Time-series RNA metabarcoding of the active Populus tremuloides root microbiome reveals hidden temporal dynamics and dormant core members

Dormancy is a key life history stage of many microbes that involves existence in a metabolically inactive state. Although dormant taxa may contribute little to community function, most microbial metabarcoding community surveys of microbial systems target DNA which captures dormant and dead taxa in addition to the active and living fraction of the community. RNA metabarcoding offers the potential to delineate the active fraction of microbial communities. Transitions between dormancy and activity may also serve as a rapid response mechanism for communities for communities to alter their function faster than turnover in community composition. Additionally, a focus on active microbiomes may provide further insight into which taxa should be prioritized as part of the core microbiome. We used DNA and RNA metabarcoding in samples collected as a time-series of the quaking aspen (Populus tremuloides) root microbiome across a natural environmental gradient to identify the degree of spatiotemporal dynamism in active and total microbial communities and to document the extent of dormancy in the core microbiome. We found that active bacterial and fungal communities were more temporally dynamic than total communities, while total communities exhibited a stronger response to spatial variability in site conditions. Additionally, we found that core microbiome members are frequently inactive, resulting in a reduced active core microbiome. This study stresses the need to focus on turnover in the active microbial community to detect variation in microbial communities in time-series and to use microbial activity levels as a key determinant of core microbiome membership.