Microbial Changes with Depth, Treatment, and Time in the Northern Minnesota SPRUCE Experiment


Spencer Roth1, Eric Johnston1, Laurel Kluber1, Susannah Tringe3,4, Emiley Eloe-Fadrosh3,4, Joel Kostka2, Paul J. Hanson1, Christopher Schadt1* (schadtcw@ornl.gov)


1Oak Ridge National Laboratory, Oak Ridge, TN; 2School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA; 3DOE Joint Genome Institute, Berkeley, CA; 4Lawrence Berkeley National Laboratory, Berkeley, CA



Peatlands store one third of all global terrestrial organic matter and are thus especially important to understand in response to changing climatic patterns. To understand how peatlands may respond, the SPRUCE experiment in northern Minnesota has applied whole-ecosystem, above- and belowground warming ranging from +0° to +9°C above ambient since 2015. Part of these efforts involves investigating changes in microbial communities within peat in the SPRUCE experimental plots with depth and years of warming in the acrotelm and catolem peat using metagenome assembled genomes (MAGs) resolved via shotgun metagenomic approaches. Over the course of three sampling years (2015, 2016, and 2018), each encompassing multiple depth increments (10-20, 40-50, 100-125 and 150-175 cm), scientists have recovered approximately 800 medium- and high-quality dereplicated MAGs across the SPRUCE treatment enclosures. Overall microbial community composition and MAG alpha-diversity show strong depth stratification, likely primarily driven by variation in redox conditions across the peatland depth profile. MAG communities are dominated by Acidobacteriaota that constitute >30% of MAGs recovered across samples and years, with members of Proteobacteria, Actinobacteriota, and Verrucomicrobiota composing the majority of the remainder (at 15%, 11%, and 7% respectively). Identified methanogens also vary with depth but peak in the 40–50 cm increment and are dominated by MAGs similar to Candidatus Methanoflorens stordalenmirensis (rice cluster II) that are often reported as important in other diverse systems but remain uncultured to date. Within depths approximately 100 MAGs were identified whose abundance was significantly correlated with temperature across temperature treatments, although the overall variance explained by warming treatments is low at around 4%. Previously published research from SPRUCE has revealed shifts in soil metabolomes and increased carbon dioxide (CO2) and methane (CH4) production with warming; however, in earlier years these seem to have been primarily driven by changes in recently fixed carbon (C) likely in porewater dissolved organic carbon pools, with C in CO2 and CH4 derived from older peat-derived pools only increasing in the most recent years. Ongoing efforts to characterize the extent of any further change over time in 2022 samples, as well as planned similar efforts for the end of the experiment in 2025, will investigate how any continued functional shifts in microbial communities may drive these and other changes in these important ecosystems.