Experimental Insight into the Enigmatic Persistence of Marine Refractory Dissolved Organic Matter

Aquatic dissolved organic matter (DOM) is a crucial component of the global carbon cycle, and the extent to which DOM escapes mineralization is important for the transport of organic carbon from the continents to the ocean. DOM persistence strongly depends on its molecular properties, but little is known about which specific properties cause the continuum in reactivity among different dissolved molecules. We investigated how DOM fractions, separated according to their hydrophobicity, differ in biodegradability across three different inland water systems. We found a strong negative relationship between hydrophobicity and biodegradability, consistent for the three systems. The most hydrophilic fraction was poorly recovered by solid-phase extraction (SPE) (3–28% DOC recovery) and was thus selectively missed by mass spectrometry analysis during SPE. The change in DOM composition after incubation was very low according to SPE–ESI (electrospray ionization)–mass spectrometry (14% change, while replicates had 11% change), revealing that this method is sub-optimal to assess DOM biodegradability, regardless of fraction hydrophobicity. Our results demonstrate that SPE–ESI mass spectrometry does not detect the most hydrophilic and most biodegradable species. Hence, they question our current understanding of the relationships between DOM biodegradability and its molecular composition, which is built on the use of this method.

Section: Resultsmentioning

confidence: 70%

Hydrophilic Species Are the Most Biodegradable Components of Freshwater Dissolved Organic Matter

Grasset,

Groeneveld,

Tranvik

et al. 2023

“…The abundance of CRAM has been reported to increase with irradiation and be recalcitrant to biodegradation, , thereby making their presence important to carbon biogeochemical cycling. In fact, around ∼8% of bulk DOC in the open ocean is CRAM. , CRAM is considered intrinsically recalcitrant, likely because of the complex three-dimensional structure-containing saturated groups (e.g., sp 3 -hybridized) causing steric hindrance and subsequently reducing reactivity. Here, we do not consider CRAM in our assessment of SRFA; however it is an important component to consider when considering marine ecosystems …”

Section: Dom Composition and Impact On Observable Photochemical And O...mentioning

confidence: 99%

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties

Buckley,

McKay,

Leresche

et al. 2024

Despite the widespread use of photochemical and optical properties to characterize dissolved organic matter (DOM), a significant gap persists in our understanding of the relationship among these properties. This study infers the molecular basis for the optical and photochemical properties of DOM using a comprehensive framework and known structural moieties within DOM. Utilizing Suwannee River Fulvic Acid (SRFA) as a model DOM, carboxylated aromatics, phenols, and quinones were identified as dominant contributors to the absorbance spectra, and phenols, quinones, aldehydes, and ketones were identified as major contributors to radiative energy pathways. It was estimated that chromophores constitute ∼63% w/w of dissolved organic carbon in SRFA and ∼47% w/w of overall SRFA. Notably, estimations indicate the pool of fluorescent compounds and photosensitizing compounds in SRFA are likely distinct from each other at wavelengths below 400 nm. This perspective offers a practical tool to aid in the identification of probable chemical groups when interpreting optical and photochemical data and challenges the current “black box” thinking. Instead, DOM photochemical and optical properties can be closely estimated by assuming the DOM is composed of a mixture of individual compounds.

“…Although the microbial activities could not be completely prevented, previous biological incubation comparisons demonstrated only trivial changes in the bacterial abundance after filtering during the 14-day observation, implying that bacteria-dominated microbial activities did not serve as the primary factor controlling DOM transformation. 27,28 We suggest that constant monitoring of the bacterial abundance during photoirradiation would be better in future research.…”

Section: Introductionmentioning

confidence: 99%

Exploring the Complexities of Dissolved Organic Matter Photochemistry from the Molecular Level by Using Machine Learning Approaches

Zhao

Chen

et al. 2023

Self Cite

Dissolved organic matter (DOM) sustains a substantial part of the organic matter transported seaward, where photochemical reactions significantly affect its transformation and fate. The irradiation experiments can provide valuable information on the photochemical reactivity (photolabile, photoresistant, and photoproduct) of molecules. However, the inconsistency of the fate of irradiated molecules among different experiments curtailed our understanding of the roles the photochemical reactions have played, which cannot be properly addressed by traditional approaches. Here, we conducted irradiation experiments for samples from two large estuaries in China. Molecules that occurred in irradiation experiments were characterized by the Fourier transform ion cyclotron resonance mass spectrometry and assigned probabilistic labels to define their photochemical reactivity. These molecules with probabilistic labels were used to construct a learning database for establishing a suitable machine learning (ML) model. We further applied our well-trained ML model to "unmatched" (i.e., not detected in our irradiation experiments) molecules from five estuaries worldwide, to predict their photochemical reactivity. Results showed that numerous molecules with strong photolability can be captured solely by the ML model. Moreover, comparing DOM photochemical reactivity in five estuaries revealed that the riverine DOM chemistry largely determines their subsequent photochemical transformation. We offer an expandable and renewable approach based on ML to compatibly integrate existing irradiation experiments and shed insight into DOM transformation and degradation processes.