Molecular Insights on Dissolved Organic Matter Transformation by Supraglacial Microbial Communities

Snow
overlays the majority of Antarctica and is an important repository
of dissolved organic matter (DOM). DOM transformations by supraglacial
microbes are not well understood. We use ultrahigh resolution mass
spectrometry to elucidate molecular changes in snowpack DOM by in
situ microbial processes (up to 55 days) in a coastal Antarctic site.
Both autochthonous and allochthonous DOM is highly bioavailable and
is transformed by resident microbial communities through parallel
processes of degradation and synthesis. DOM thought to be of a more
refractory nature, such as dissolved black carbon and carboxylic-rich
alicyclic molecules, was also rapidly and extensively reworked. Microbially
reworked DOM exhibits an increase in the number and magnitude of N-,
S-, and P-containing formulas, is less oxygenated, and more aromatic
when compared to the initial DOM. Shifts in the heteroatom composition
suggest that microbial processes may be important in the cycling of
not only C, but other elements such as N, S, and P. Microbial reworking
also produces photoreactive compounds, with potential implications
for DOM photochemistry. Refined measurements of supraglacial DOM and
their cycling by microbes is critical for improving our understanding
of supraglacial DOM cycling and the biogeochemical and ecological
impacts of DOM export to downstream environments.