Determination of apparent quantum yield spectra for the formation of biologically labile photoproducts (original) (raw)
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Biogeosciences Discussions, 2015
In order to assess the production of biologically labile photoproducts (BLPs) from non-labile riverine dissolved organic carbon (DOC), we collected water samples from ten major rivers, removed labile DOC and mixed the residual non-labile DOC with artificial seawater for microbial and photochemical experiments. Bacteria grew on non-labile DOC with a growth efficiency of 11.5% (mean; range from 3.6 to 15.3%). Simulated solar radiation transformed a part of non-labile DOC into BLPs, which stimulated bacterial respiration and production, but did not change bacterial growth efficiency (BGE) compared to the non-irradiated dark controls. In the irradiated water samples, the amount of BLPs stimulating bacterial production depended on the photochemical bleaching of chromophoric dissolved organic matter (CDOM). The apparent quantum yields for BLPs supporting bacterial production ranged from 9.5 to 76 (mean 39) (μmol C mol photons<sup>−1</sup>) at 330 nm. The corresponding values f...
Rate and apparent quantum yield of photodissolution of sedimentary organic matter
Limnology and Oceanography, 2012
We quantified rates of photochemical dissolution (photodissolution) of organic carbon in coastal Louisiana suspended sediments, conducting experiments under well-defined conditions of irradiance and temperature. Optical properties of the suspended sediments were characterized and used in a radiative transfer model to compute irradiances within turbid suspensions. Photodissolution rate increased with temperature (T), with activation energy of 32 6 7 kJ mol 21 , which implicates indirect (non-photochemical) steps in the net reaction. In most samples, dissolved organic carbon (DOC) concentration increased approximately linearly with time over the first 4 h of irradiation under broadband simulated sunlight, after higher rates in the initial hour of irradiation. Four-hour rates ranged from 2.3 mmol DOC m 23 s 21 to 3.2 mmol DOC m 23 s 21 , but showed no relation to sample origin within the study area, organic carbon or reducible iron content, or mass-specific absorption coefficient. First-hour rates were higher-from 3.5 mmol DOC m 23 s 21 to 7.8 mmol DOC m 23 s 21-and correlated well with sediment reducible iron (itself often associated with organic matter). The spectral apparent quantum yield (AQY) for photodissolution was computed by fitting DOC photoproduction rates under different spectral irradiance distributions to corresponding rates of light absorption by particles. The photodissolution AQY magnitude is similar to most published dissolved-phase AQY spectra for dissolved inorganic carbon photoproduction, which suggests that in turbid coastal waters where particles dominate light absorption, DOC photoproduction from particles exceeds photooxidation of DOC.
Geophysical Research Letters, 2004
Quantum yield spectra for the photochemical formation of biologically labile photoproducts from dissolved organic matter (DOM) have not been available previously, although they would greatly facilitate attempts to model photoproduct formation rates across latitudinal, seasonal, and depth-related changes in spectral irradiance. Apparent quantum yield spectra were calculated for two coastal environments from the southeastern United States using postirradiation bacterial respiration as a measure of total labile photoproduct formation and a cutoff filter method to model spectral dependence. As has been the case for previously studied classes of DOM photoproducts (i.e., dissolved inorganic carbon, CO, and H 2 O 2), ultraviolet (UV)-B irradiance was significantly more efficient at forming labile photoproducts (i.e., compounds readily assimilated by marine bacterioplankton) than UV-A and visible irradiance. Calculations of DOM photoproduct formation in southeastern U.S. coastal surface waters indicate a formation ratio for biologically labile photoproducts : CO of 13 : 1. The slope of a natural log plot of the apparent quantum yield spectrum obtained for biologically labile photoproducts was similar to that for CO (0.028 nm Ϫ1 vs. 0.034 nm Ϫ1). Modeled kinetic rates therefore indicate that the production ratio of these photoproduct classes is approximately maintained despite variations in the solar spectrum that occur with depth in a water column or distance from shore. Application of the apparent quantum yield to coastal regions worldwide predicts an annual formation rate of biologically labile photoproducts in coastal waters of 206 ϫ 10 12 g C.
Increases in the longwave photobleaching of chromophoric dissolved organic matter in coastal waters
Limnology and Oceanography, 2009
Salinity effects on the photobleaching of chromophoric dissolved organic matter (CDOM) due to coastal mixing were investigated through a comparative study of surrogate and surface-water CDOM. Suwannee River humic acid (SRHA) and ultrafiltered river dissolved organic matter (UDOM) added to mixtures of river and seawater permeates (,1 kDa) that varied in salinity from 0 to 33 to mimic coastal mixing. Surface-water CDOM was collected from the Chesapeake Bay in January, June, and September 2002. Shortwave CDOM absorption loss (e.g., 280 nm) did not change with salinity; however, longwave CDOM absorption loss (e.g., 440 nm) often decreased by 10% to 40% with salinity. Apparent quantum yields for average absorption loss from 280 to 550 nm (Q avg ) increased with salinity for both surrogate and surface-water CDOM, providing evidence for an effect of salinity independent of light absorption among different samples. Further, hydrogen peroxide photoproduction from UDOM increased from 15 to 368 nmol L 21 h 21 with salinity, even though pH values were circumneutral. A kinetic model demonstrated that, at circumneutral pH and iron concentrations expected for the Chesapeake Bay, photo-Fenton chemistry could not explain the increase in hydrogen peroxide production quantum yields (Q hp ) with salinity. Using Q avg for the SRHA and UDOM surrogates, a model of the change in surface-water CDOM photoreactivity in the Chesapeake Bay as a function of salinity suggested additional CDOM inputs for the lower Chesapeake Bay. Because estuarine mixing increases photobleaching of longwave CDOM absorption, the modeling of absorption coefficients above 400 nm may underestimate dissolved organic matter in coastal waters. 1 Corresponding author (chris_osburn@ncsu.edu).
Formation of planktonic chromophoric dissolved organic matter in the ocean
Marine Chemistry, 2018
Chromophoric dissolved organic matter (CDOM) is an important fraction of the marine carbon cycle that controls most light absorption and many photochemical and biological processes in the ocean. Despite its importance, the chemical basis for the formation of oceanic CDOM remains unclear. Currently, CDOM's optical properties are best-described by an electronic interaction (EI) model of charge transfer (CT) complexes which form between electron-rich donors and electron-poor acceptors. While terrigenous compounds such as lignin best fit this model, planktonic sources of CDOM have not yet been tested. Here, we have tested CDOM formed during an incubation experiment using a natural phytoplankton assemblage and throughout active growth, stationary phase and algal biomass decomposition. Absorbance of the derived planktonic CDOM generally decreased with increasing wavelength, similar to the reference Pony Lake (PLFA) and Suwanee River (SRFA) fulvic acid solutions used as models of terrigenous CDOM. Further, after 60 d of microbial degradation in the dark, CDOM exhibited fluorescence emission maxima continuously red-shifted into the visible band, consistent with PLFA and SRFA. Reduction of carbonyl-containing groups, key to CT complex formation, with sodium borohydride (NaBH 4) produced coherent results in planktonic CDOM and reference FAs. Absorption at 350 nm decreased by 50% for planktonic CDOM and by 30% for PLFA and SRFA, with corresponding increases in spectral slope (S) values, indicating preferential loss of absorption well into the visible. Fluorescence likewise responded with enhanced emission at shorter wavelengths. Apparent quantum yields (Φ) were similarly affected. Results from our work support prior observations that phytoplankton and bacteria are important sources of CDOM that color the ocean's "twilight zone". We hypothesize that microbial processing of a variety of source substrates into more complex compounds represented as planktonic CDOM likely represents a semi-refractory pool of DOM in the ocean.
Organic Geochemistry, 2021
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