Compositional and spectroscopic analysis of dissolved organic matter samples from Everglades periphyton and water

Periphyton is a ubiquitous niche in aquatic environments and can be a significant source of dissolved organic matter (DOM) production and leaching, especially in such environment as the Everglades, a slow-water flow wetland in Florida, USA. We employed an array of methods, including compositional analysis, 3-dimensional excitation emission matrix (3-D EEM) fluorescence spectroscopy, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, to perform quantitative and qualitative analyses on the DOM produced by periphyton and DOM in surrounding surface water and periphyton overlying water for comparison purposes. Higher dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) contents in periphyton pore water than surface water and periphyton overlying water indicated the remarkable contribution from periphyton-produced DOM. Higher total protein, carbohydrate, and thiol contents in periphyton pore water than in surface water and periphyton overlying water underscored the possibility of periphyton pore water DOM leached from periphyton. These results agreed with 3-D EEM and ATR-FTIR analyses that showed the prevalence of possible microbial source of periphyton pore water DOM as indicated by higher fluorescence index (FI) than surface water and periphyton overlying water. Similarly, the size-fractionated DOM from surface water demonstrated terrestrial sources, and periphyton pore water demonstrated microbial sources regardless of their differences in size based on their FI values. The types of periphyton affect the production and composition of DOM, as evidenced by higher total protein, carbohydrate, and chlorophyll-a (Chl-a) contents in floating mat on the water surface than in epiphyton attached to submerged phytoplankton, probably because the former is photo-synthetically more productive than the latter due to different light availability. This study provided fundamental information on periphyton DOM that is essential for further investigating its role in carbon cycle and its biogeochemistry.

Relating natural organic matter conformation, metal complexation, and photophysics

We investigated the relationship between changes in fluorescence intensity and in fluorescence anisotropy for Suwannee River Natural Organic Matter (SRNOM) due to the formation of NOM-metal complexes with divalent and trivalent metals commonly present in both fresh water and sea water environments. We chose metal ions whose complexes give rise to both fluorescence quenching (Fe3+, Cu2+) and fluorescence enhancement (Al3+, Mg2+). Stern–Volmer type analyses quantified the changes in the SRNOM fluorescence as a function of metal concentration. All metals display strong complexation with SRNOM, associated with their effect on fluorescence. Experiments with Fe3+ further show strong effects due to NOM aggregation at all but the lowest metal concentrations studied here. There was little to no change in the conformation of SRNOM as inferred from fluorescence anisotropy caused by increasing metal concentration. These results suggest that there is no correlation between photophysical changes and conformational changes in NOM associated with complexation by the metal ions.

Chromophoric dissolved organic matter in inland waters: Present knowledge and future challenges

Chromophoric dissolved organic matter (CDOM) plays an important role in the biogeochemical cycle and energy flow of aquatic ecosystems. Thus, systematic and comprehensive understanding of CDOM dynamics is critically important for aquatic ecosystem management. CDOM spans multiple study fields, including analytical chemistry, biogeochemistry, water color remote sensing, and global environmental change. Here, we thoroughly summarize the progresses of recent studies focusing on the characterization, distribution, sources, composition, and fate of CDOM in inland waters. Characterization methods, remote sensing estimation, and biogeochemistry cycle processes were the hotspots of CDOM studies. Specifically, optical, isotope, and mass spectrometric techniques have been widely used to characterize CDOM abundance, composition, and sources. Remote sensing is an effective tool to map CDOM distribution with high temporal and spatial resolutions. CDOM dynamics are mainly determined by watershed-related processes, including rainfall discharge, groundwater, wastewater discharges/effluents, and biogeochemical cycling occurring in soil and water bodies. We highlight the underlying mechanisms of the photochemical degradation and microbial decomposition of CDOM, and emphasize that photochemical and microbial processes of CDOM in inland waters accelerate nutrient cycling and regeneration in the water column and also exacerbate global warming by releasing greenhouse gases. Future study directions to improve the understanding of CDOM dynamics in inland waters are proposed. This review provides an interdisciplinary view and new insights on CDOM dynamics in inland waters.

In-situ fluorescence spectroscopy indicates total bacterial abundance and dissolved organic carbon

We explore in-situ fluorescence spectroscopy as an instantaneous indicator of total bacterial abundance and faecal contamination in drinking water. Eighty-four samples were collected outside of the recharge season from groundwater-derived water sources in Dakar, Senegal. Samples were analysed for tryptophan-like (TLF) and humic-like (HLF) fluorescence in-situ, total bacterial cells by flow cytometry, and potential indicators of faecal contamination such as thermotolerant coliforms (TTCs), nitrate, and in a subset of 22 samples, dissolved organic carbon (DOC). Significant single-predictor linear regression models demonstrated that total bacterial cells were the most effective predictor of TLF, followed by on-site sanitation density; TTCs were not a significant predictor. An optimum multiple-predictor model of TLF incorporated total bacterial cells, nitrate, nitrite, on-site sanitation density, and sulphate (r² 0.68). HLF was similarly related to the same parameters as TLF, with total bacterial cells being the best correlated (ρ_s 0.64). In the subset of 22 sources, DOC clustered with TLF, HLF, and total bacterial cells, and a linear regression model demonstrated HLF was the best predictor of DOC (r² 0.84). The intergranular nature of the aquifer, timing of the study, and/or non-uniqueness of the signal to TTCs can explain the significant associations between TLF/HLF and indicators of faecal contamination such as on-site sanitation density and nutrients but not TTCs. The bacterial population that relates to TLF/HLF is likely to be a subsurface community that develops in-situ based on the availability of organic matter originating from faecal sources. In-situ fluorescence spectroscopy instantly indicates a drinking water source is impacted by faecal contamination but it remains unclear how that relates specifically to microbial risk in this setting.

Characterizing the interactions between copper ions and dissolved organic matter using fluorescence excitation-emission matrices with two-dimensional Savitzky-Golay second-order differentiation

Monitoring dissolved organic matter (DOM) content in aquatic environments is crucial for not only understanding the dynamics of heavy metals but also predicting their bioavailability. Fluorescence spectroscopy is typically employed to characterise DOM. Here, the interaction between DOM and trace metals was investigated by combining excitation-emission matrix (EEM) quenching with two-dimensional Savitzky-Golay second-order differentiation (2D-SG-2nd-df) analysis. The 2D-SG-2nd-df analysis decomposed the EEM spectra of commercial humic acid (HA) samples into six separate fluorescence peaks, which agreed with the results obtained through conventional parallel factor (PARAFAC) analysis. Compared with PARAFAC modeling, the 2D-SG-2nd-df approach provided more valid and reliable results when the dataset contained distinct samples. Moreover, since the results obtained from 2D-SG-2nd-df for each sample are independent, shifts in the peak wavelength can be reproduced more efficiently using this method. Triplicate titration experiments showed clear differences in HA-copper interactions for samples with different HA composition and molecular weight. The binding strength between copper and low-molecular-weight DOM in water was weaker than that observed for high-molecular-weight DOM. The results obtained in this study will serve as a basis for applying 2D-SG-2nd-df not only to DOM but also to other samples studied using EEM measurements.

Streams with Riparian Forest Buffers versus Impoundments Differ in Discharge and DOM Characteristics for Pasture Catchments in Southern Amazonia

Forest to pasture land use change following deforestation in Southern Amazonia can result in changes to stream water quality. However, some pasture streams have riparian forest buffers, while others are dammed for farm ponds. Stream corridor management can have differential effects on hydrology and dissolved organic matter (DOM) characteristics. We examined rainfall-runoff patterns and DOM characteristics in a pasture catchment with a forested riparian buffer, and an adjacent catchment with an impoundment. Total streamflow was 1.5 times higher with the riparian buffer, whereas stormflow represented 20% of total discharge for the dammed stream versus 13% with buffer. Stream corridor management was also the primary factor related to DOM characteristics. In the impounded catchment, DOM was found to be less structurally complex, with lower molecular weight compounds, a lesser degree of humification, and a larger proportion of protein-like DOM. In the catchment with a forested buffer, DOM was dominated by humic-like components, with fluorescence characteristics indicative of DOM derived from humified soil organic matter under native vegetation. Our results suggest that differences in stream corridor management can have important implications for carbon cycling in headwater pasture catchments, and that such changes may have the potential to influence water quality downstream in the Amazon basin.

Differences in photochemistry between seawater and freshwater for two natural organic matter samples

We report changes in the excitation and resolved fluorescence spectra, inferred triplet formation and singlet oxygen formation abilities of two different Natural Organic Matter samples (NOM) in seawater vs. freshwater or NaCl solution. In artificial seawater solution (but not in NaCl solution), the natural water-derived NOM samples Suwannee River Natural Organic Matter (SRNOM) and Nordic Reservoir Natural Organic Matter (NRNOM) display large enhancements in fluorescence intensity. Nearly identical spectra are seen when seawater is replaced by solutions of Mg2+ at its seawater concentration, consistent with magnesium binding to ligand sites of the natural organic matter giving rise to different photophysics. Fluorescence anisotropy measurements show a decrease in anisotropy of SRNOM and NRNOM in seawater, also consistent with Mg2+ binding. Different effects of Mg2+ are seen when the different NOM samples are illuminated: NRNOM exhibits increased formation of its triplet state and also quenching of its triplet by oxygen, compared to its photochemistry in the absence of Mg2+, while SRNOM exhibits a reduction in triplet formation in the presence of Mg2+. These observations imply that the photochemistry of NOM in seawater may be very different from what is expected based on freshwater or NaCl solution measurements.

Microbial Processing and Production of Aquatic Fluorescent Organic Matter in a Model Freshwater System

Organic matter (OM) has an essential biogeochemical influence along the hydrological continuum and within aquatic ecosystems. Organic matter derived via microbial processes was investigated within a range of model freshwater samples over a 10-day period. For this, excitation-emission matrix (EEM) fluorescence spectroscopy in combination with parallel factor (PARAFAC) analysis was employed. This research shows the origin and processing of both protein-like and humic-like fluorescence within environmental and synthetic samples over the sampling period. The microbial origin of Peak T fluorescence is demonstrated within both synthetic samples and in environmental samples. Using a range of incubation temperatures provides evidence for the microbial metabolic origin of Peak T fluorescence. From temporally resolved experiments, evidence is provided that Peak T fluorescence is an indication of metabolic activity at the microbial community level and not a proxy for bacterial enumeration. This data also reveals that humic-like fluorescence can be microbially derived in situ and is not solely of terrestrial origin, likely to result from the upregulation of cellular processes prior to cell multiplication. This work provides evidence that freshwater microbes can engineer fluorescent OM, demonstrating that microbial communities not only process, but also transform, fluorescent organic matter.

Combining parallel factor analysis and machine learning for the classification of dissolved organic matter according to source using fluorescence signatures

Parallel factor (PARAFAC) analysis of dissolved organic matter (DOM) fluorescence has facilitated a surge of investigation into its biogeochemical cycling. However, rigorous, PARAFAC-based methods for holistically distinguishing DOM sources are lacking. This study classified 1029 PARAFAC-analyzed excitation-emission matrices (EEMs) measured using DOM isolated from 24 different leaf leachates, rivers, and organic matter standards using four machine learning methods (MLM). EEMs were also divided into subsets to assess the impact of experimental treatments (i.e. whole EEMs, size fractionation, mixtures, quenching) and dataset properties (i.e. different numbers of EEMs from each leachate/river) on classification. A split-half validated, 10-component PARAFAC model was extended to 12 components to remove consistent peaks evident in model residuals. The 12-component model performed better than the 10-component model, correctly classifying up to 80 additional EEMs, when the dataset included size-fractionated DOM or several different sources (i.e. many leaf species and rivers); however, the 10-component model performed better for whole-sample EEMs when comparing leaf leachates to rivers. The MLM correctly classified whole EEMs of riverine DOM by source with up to 87.0% accuracy, leachates with up to 92.5% accuracy, and distinguished leachates from rivers with 97.2% accuracy. A difference of up to 17.3% in classification accuracy was observed depending on the MLM method used with the following order: multilayer perceptron = support vector machine > k-nearest neighbours ≫ decision tree; however, performances differed widely depending on the data subset. Classification accuracy for whole and size-fractionated rivers compared to whole and size-fractionated leachates using N-way partial least-squares discriminant analysis (NPLS-DA; 97.7%) was similar to that achieved using MLM. Combining MLM with PARAFAC is an effective method for classifying DOM based on its fluorescence signature because PARAFAC can isolate meaningful fluorescent species and unlike PLSDA, MLM constructs a single model which simultaneously classifies EEMs as belonging to one of several categories. A complete accounting of carbon flows through ecosystems should include the processes and sources that contribute to the disparate fluorescence signatures of riverine and leached DOM.

Associations Between the Molecular and Optical Properties of Dissolved Organic Matter in the Florida Everglades, a Model Coastal Wetland System

Optical properties are easy-to-measure proxies for dissolved organic matter (DOM) composition, source, and reactivity. However, the molecular signature of DOM associated with such optical parameters remains poorly defined. The Florida coastal Everglades is a subtropical wetland with diverse vegetation (e.g., sawgrass prairies, mangrove forests, seagrass meadows) and DOM sources (e.g., terrestrial, microbial, and marine). As such, the Everglades is an excellent model system from which to draw samples of diverse origin and composition to allow classically-defined optical properties to be linked to molecular properties of the DOM pool. We characterized a suite of seasonally- and spatially-collected DOM samples using optical measurements (EEM-PARAFAC, SUVA254, S275-295, S350-400, SR, FI, freshness index, and HIX) and ultrahigh resolution mass spectrometry (FTICR-MS). Spearman's rank correlations between FTICR-MS signal intensities of individual molecular formulae and optical properties determined which molecular formulae were associated with each PARAFAC component and optical index. The molecular families that tracked with the optical indices were generally in agreement with conventional biogeochemical interpretations. Therefore, although they represent only a small portion of the bulk DOM pool, absorbance, and fluorescence measurements appear to be appropriate proxies for the aquatic cycling of both optically-active and associated optically-inactive DOM in coastal wetlands.

Climate-driven terrestrial inputs in ultraoligotrophic mountain streams of Andean Patagonia revealed through chromophoric and fluorescent dissolved organic matter

Fluvial networks transport a substantial fraction of the terrestrial production, contributing to the global carbon cycle and being shaped by hydrologic, natural and anthropogenic factors. In this investigation, four Andean Patagonian oligotrophic streams connecting a forested catchment (~125km(2)) and draining to a double-basin large and deep lake (Lake Moreno complex, Northwestern Patagonia), were surveyed to analyze the dynamics of the allochthonous subsidy. The results of a 30month survey showed that the catchment supplies nutrients and dissolved organic matter (DOM) to the streams. The eruption of the Puyehue-Cordón Caulle at the beginning of the study overlapped with seasonal precipitation events. The largest terrestrial input was timed with precipitation which increased particulate materials, nutrients and DOM through enhanced runoff. Baseline suspended solids and nutrients were very low in all the streams (suspended solids: ~1mg/L; total nitrogen: ~0.02mg/L; total phosphorus: ~5μg/L), increasing several fold with runoff. Baseline dissolved organic carbon concentrations (DOC) ranged between 0.15 and 1mg/L peaking up to three-fold. Chromophoric and fluorescent analyses characterized the DOM as of large molecular weight and high aromaticity. Parallel factor modeling (PARAFAC) of DOM fluorescence matrices revealed three components of terrestrial origin, with certain degree of microbial processing: C1 and C2 (terrestrial humic-like compounds) and C3 (protein-like and pigment derived compounds). Seasonal changes in MOD quality represent different breakdown stages of the allochthonous DOM. Our survey allowed us to record and discuss the effects of the Puyehue-Cordón Caulle eruption, showing that due to the high slopes, high current and discharge of the streams the volcanic material was rapidly exported to the Moreno Lake complex. Overall, this survey underscores the magnitude and timing of the allochthonous input revealing the terrestrial subsidy to food webs in Patagonian freshwaters, which are among the most oligotrophic systems of the world.

Comparison of Two PARAFAC Models of Dissolved Organic Matter Fluorescence for a Mid-Atlantic Forested Watershed in the USA

The composition of dissolved organic matter (DOM) in a mid-Atlantic forested watershed was evaluated using two fluorescence models—one based on previously validated model (Cory and McKnight, 2005) and the other developed specifically for our study site. DOM samples for the models were collected from multiple watershed sources over a two-year period. The previously validated parallel factor analysis (PARAFAC) model had 13 DOM components whereas our site-specific model yielded six distinct components including two terrestrial humic-like, two microbial-derived humic-like, and two protein-like components. The humic-like components were highest in surficial watershed sources and decreased from soil water to groundwater whereas the protein-like components were highest for groundwater sources. Discriminant analyses indicated that our site-specific model was more sensitive to subtle differences in DOM and the sum of the humic- and protein-like constituents yielded more pronounced differences among watershed sources as opposed to the prevalidated model. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations and selected DOM metrics were also more strongly correlated with the site-specific model components. These results suggest that while the pre-validated model may capture broader trends in DOM composition and allow comparisons with other study sites, a site-specific model will be more sensitive for characterizing within-site differences in DOM.

Identifying fluorescent pulp mill effluent in the Gulf of Maine and its watershed

Using fluorescence spectroscopy and parallel factor analysis (PARAFAC) we characterized and modeled the fluorescence properties of dissolved organic matter (DOM) in samples from the Penobscot River, Androscoggin River, Penobscot Bay, and the Gulf of Maine (GoM). We analyzed excitation-emission matrices (EEMs) using an existing PARAFAC model (Cory and McKnight, 2005) and created a system-specific model with seven components (GoM PARAFAC). The GoM PARAFAC model contained six components similar to those in other PARAFAC models and one unique component with a spectrum similar to a residual found using the Cory and McKnight (2005) model. The unique component was abundant in samples from the Androscoggin River immediately downstream of a pulp mill effluent release site. The detection of a PARAFAC component associated with an anthropogenic source of DOM, such as pulp mill effluent, demonstrates the importance for rigorously analyzing PARAFAC residuals and developing system-specific models.