# Dissolved Organic Matter in the Coastal Ocean Is Structurally More Diverse Than in Terrestrial Systems, as Shown in an Amazonian Mangrove Estuary

## TL;DR

This study shows that dissolved organic matter in coastal oceans has more structural diversity than in terrestrial systems, based on findings from an Amazonian mangrove estuary.

## Contribution

The study reveals that coastal marine DOM is structurally more diverse than terrigenous DOM due to molecular pathways like (photo)oxidation and sulfur incorporation.

## Key findings

- Aromatic compounds decreased from river to coastal ocean, while aliphatic compounds increased.
- Coastal marine DOM showed greater structural diversity despite fewer molecular formulas.
- Sulfurized compounds were linked to urban and mangrove sediment sources.

## Abstract

Dissolved organic matter (DOM) cycling across the land-ocean
continuum
is highly complex, and our limited understanding of DOM molecular
transformations hinders a full assessment of land-ocean connectivity
in the global carbon cycle. Here, we applied one- and two-dimensional
high-field 1H nuclear magnetic resonance (NMR) spectroscopy
and ultrahigh-resolution mass spectrometry (FT-ICR-MS) to investigate
sources and transformations of solid-phase extractable DOM along
an Amazonian mangrove-fringed river-to-ocean transect. Relative abundances
of aromatic compounds decreased from the river to the coastal ocean,
whereas aliphatic compounds increased. NMR spectroscopic features,
commonly associated with carbohydrates, are probably related to flavonoid-
and lignin-derived structural motifs. These structural features were
more readily detected by 1H NMR spectroscopy, whereas aromatics
were more effectively detected by FT-ICR-MS. We tentatively identified
polycyclic aromatic sulfur-containing compounds as being predominantly
derived from urban areas, whereas sulfurized aliphatic compounds originated
from sulfidic mangrove sediments. Surprisingly, while the number of
DOM molecular formulas decreased along the river-to-coastal continuum,
coastal marine DOM exhibited greater structural diversity than terrigenous
DOM. Here, we showed that the interplay of distinct molecular pathways,
particularly (photo)­oxidation processes and sulfur incorporation,
structurally diversifies DOM in coastal marine environments.