Indiana University Bloomington Bloomington, Indiana, United States
Abstract: Background/Question/Methods
Mark-recapture approaches are common for studying animal and plant populations but are often not performed in microbial ecology. Recent advances in molecular barcoding provide the opportunity to track microbial lineages through space and time. Here, we address long-standing questions about the contributions of variation to symbiotic partner quality using molecular barcoding. Rhizobia are symbiotic bacteria that fix nitrogen for leguminous plants. Much of our knowledge of their lifecycle focuses on their symbiotic phase, despite their extensive time spent in the soil. We conducted a greenhouse experiment to test for a hypothesized trade-off between rhizobia fitness in soil versus plant roots. We introduced unique barcodes into high-quality and low-quality mutualists that allow us to track each strain in soil and nodules. We predicted that poor-quality symbionts invest in their free-living soil phase, while superior symbionts have higher fitness in root nodules. Our experiment involved inoculating rhizobia individually and together in soil with and without the plant host present to identify trade-offs in rhizobia fitness over plant developmental stages to gain insight into functional differences between each rhizobial symbiont.
Results/Conclusions
The high-quality symbiont had reduced fitness in the bulk soil environment compared to poor-quality mutualist. Yet, rhizobia fitness exhibited the reverse trend in the nodule environment with the high-quality symbiont outcompeting the poor-quality symbiont, which supports our hypothesized trade-off. To further understand functional differences among rhizobia strains, we examined several microbial-responsive plant traits. When symbionts were inoculated individually, plant traits were significantly increased with respect to the high-quality symbiont compared to the poor-quality rhizobia strain. However, when co-inoculated, plant traits indicated non-additive negative effects on plant fitness suggesting that rhizobia compete with one another. The abundance of each symbiont is assessed with mark-recapture technology to identify how the presence of the plant alters the rhizobial community in soil. Our data highlights how microbial mark-recapture can provide insight into the lifecycle of microbes to address long-standing and difficult to address questions.