concerned with analyzing changes in trait-based strategies and compositional differences across a range of biochemical conditions in the rhizosphere.