Relating Molecular Properties to the Persistence of Marine Dissolved Organic Matter with Liquid Chromatography–Ultrahigh-Resolution Mass Spectrometry

Marine dissolved organic matter (DOM)
contains a complex mixture
of small molecules that eludes rapid biological degradation. Spatial
and temporal variations in the abundance of DOM reflect the existence
of fractions that are removed from the ocean over different time scales,
ranging from seconds to millennia. However, it remains unknown whether
the intrinsic chemical properties of these organic components relate
to their persistence. Here, we elucidate and compare the molecular
compositions of distinct DOM fractions with different lability along
a water column in the North Atlantic Gyre. Our analysis utilized ultrahigh-resolution
Fourier transform ion cyclotron resonance mass spectrometry at 21
T coupled to liquid chromatography and a novel data pipeline developed
in CoreMS that generates molecular formula assignments and metrics
of isomeric complexity. Clustering analysis binned 14 857 distinct
molecular components into groups that correspond to the depth distribution
of semilabile, semirefractory, and refractory fractions of DOM. The
more labile fractions were concentrated near the ocean surface and
contained more aliphatic, hydrophobic, and reduced molecules than
the refractory fraction, which occurred uniformly throughout the water
column. These findings suggest that processes that selectively remove
hydrophobic compounds, such as aggregation and particle sorption,
contribute to variable removal rates of marine DOM.