Photochemical and microbial degradation of dissolved lignin phenols: Implications for the fate of terrigenous dissolved organic matter in marine environments

Abstract

[1] Molecular level characterizations of dissolved lignin were conducted in Mississippi River plume waters to study the impact of various removal mechanisms (photooxidation, microbial degradation, and flocculation) on dissolved organic material (DOM) concentrations and compositions. Prior to analysis, dissolved (<0.2-mm pore size) samples were size fractionated by ultrafiltration into high molecular weight (HMW; >1 kDalton) and low molecular weight (LMW; <1 kDalton) components. At salinities <25 psu, flocculation and microbial degradation were the primary factors affecting lignin concentrations. At salinities >25 psu, photooxidation was a dominant factor influencing lignin compositions and concentrations. Diagnostic indicators of photooxidation include a sharp decrease in the percentage of lignin in the HMW size fraction, changes in ratios of syringyl to vanillyl phenols, and increases in LMW acid:aldehyde ratios for both vanillyl and syringyl phenols. A 10-day incubation experiment with plume water indicated rates of microbial degradation of dissolved lignin that were $30% of photooxidation rates in surface waters. These results highlight the importance of microbial as well as photochemical processes in the cycling of terrigenous DOM in coastal waters. Neither flocculation nor microbial degradation significantly altered lignin composition, suggesting that composition is primarily determined by source and photochemical transformation. Overall, high removal rates indicate the potential importance of terrigenous DOM as a carbon and nutrient source in the coastal ocean. Strong correlations between absorption coefficients at 350 nm and dissolved lignin demonstrate the potential for using absorption to trace terrigenous DOM in coastal environments with significant riverine input. Citation: Hernes, P. J., and R. Benner, Photochemical and microbial degradation of dissolved lignin phenols: Implications for the fate of terrigenous dissolved organic matter in marine environments,

Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

“…After only a 2 weeks incubation under ambient solar conditions, we observed changes in S R (increase of 184%) and SUVA 254 (decrease of 20%) that indicated a decrease in the average aromaticity and molecular weight of the DOM pool (Weishaar et al 2003;Helms et al 2008). Increases in the acid:aldehyde ratios for both the vanillyl [(Ad:Al) V ] and syringyl [(Ad:Al) S ] lignin phenol fractions also agreed with the results of previous studies of lignin photodegradation (e.g., Spencer et al 2009), as did a negative relationship between (Ad:Al) V (elevated) and S:V (decreased) that resulted from known differences in photodegradation rates among phenols (Opsahl and Benner 1998;Hernes and Benner 2003). In addition, decreases in DOC concentrations (−10%) and total lignin phenol concentrations (Σ 8 ; −42%) indicated that a portion of the Fig.…”

“…Dried samples were resuspended in 2 mL of 2 N NaOH and transferred to Teflon-lined acid digestion vessels (Parr Instruments). A further 11 mL of 2N NaOH was added to the sample, along with ∼500 mg cupric oxide and ∼50 mg ferrous ammonium sulphate, before samples were hydrolysed at 150°C for 3 h. Trans-cinnamic acid was added as a recovery standard, and the products of the lignin oxidation were quantified using a five-point calibration curve of eight lignin phenol standards (Hernes and Benner 2003). Phenols were separated using a Varian 431-GC gas chromatograph, followed by phenol detection using a Varian 220-MS ion-trap mass spectrometer operated in the electron ionization (EI) mode using MS-MS (tandem mass spectrometer).…”

Compositions and constituents of freshwater dissolved organic matter isolated by reverse osmosis

“…The aromatic C (109-162 ppm) of the HMW DOM decreased along a salinity gradient (Abdulla et al, 2010), whereas that of DOM increased slightly. This suggests that a fraction of dissolved lignin in HMW DOM may be degraded to LMW lignin, rather than remineralization, which is consistent with a finding based on lignin biomarkers in HMW and LMW DOM (Hernes and Benner, 2003). It also supports the hypothesis that alteration of HMW DOM is an important process in the estuarine mixing zone, in addition to the mixing of river and marine sources (Guo et al, 2009).…”

“…The elevated aromatic and methoxyl C contents suggested a noticeable proportion of dissolved lignin in HMW DOM. This observation was consistent with higher lignin phenol contents in HMW DOM than in LMW DOM from the Mississippi River by using molecular-level analysis (Hernes and Benner, 2003). The simulated compositions of LMW DOM by difference between the RO DOM and HMW DOM were involved in the following discussions to elucidate the transformation mechanisms of DOM during estuarine transition.…”

“…This may indicate a non-conservative mixing behavior (i.e., a simple mixing combined with source and biogeochemical alteration) of DOM during the transport to oceans. The behavior for HMW DOM along estuary was also investigated (Abdulla et al, 2010;Hernes and Benner, 2003). However, some compositional changes of DOM differed from those of HMW DOM.…”