Uncovering the metabolic processes driving microbial activity beneath the snowpack and mechanisms of microbial biomass turnover and nitrogen release following snowmelt

In mountainous systems with seasonal snow-cover, snowmelt coincides with a large pulse of nutrients originating from microbial mineralization of organic matter beneath the snowpack, and from turnover of microbial biomass during snowmelt. Vegetation phenology in these systems is regulated by environmental cues such as photoperiod, soil and air temperature, such that plant nutrient uptake and biomass production are highest, and typically in sync with microbial biomass turnover and nutrient release. The metabolic processes regulating organic matter mineralization and assimilation under-snow by soil bacteria and fungi, as well as the mechanisms of microbial biomass turnover during snowmelt, have not been characterized previously. As part of the Watershed Function Scientific Focus Area (SFA) at the Lawrence Berkeley National Laboratory (LBNL), one goal is to determine the metabolic processes fueling microbial activity beneath the snowpack and the factors contributing to microbial biomass turnover and N release following snowmelt. Using a range of approaches including metagenomics, metatranscriptomics, and spectrometric methods such as FT-ICR MS and NMR, we are building a conceptual model of microbial metabolism and turnover in these systems. Initial results suggest that fermentation is an important process under snow and immediately following snowmelt with volatile fatty acids (acetate, formate) and other volatile organic compounds (VOCs), with ethanol, acetone, methanol and isopropanol also prominent. Colorimetric assays indicated an increase in primary amine concentrations following snowmelt and corresponded with FT-ICR MS detection of polypeptides and amino sugars. NMR detection of mono-, di-, and tri-methylamine also indicated these as potentially important organic N species. Other compounds known to function as cryoprotectants (e.g., sarcosine) were also detected. Analysis of recovered microbial genomes and their transcriptomes are ongoing and are being used to identify the microorganisms and pathways involved in organic matter transformation under snow and the mechanisms of microbial biomass turnover that likely fuels early season plant productivity.