Unifying Concepts Linking Dissolved Organic Matter Composition to Persistence in Aquatic Ecosystems

Area of Science:

• Environmental Chemistry
• Biogeochemistry
• Organic Geochemistry

Background:

• The composition and reactivity of dissolved organic matter (DOM) are critical for understanding aquatic systems' role in the global carbon cycle.
• However, unifying concepts that explain the molecular composition of DOM remain elusive.
• Aquatic DOM originates from both external (allochthonous) and internal (autochthonous) sources, each with distinct characteristics.

Purpose of the Study:

• To establish unifying concepts for the molecular composition of DOM.
• To link DOM composition to its reactivity and role in the carbon cycle.
• To investigate how DOM composition varies across diverse aquatic ecosystems and source types.

Main Methods:

• Characterization of 37 DOM isolates from diverse aquatic environments (US, Pacific Ocean, Antarctic lakes).
• Analysis of stable and radiocarbon isotopes (δ¹³C-DOC and Δ¹⁴C-DOC).
• Measurement of optical properties (absorbance and fluorescence, including SUVA₂₅₄).
• Determination of molecular composition using ultrahigh-resolution mass spectrometry.

Main Results:

• Modern Δ¹⁴C-DOC and optical properties (e.g., SUVA₂₅₄) correlated with polyphenolic and polycyclic aromatic compounds, indicative of allochthonous DOM.
• Enriched δ¹³C-DOC and optical properties correlated with aliphatic compounds, characteristic of autochthonous DOM.
• Autochthonous DOM from different locations (Pacific Ocean vs. Antarctic lakes) showed distinct molecular signatures due to varying degradation levels.
• A consistent compositional shift with aging was observed across all sites, with certain biomolecules persisting over time.

Conclusions:

• Molecular composition, isotopic signatures, and optical properties provide a framework for understanding DOM.
• DOM composition is strongly linked to its source (allochthonous vs. autochthonous) and degradation state.
• Aging processes lead to predictable shifts in DOM molecular makeup, impacting its environmental fate and reactivity.