Dissolved Organic Matter Composition Determines Its Susceptibility to Complete and Partial Photooxidation within Lakes

Abstract

Dissolved organic matter (DOM) plays an important role in carbon cycling within inland surface waters. Under sunlight irradiation, DOM undergoes complete photooxidation to produce carbon dioxide (CO2) and partial photooxidation that alters the molecular composition of DOM. However, a mechanistic understanding of the relationship between DOM composition and its susceptibility to partial and complete photooxidation in surface waters is currently lacking. This work combines light exposure experiments with high-resolution mass spectrometry to investigate DOM photooxidation using two DOM isolates and DOM from 16 lakes that vary in trophic status and size. High ratios of oxygen consumption to dissolved inorganic carbon (DIC) production demonstrate that all samples undergo extensive partial photooxidation. At the molecular level, more oxidized, aromatic DOM formulas are associated with oxygen consumption and DIC production. Bulk level measurements indicate that DOM becomes less aromatic and lower in apparent molecular weight following partial photooxidation, and there is molecular level evidence of oxygen addition and loss of CO2 in all samples. However, formulas most susceptible to photooxidation vary depending on the initial DOM composition. Collectively, this work provides insights into the relationship between DOM composition and photooxidation, which has important implications for carbon cycling in diverse surface waters.

Limitations of conventional approaches to identify photochemically produced reactive intermediates involved in contaminant indirect photodegradation

“…The photochemical production of 3 DOM, 1 O 2 , and ˙OH also ranges widely in these diverse samples, which is expected because the composition of DOM influences its photoreactivity. 21,24,25,37 The quantum yield coefficient of 3 DOM ( f TMP ) ranges from 11.6–87.1 M −1 (average = 36.1 ± 17.9 M −1 ) and the quantum yield of 1 O 2 ranges from (0.6–8.27) × 10 −2 (average = (3.4 ± 1.9) × 10 −2 ; Table S6†). These parameters are highly correlated with each other 37 and the highest values are observed in urban and wastewater-impacted samples.…”

“…DOM from each water sample was extracted via solid-phase extraction and eluted in methanol as described previously. 25,37,48 Extracts were diluted in 50 : 50 acetonitrile : water and directly injected into a Bruker FT-ICR MS (Solarix XR 12T) using negative mode electrospray ionization. Signals were exported, linearly calibrated, and compared with potential chemical formulae containing combinations of 13 C 0–1 12 C 5–80 H 0–120 O 0–80 N 0–3 S 0–1 .…”

“…For example, concentrations range from 1–3 mg C L −1 in oligotrophic systems to 10–>35 mg C L −1 in rivers and wetlands. 19–25 The composition of DOM is determined by its source ( e.g. , terrestrial and/or microbial) and its extent of biological and photochemical processing.…”

Influence of Divalent Cation Inhibition and Dissolved Organic Matter Enhancement on Phenol Oxidation Kinetics by Manganese Oxides

“…We hypothesize that the composition of DOM influences its ability to either increase or decrease the phenol compound oxidation rates. DOM composition is a function of source inputs and environmental processing and therefore can vary widely within the same water body or across different systems. − DOM composition influences its reactivity with sunlight, ,,− chemical disinfectants (e.g., chlorine and ozone), − and manganese oxides. ,, For example, highly aromatic DOM undergoes greater changes following oxidation with manganese oxides than more aliphatic DOM. , This composition-dependent transformation means DOM from different environmental systems (e.g., wastewater vs dystrophic lakes) likely results in divergent impacts on the oxidation of phenolic contaminants by manganese oxides; however, evidence for this relationship is lacking.…”

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

“…Specifically, deterministic processes result from environmental selection, whereby biotic or abiotic reactivity either favors or hinders the presence of specific compounds. For instance, sunlight irradiation causes the conversion of aromatic compounds into more saturated components. − In addition to photochemistry, there are many more deterministic processes that can induce directional changes in DOM compositions, such as microbial preferential metabolism, salinity-induced flocculation, mineral adsorption/desorption, and specific organic matter inputs from soil and atmosphere . In cases of robust determinism, the DOM spatiotemporal distribution should exhibit simplicity, regularity, and predictability.…”