Quantifying Stochastic Processes in Shaping Dissolved Organic Matter Pool with High-Resolution Mass Spectrometry

Natural dissolved organic matter (DOM) represents a ubiquitous
molecular mixture, progressively characterized by spatiotemporal resolution.
However, an inadequate comprehension of DOM molecular dynamics, especially
the stochastic processes involved, hinders carbon cycling predictions.
This study employs ecological principles to introduce a neutral theory
to elucidate the fundamental processes involving molecular generation,
degradation, and migration. A neutral model is thus formulated to
assess the probability distribution of DOM molecules, whose frequencies
and abundances follow a β-distribution relationship. The neutral
model is subsequently validated with high-resolution mass spectrometry
(HRMS) data from various waterbodies, including lakes, rivers, and
seas. The model fitting highlights the prevalence of molecular neutral
distribution and quantifies the stochasticity within DOM molecular
dynamics. Furthermore, the model identifies deviations of HRMS observations
from neutral expectations in photochemical and microbial experiments,
revealing nonrandom molecular transformations. The ecological null
model further validates the neutral modeling results, demonstrating
that photodegradation reduces molecular stochastic dynamics at the
surface of an acidic pit lake, while random distribution intensifies
at the river surface compared with the porewater. Taken together,
the DOM molecular neutral model emphasizes the significance of stochastic
processes in shaping a natural DOM pool, offering a potential theoretical
framework for DOM molecular dynamics in aquatic and other ecosystems.