Microbial labilization and diversification of pyrogenic dissolved organic matter
Authors: Goranov Aleksandar I. , Wozniak Andrew S. , Bostick Kyle W. , Zimmerman Andrew R. , Mitra Siddhartha , Hatcher Patrick G. 
Published: March 2022

Abstract. With the increased occurrence of wildfires around the world, interest in the
chemistry of pyrogenic organic matter (pyOM) and its fate in the environment
has increased. Upon leaching from soils by rain events, significant amounts
of dissolved pyOM (pyDOM) enter the aquatic environment and interact with
microbial communities that are essential for cycling organic matter within
the different biogeochemical cycles. To evaluate the biodegradability of
pyDOM, aqueous extracts of laboratory-produced biochars were incubated with
soil microbes, and the molecular changes to the composition of pyDOM were
probed using ultrahigh-resolution mass spectrometry (Fourier transform–ion cyclotron resonance–mass spectrometry). Given that solar irradiation
significantly affects the composition of pyDOM during terrestrial-to-marine
export, the effects of photochemistry were also evaluated in the context of
pyDOM biodegradability. Ultrahigh-resolution mass spectrometry revealed that many different (both
aromatic and aliphatic) compounds were biodegraded. New labile compounds were produced, 22 %–40 % of which were peptide-like. These results
indicated that a portion of pyDOM has been labilized into microbial biomass
during the incubations. Fluorescence excitation–emission matrix spectra
revealed that some fraction of these new bio-produced molecules is
associated with proteinaceous fluorophores. Two-dimensional 1H–1H
total correlation nuclear magnetic resonance
(NMR) spectroscopy identified a peptidoglycan-like backbone
within the microbially produced compounds. These results are consistent with
previous observations of peptidoglycans within the soil and ocean nitrogen
cycles where remnants of biodegraded pyDOM are expected to be observed. Interestingly, the exact nature of the bio-produced organic matter was found
to vary drastically among samples indicating that the microbial
consortium used may produce different exudates based on the composition of the
initial pyDOM. Another potential explanation for the vast diversity of
molecules is that microbes only consume low molecular-weight compounds, but
they also produce reactive oxygen species (ROS), which initiate oxidative
and recombination reactions that degrade high molecular-weight compounds and produce new molecules. Some of the
bio-produced molecules (212–308 molecular formulas) were identified in estuarine and marine (surface and abyssal oceanic), and 81–192 of these formulas were of molecular
composition attributed to carboxyl-rich alicyclic molecules (CRAM). These
results indicate that some of the pyDOM biodegradation products have an
oceanic fate and can be sequestered into the deep ocean. The observed
microbially mediated diversification of pyDOM suggests that pyDOM
contributes to the observed large complexity of natural organic matter
observed in riverine and oceanic systems. More broadly, our research shows
that pyDOM can be substrate for microbial growth and be incorporated into
environmental food webs within the global carbon and nitrogen cycles.