Optical and molecular indices of dissolved organic matter for estimating biodegradability and resulting carbon dioxide production in inland waters: A review

Biodegradable dissolved organic carbon (BDOC) constitutes the most labile fraction of dissolved organic matter (DOM), which also functions as a source of CO₂ emissions from inland waters. However, no systematic review is available on DOM indicators of BDOC and CO₂ production potential. Optical and molecular indices can be used to track small changes in DOM composition during biodegradation. In this review, we identified four different methods for measuring BDOC together with their strengths and limitations. In addition, we discuss the potential of using documented optical indices based on absorption and fluorescence spectroscopy and molecular indices based on Fourier transform ion cyclotron mass spectrometry as proxies for estimating BDOC and biodegradation-induced CO₂ production based on previously reported relationships in the literature. Many absorbance- and fluorescence-based indices showed inconsistent relationships with BDOC depending on watershed characteristics, hydrology, and anthropogenic impacts. Nevertheless, several indices, including specific UV absorbance at 254 nm (SUVA₂₅₄), humification index (HIX), and terrestrial humic-like fluorescent DOM (FDOM) components, tended to have negative relationships with BDOC in tropical and temperate watersheds under baseflow or drought periods. Protein-like FDOM exhibited the strongest correlation with BDOC in different systems, except during storms and flood events. Despite the limited number of studies, DOM molecular indices exhibited consistent relationships with BDOC, suggesting that the relative abundance of aliphatic formulas and the molecular lability index could act as reliable proxies. The DOM optical indices explain up to 96% and 78% variability in BDOC and CO₂, respectively; nonetheless, there were limited studies on molecular indices, which explain up to 74% variability in BDOC. Based on literature survey, we recommend several sensitive indices such as SUVA₂₅₄, HIX, and terrestrial humic- and protein-like FDOM, which could be useful indicators of BDOC and dissolved CO₂ in inland water. Future research should incorporate a wider range of geographic regions with various land use, hydrology, and anthropogenic disturbances to develop system- or condition-specific DOM optical or molecular proxies for better prediction of BDOC and CO₂ emissions.

Biodegradable dissolved organic carbon shapes bacterial community structures and co-occurrence patterns in large eutrophic Lake Taihu

Interactions between dissolved organic matter (DOM) and bacteria are central in the biogeochemical cycles of aquatic ecosystems; however, the relative importance of biodegradable dissolved organic carbon (BDOC) compared with other environmental variables in structuring the bacterial communities needs further investigation. Here, we investigated bacterial communities, chromophoric DOM (CDOM) characteristics and physico-chemical parameters as well as examined BDOC via bioassay incubations in large eutrophic Lake Taihu, China, to explore the importance of BDOC for shaping bacterial community structures and co-occurrence patterns. We found that the proportion of BDOC (%BDOC) correlated significantly and positively with the DOC concentration and the index of the contribution of recent produced autochthonous CDOM (BIX). %BDOC, further correlated positively with the relative abundance of the tryptophan-like component and negatively with CDOM aromaticity, indicating that autochthonous production of protein-like CDOM was an important source of BDOC. The richness of the bacterial communities correlated negatively with %BDOC, indicating an enhanced number of species in the refractory DOC environments. %BDOC was identified as a significant stronger factor than DOC in shaping bacterial community composition and the co-occurrence network, suggesting that substrate biodegradability is more significant than DOC quantity determining the bacterial communities in a eutrophic lake. Environmental factors explained a larger proportion of the variation in the conditionally rare and abundant subcommunity than for the abundant and the rare bacterial subcommunities. Our findings emphasize the importance of considering bacteria with different abundance patterns and DOC biodegradability when studying the interactions between DOM and bacteria in eutrophic lakes.

Fluorescence characteristics and biodegradability of dissolved organic matter (DOM) leached from non-point sources in southeastern China

Under the increasingly intensive measures for surface water restoration in China, point source discharge has been strictly regulated; however, for non-point sources, which constitute a large part of surface water pollutants, effective control has been difficult to reach. A comprehensive understanding of the characteristics of non-point source pollutants is essential for surface water improvement programs of cities such as Ningbo, on the southeast coast of China. Ningbo has made tremendous efforts in the past few years to control point source pollutants, but available data and management strategies on the non-point source pollutants are still limited. To this end, leachates of representative non-point source samples from the territory of Ningbo, including cropland and wetland soil, urban channel sediment, and poultry manure, were examined and compared focusing on the fluorescence characteristics and biodegradability of the dissolved organic matter (DOM). Results indicated that biodegradable dissolved organic carbon (BDOC) accounting for the total DOC was 46.7 ± 0.7% for cropland, wetland (56.3 ± 6.8%), non-sewage channel (60.1 ± 0.4%), sewage channel (74.5 ± 1.1%), and poultry manure (62.7 ± 4.5%). The leachates of the studied samples showed significant differences in both the amount and composition of DOM. However, a fluorescence component representing tryptophan-like substances identified by the excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis effectively predicted the BDOC variations among the studied samples. Moreover, under the studied nutrient concentrations, which were equivalent to Grade III water quality in China, nutrient limitation of microbial degradation was not observed. Threats to water quality, especially excessive consumption of dissolved oxygen, could be posed by the non-point source leachates due to their high bioavailability, large distribution, and weak nutrient restraint. Further investigations, including a quantitative evaluation of the non-point source pollution contribution, and pollutant blocking techniques are required.

Biotransformation of trace organic chemicals in the presence of highly refractory dissolved organic carbon

Previous studies demonstrated that the transformation of trace organic chemicals (TOrCs) in managed aquifer recharge (MAR) systems is favored under carbon-limited and oxic redox conditions especially, if the dissolved organic carbon (DOC) serving as primary substrate has a refractory character. Since co-metabolism is suggested to be the dominant removal mechanism, it is hypothesized that TOrCs transformation is controlled by the concentration of the refractory carbon under oxic redox conditions. A laboratory-scale soil column experiment mimicking MAR was established to investigate the influence of two different concentrations of highly refractory carbon sources on TOrCs transformation, namely drinking water (DW) and drinking water augmented with humic acid (DW + HA). Oxic redox conditions and carbon-limitation were present in both systems (ΔDOC_DW+HA ≈ 0.6-0.7 mg/L; ΔDOC_DW ≈ 0.1 mg/L). Of the 12 TOrCs investigated seven exhibited moderate to efficient transformation in both systems with only one compound (diclofenac) showing significantly enhanced (co-metabolic) biotransformation by adding humic acids as primary growth substrate. It is postulated that transformation of some TOrCs is characterized by metabolic degradation under starving conditions (ΔDOC ≤ 0.1 mg/L). By comparing the transformation efficiency of selected TOrCs with previous studies operated under carbon-limited and oxic conditions, an inconsistent behavior of some compounds was observed. These results demonstrate that key factors triggering the transformation of TOrCs are still poorly understood and thus, further investigations regarding the biodegradation pathways of TOrCs, upregulation of key enzymes by the microbial community but also more detailed analysis of the composition of the biodegradable DOC are needed.

Seasonally related effects on natural organic matter characteristics from source to tap in Korea

In this study, natural organic matter (NOM) characteristics were investigated over three years of monthly monitoring to determine the effect of seasonal variations on NOM levels from source to tap. Liquid chromatography with organic carbon detection (LC-OCD) was used to determine NOM characteristics and the level of reduction of biodegradable dissolved organic carbon (BDOC). The average dissolved organic matter concentration in the source water (Lake Paldang, Korea) was not significantly different between summer and winter. However, the distribution of NOM components, such as biopolymers, building blocks, low molecular weight (MW) neutrals and acids, identified by LC-OCD, varied seasonally. While high MW NOM was preferentially removed by coagulation/sedimentation/rapid sand filtration (CSR), no seasonal effects were observed on the removal of high MW NOM. CSR and biological activated carbon (BAC) filtration showed a better efficiency of BDOC removal in winter and summer, respectively. High concentrations of chlorine used in the treatment plants in summer resulted in 10% higher DOC concentrations during disinfection. Overall NOM removal efficiencies from source to tap were 45% and 35% for summer and winter, respectively. Principal component analysis also indicated that seasonal variations (principal component 1) showed the strongest positive correlation with the overall performance of water treatment. The long-term monitoring of drinking water treatment processes showed that seasonal variations were important factors affecting NOM characteristics during water treatment.

Application of UV absorbance and fluorescence indicators to assess the formation of biodegradable dissolved organic carbon and bromate during ozonation

This study examined the significance of changes of UV absorbance and fluorescence of dissolved organic matter (DOM) as surrogate indicators for assessing the formation of bromate and biodegradable dissolved organic carbon (BDOC) during the ozonation of surface water and wastewater effluent. Spectroscopic monitoring was carried out using benchtop UV/Vis and fluorescence spectrophotometers and a newly developed miniature LED UV/fluorescence sensor capable of rapidly measuring UVA280 and protein-like and humic-like fluorescence. With the increase of O₃/DOC mass ratio, the plots of BDOC formation were characterized of initial lag, transition slope and final plateau. With the decrease of UV absorbance and fluorescence, BDOC concentrations initially increased slowly and then rose more noticeably. Inflection points in plots of BDOC versus changes of spectroscopic indicators were close to 35-45% loss of UVA254 or UVA280 and 75-85% loss of humic-like fluorescence. According to the data from size exclusion chromatography (SEC) with organic carbon detection and 2D synchronous correlation analyses, DOM fractions assigned to operationally defined large biopolymers (apparent molecular weight, AMW>20 kDa) and medium AMW humic substances (AMW 5.5-20 kDa) were transformed into medium-size building blocks (AMW 3-5.5 kDa) and other smaller AMW species (AMW<3 kDa) associated with BDOC at increasing O₃/DOC ratios. Appreciable bromate formation was observed only after the values of UVA254, UVA280 and humic-like fluorescence in O₃-treated samples were decreased by 45-55%, 50-60% and 86-92% relative to their respective initial levels. No significant differences in plots of bromate concentrations versus decreases of humic-like fluorescence were observed for surface water and wastewater effluent samples. This was in contrast with the plots of bromate concentration versus UVA254 and UVA280 which exhibited sensitivity to varying initial bromide concentrations in the investigated water matrixes. These results suggest that measurements of humic-like fluorescence can provide a useful supplement to UVA indices for characterization of ozonation processes.

Role of primary substrate composition and concentration on attenuation of trace organic chemicals in managed aquifer recharge systems

This study was undertaken to investigate the role of primary substrate composition and concentration on the attenuation of biodegradable emerging trace organic chemicals (TOrCs) in simulated managed aquifer recharge (MAR) systems. Four sets of soil columns were established in the laboratory, each receiving synthetic feed solutions comprising different ratios and concentrations of peptone-yeast and humic acid as the primary substrate to investigate the effect on removal of six TOrCs (atenolol, caffeine, diclofenac, gemfibrozil, primidone, and trimethoprim). Based on abiotic control experiments, adsorption was not identified as a significant attenuation mechanism for primidone, gemfibrozil and diclofenac. Caffeine, atenolol and trimethoprim displayed initial adsorptive losses, however, adsorption coefficients derived from batch tests confirmed that adsorption was limited and in the long-term experiment, biodegradation was the dominant attenuation process. Within a travel time of 16 h, caffeine - an easily degradable compound exhibited removal exceeding 75% regardless of composition or concentration of the primary substrate. Primidone - a poorly degradable compound, showed no removal in any column regardless of the nature of the primary substrate. The composition and concentration of the primary substrate, however, had an effect on attenuation of moderately degradable TOrCs, such as atenolol, gemfibrozil and diclofenac, with the primary substrate composition seeming to have a larger impact on TOrC attenuation than its concentration. When the primary substrate consisted mainly of refractory substrate (humic acid), higher removal of the moderately degradable TOrCs was observed. The microbial communities in the columns receiving more refractory carbon, were noted to be more diverse and hence likely able to express a wider range of enzymes, which were more suitable for TOrC transformation. The effect of the primary substrate on microbial community composition, diversity and gene expression potential confirmed its influence on TOrC degradation.