Assistant Profersor University of California, San Diego, California, United States
Abstract: Plant-microbe interactions regulate ecosystem function, carbon cycling, and energy flux among trophic levels within and across ecosystem boundaries. We have previously shown that red alder (Alnus rubra) leaf litter decomposes faster at local sites than more distant sites within riverine and riparian systems. This phenomenon known as the “Home Field Advantage” occurs in part due to spatial variation in red alder secondary leaf metabolites, locally adjusted microbial communities and/or locally adapted microbial populations. Further, early successional bacterial communities inhabiting decomposing A. rubra leaves are comprised of taxa within the Burkholderiales, which are known for their capacity to degrade aromatic compounds, such as plant secondary metabolites. In this study, we quantify microbial metabolic activity in response to intraspecific variation in host derived secondary metabolites throughout decomposition. We used a reciprocal transplant experimental design, where single genotype leaf packs (n = 20) were submerged at home and away sites on the Sekiu and Hoko Rivers. For each genotype, secondary metabolites were extracted from dried leaf tissue with 70% methanol over 14 days. Leaf packs were submerged for a total of 20 days and microbial decomposer communities were collected every five days from these decomposing leaves. In-situ tetrazolium assays were designed to evaluate microbial metabolic activity at each time point via a colorimetric change. Changes in bacterial cell abundance and community composition were described by flow cytometry and 16S and ITS1 rRNA amplicon sequencing.
We found that leaves sourced from local trees decomposed faster compared to those sourced from trees growing in the riparian zone of a neighboring river (µHFA Index [HFAI] = 13.63%, where 78.57% of reciprocal comparisons showed positive HFAI). Further, the home-field advantage pattern was evident in our in-situ microbial metabolism assays. Specifically, we found that secondary leaf metabolites are degraded more quickly when paired with microbial communities originating from the same site as the source tree, suggesting microbial metabolism is adjusted to most efficiently degrade secondary metabolites from local plants. While this home-field advantage pattern was evident at each stage of decomposition, across all tree genotypes, late-stage microbial communities tended to degrade host derived secondary metabolites more quickly than early successional communities (p < 0.001). These results suggest that as decomposition progresses, more recalcitrant compounds may become more readily available for metabolism by a consortia of microbial decomposers. Linking microbial metabolism with spatial variability in host derived compounds could shed light on mechanisms that mediate ecosystem productivity and function.
