Fluorescence Quantum Yields of Natural Organic Matter and Organic Compounds: Implications for the Fluorescence-based Interpretation of Organic Matter Composition

Abstract

Absorbance and fluorescence spectroscopy are economical tools for tracing the supply, turnover and fate of dissolved organic matter (DOM). The colored and fluorescent fractions of DOM (CDOM and FDOM, respectively) are linked by the apparent fluorescence quantum yield (AQY) of DOM, which reflects the likelihood that chromophores emit fluorescence after absorbing light. Compared to the number of studies investigating CDOM and FDOM, few studies have systematically investigated AQY spectra for DOM, and linked them to fluorescence quantum yields () of organic compounds. To offer a standardized approach, a MATLAB toolbox for the determination of apparent quantum yields of DOM (aquaDOM), featuring two calculation approaches, was developed and used to derive AQYs for samples from the Norwegian Sea. of the organic compounds varied between 0.00079 and 0.35, whereas the average AQY for DOM samples at 350 nm was 0.011 ± 0.003. The AQY at 350 nm increased with depth, while the AQY at 250 nm showed no trend. Laboratory tests indicated that of compound mixtures are additive and represent an intermediate of the constituents. Additionally, the presence of non-fluorescent chromophores greatly suppressed calculated AQYs. Similar trends in the DOM AQY at 350 nm were observed in natural samples. We therefore hypothesize that fluorescence AQYs can indicate changes in the relative abundances of CDOM and FDOM. Additionally, the optical properties of 15 potential DOM constituents were determined and compared to more than 200 modeled spectra (PARAFAC components) in the OpenFluor database. Apparent matches, based on spectral similarity, were subsequently evaluated using molar fluorescence and absorbance. Five organic compounds were potential matches with PARAFAC components from 16 studies; however, the ability to confirm matches was limited due to multiple compounds exhibiting very similar spectra. This reiterates the fact that spectral similarity alone is insufficient evidence of the presence of particular compounds, and additional evidence is required.

Regional Groundwater and Storms Are Hydrologic Controls on the Quality and Export of Dissolved Organic Matter in Two Tropical Rainforest Streams, Costa Rica

“…While no statistical match was found in OpenFluor, its primary Ex/Em corresponds to a degradation component that has been produced photochemically (Stedmon et al, ). Components 5 and 6 exhibited fluorescence that often is attributed to protein‐like substances but also shares characteristics with phenolic material from lignin and tannin (Hernes et al, ; Stubbins et al, ; Wünsch et al, ). C5 matched with 14 models ranging from the open ocean to lakes, rivers, and estuaries (e.g., Catalá et al, ; Osburn & Stedmon, ; Yamashita, Maie, et al, ; Yamashita, Scinto, et al, ).…”

“…C6 provided no matches in OpenFluor which presently lacks models of throughfall. The spectral properties of C6 are, however, quite similar to other plant‐like aromatic compounds such as lignin phenols (Hernes et al, ; Wünsch et al, ).…”

Convergence of Terrestrial Dissolved Organic Matter Composition and the Role of Microbial Buffering in Aquatic Ecosystems

“…The PARAFAC analysis revealed the presence of 10 fluorescent components within the EEMs (Figure S2), nine of which were reported in previous studies according to the OpenFluor online library based on a Tucker congruence exceeding 0.95 (http://www.openfluor.org; Murphy et al, ). Among the nine previously reported components, five are typically assigned as terrestrial humic‐like peaks (C1 ex/em: <250 nm, 444 nm; C5 ex/em: 320 nm, 430 nm; C6 ex/em: <250/340 nm, 465 nm; C8 ex/em: 275/410 nm, 518 nm; and C9 ex/em: 365 nm, 439 nm), two as tyrosine‐like (C2 ex/em: 280 nm, 309 nm) or tryptophan‐like fluorescence (C10 ex/em: 280 nm, 338 nm), one as microbial humic‐like spectra (C4 ex/em: 305 nm, 386 nm), and one (C7 ex/em: 260 nm, 336 nm) previously assigned as coniferyl alcohol (Wünsch et al, ; Table S1). Although it yielded no match in OpenFluor, Component C3 (ex/em: 390 nm, 475 nm) fluoresces in the broad humic‐like peak C region typical of terrestrial sources (Fellman et al, ).…”

Formation of Chromophoric Dissolved Organic Matter by Bacterial Degradation of Phytoplankton-Derived Aggregates

“…The prominence of the protein-like peak strongly resembles tryptophan, which has been shown to be dominant during phytoplankton growth (Stedmon and Markager, 2005;Murphy et al, 2008;Jørgensen et al, 2011). Distinct fluorescence in the region of peaks A and C have been attributed to fluorophores present in derivatives of benzoic acids (including phenols) and flavins, respectively (Wolfbeis, 1985;Wünsch et al, 2015). Other possibilities include siderophores (Fukuzaki et al, 2014).…”